Transcript RJR P2 Revision slides - abbreviated
P2
LO: calculate the forces acting on an object
Forces between objects
When two objects push or pull on each other, they exert equal and opposite forces on one another
e.g. you are all pushing down on the floor, but the floor is also pushing up on you….if it didn’t you’d fall straight through the floor!
LO: calculate the forces acting on an object
Resultant force
If you have multiple forces acting on an object, you can replace them with one single force that has the effect of all the other forces combined together. This single force is called the
resultant force
LO: calculate the forces acting on an object
Calculating the resultant
A rocket produces a thrust of 2000N. It has a weight of 1000N. What is the resultant force acting on the rocket?
LO: calculate the forces acting on an object
Calculating the resultant
A rocket producing a resultant force of 1000N hits a wall, causing it to come to a stop. What force does the wall exert on the rocket and the rocket exert on the wall. Explain the reasoning for your answer.
LO: calculate the forces acting on an object
Calculating the resultant
A car of weight 5000N produces a driving force of 2000N. It experiences friction force from the ground of 500N and air resistance of 300N. What are the horizontal and vertical resultant forces acting on the car?
LO: calculate the forces acting on an object
Rules for calculating the resultant
1. Forces that act in the
same direction
can be
added
together 2. Forces that act
opposite
to each other must be
taken away
3. Forces that act vertically and horizontally
CAN NOT
be added and taken away from each other and
MUST
be considered separately.
LO: calculate the forces acting on an object
Effects of forces 1
• • The resultant force on a stationary (not moving) object is zero!
If a resultant force is applied to an object, it will
accelerate in the direction of the force
LO: calculate the forces acting on an object
Effects of forces 2
• • If an object is moving with constant speed, the resultant force on it is zero If a resultant force is applied to a moving object, it will
accelerate in the direction of the force
LO: calculate the forces acting on an object
Calculating forces
F = m x a
• • • F = force (N) m = mass (kg) a = acceleration (m/s 2 )
F m x a
LO: calculate the forces acting on an object
Example 1
A car of mass 400kg is accelerating at 5m/s 2 . What is the driving force produced by the engine?
LO: calculate the forces acting on an object
Example 2
A novice skier is being pulled along a horizontal section of a nursery slope. Given that her acceleration of 1.3m/s 2 is provided by a force of 70N, calculate her mass.
LO: calculate the forces acting on an object
Example 3
A man pushes a car with a force of 200N along a straight horizontal road. He manages to accelerate the car by 0.1m/s². Find the mass of the car.
LO: understand how to draw and interpret graphs of motion
Graphs of motion
Graphs of motion are a visual representation of the motion of a body They can either show the change in
displacement
or change in
velocity
object of an
LO: understand how to draw and interpret graphs of motion
Can you draw…
Mr R cycles into work. The journey takes him 15 minutes (900s) and is a total distance of 3km (3000m). We will try to represent his journey using a graph…
LO: understand how to draw and interpret graphs of motion
Can you draw…
1. Mr R cycles to the first traffic light, a distance of 500m away. It takes him 180 seconds to do this.
2. He waits at the traffic lights for 120 seconds while the light is red 3. When the light turns green, he cycles for 2000m without stopping. This takes him 5 minutes to do.
4. After 2000m, Mr R has to stop at another traffic light. He waits for 180 seconds.
5. Realising that he is about to be late, he sprints the last 500m in 120 seconds.
LO: understand how to draw and interpret graphs of motion
Mini-plenary
1. Usain Bolt ran the 100m race in London 2012 in approximately 9.6 seconds. He ran the first 20m in approximately 2.7 seconds after accelerating and running the final 80m in 6.9 seconds. It took him 20 metres to come to a stop, which he covered in 5 seconds. Draw a distance-time graph to show this journey.
2. Explain what a horizontal line on a distance-time graph represents
Extension: What do you think the steepness of a line on a distance-time graph represents?
LO: understand how to draw and interpret graphs of motion
Using distance-time graphs
• • • How steep the line is (
the gradient
) on a distance-time graph tells you the speed that an object is moving
The steeper the line, the faster something is moving Speed is measured in m/s
LO: understand how to draw and interpret graphs of motion
Calculating the gradient
Gradient = Gradient = Change in y Change in x ∆y ∆x
Lets have a go at working out the speed that Mr C was travelling at during his journey to school!
LO: understand how to draw and interpret graphs of motion
One last definition
Two cars are travelling on a road in opposite directions. One is travelling east at 20m/s and the other is travelling west at 20m/s. Their speeds are exactly the same.
However, their velocity’s are different. What do you think their velocity’s are?
Velocity is the speed of an object in a given direction. Two objects can have the same speed, but very different velocities
LO: understand how to draw and interpret graphs of motion
Can you draw…
Mr R has brought himself a slick new ride! He has also moved house and is now living in the leafy suburbs. His journey takes him 1200 seconds, his top speed is 50m/s and his lowest is -30m/s. Let’s plot his journey into school on a velocity-time graph…
LO: understand how to draw and interpret graphs of motion
Can you draw…
1.
2.
3.
4.
5.
6.
Mr R leaves his house. He is happily driving along on country roads at a steady speed of 30m/s for two minutes… D’oh! He’s forgotten his lunch. He turns round and drives back at 30m/s for two minutes. He is at home for 60s.
Back on the road, Mr R drives at 30m/s for 300s Now on the motorway, Mr R is able to drive at 50m/s, which he does for 5mins Coming off the motorway, he stops at a traffic light for 120s Realising he is going to be late, he steadily increases his speed for the next 180 seconds from 0 to 50m/s. He arrives JUST on time!
LO: understand how to draw and interpret graphs of motion
Task
1. What does a horizontal line on a velocity-time graph represent?
2. How do you know if an object has stopped by looking at the velocity-time graph?
3. How can you tell if an object is accelerating using a velocity time graph?
4. Draw the velocity-time graph for the following journey: 0-10s = 50m/s 10-25s = 0m/s 25-50s = 60m/s 50-80s = acceleration to 80m/s
LO: understand how to draw and interpret graphs of motion
Acceleration
Acceleration is the change in speed of a body over a given amount of time
LO: understand how to draw and interpret graphs of motion
Acceleration
Acceleration can be calculated using the following equation: Acceleration = Acceleration = a = v - u t • • • • Change in velocity Time taken Final velocity – initial velocity Time taken a = acceleration (m/s 2 ) v = final velocity (m/s) u = initial velocity (m/s) t = time (s)
LO: understand how to draw and interpret graphs of motion
Example 1
A car accelerates from a velocity of 10m/s to a velocity of 25m/s in 15 seconds. What is the acceleration of the car?
LO: understand how to draw and interpret graphs of motion
Example 2
A runner starts at rest and accelerates to a top speed of 10m/s. If he does this in 2 seconds, what is his acceleration?
LO: understand how to draw and interpret graphs of motion
Example 3
A train accelerates at 2m/s² for 30 seconds. If its initial velocity was 10m/s, calculate what the final velocity will be after 30 seconds.
LO: understand how to draw and interpret graphs of motion
Task
1. What is the acceleration of a car that starts at rest and reaches a top speed of 50m/s in 25 seconds?
2. A plane starts at rest. It takes 8 seconds to take off and accelerates at a constant rate of 10m/s². What is the final take off velocity of the aircraft?
3. A runner starting at rest reaches a speed of 11m/s in 2.2 seconds during the drive phase of his 100m sprint. What is his acceleration during this phase? Assuming that his speed remains constant for the rest of the race, sketch the velocity time graph for his journey 4. A car accelerates at 5m/s² for 12 seconds, reaching a final velocity of 80m/s. What was the car’s initial velocity before it started accelerating?
LO: understand how to draw and interpret graphs of motion
How are they linked?
Gradient = Change in y Change in x Gradient = ∆y ∆x
The gradient of a velocity time graph represent the acceleration of an object!
Go back and calculate the acceleration of Mr C in the final part of his journey
LO: understand the factors that affect the stopping distance of a car
Streamlining
Most of the resistance forces that act on a car are due to
air resistance
.
Streamlining
a car will increase the top speed, even if the engine is giving the same power output
LO: understand the factors that affect the stopping distance of a car
Stopping distance
The stopping distance of a car is the minimum distance that a car can safely stop in Stopping distance = thinking distance + braking distance
LO: understand the factors that affect the stopping distance of a car
Thinking distance
The thinking distance is the distance travelled by the vehicle in the time it takes for the driver to react
alcohol other drugs and some medicines distractions, such as mobile phones tiredness speed
LO: understand the factors that affect the stopping distance of a car
Stopping distance
The stopping distance is the distance travelled by the vehicle during the time the braking force acts
weather condition of tyres/brakes condition of road speed
LO: understand the factors that affect the stopping distance of a car
Typical stopping distances
What effect would factors such as drugs, alcohol, tiredness, higher speed, adverse weather, poor road conditions or worn out breaks have on these stopping distances?
LO: understand the factors that affect the stopping distance of a car
Braking force
Which of these would need the bigger force to stop if the stopping distance remained the same? Why?
LO: understand the factors that affect the stopping distance of a car
QWC Practice
• •
A local authority is worried about the number of road deaths occurring in the area. They have imposed a ban on mobile phones while driving, imposed a speed limit of 30mph and installed speed cameras. Explain how the changes may lead to fewer people being
• • 5-6 marks criteria:
hit by cars.
Knowledge of accurate information appropriately contextualised Detailed understanding , supported by relevant evidence and examples Answer is coherent and in an organised, logical sequence, containing a wide range of appropriate or relevant specialist terms used accurately The answer shows almost faultless spelling, punctuation and grammar .
LO: understand what is meant by terminal velocity
What is happening?
The graph below shows the velocity-time profile for a skydiver falling through the air. Discuss with the people on your pod what you think is happening and why. Think about the forces that are involved at each stage
LO: understand what is meant by terminal velocity
Moving in a fluid
Let’s think about what happens when an object moves through a fluid by considering a skydiver When the skydiver
FIRST
of the aircraft,
gravity
jumps out causes him to
accelerate
. The acceleration is a
constant
so the line on v-t graph will have an
unchanging steepness
at the beginning
LO: understand what is meant by terminal velocity
Moving in a fluid
As the speed of the skydiver
increases
, the air resistance on him
increases
. The increased air resistance causes his
acceleration
to
decrease
. However, his
velocity is still increasing
i.e. he’s speeding up slower than before, but he’s NOT slowing down.
LO: understand what is meant by terminal velocity
Moving in a fluid
After a certain amount of time, the
weight
of the skydiver and the
air resistance
on the skydiver will be
balanced
.
At this point, the skydiver will be moving at
a constant velocity
. This is the
MAXIMUM
velocity it is possible for him to move with and is known as the
TERMINAL VELOCITY
.
LO: understand what is meant by terminal velocity
Moving in a fluid
The process that we have just considered is relevant for ANY object that is moving in a fluid e.g. a car driving along a road, a plane flying at 2000ft, a submarine underwater etc.
LO: understand what is meant by terminal velocity
Moving in a fluid
The factors that will • • increase/decrease the terminal • velocity of an object are: The
driving force
that the object can generate How The
streamlined fluid
the object is that the object is travelling through
LO: understand what is meant by terminal velocity
Calculating weight
The weight of an object is the force that acts on an object due to gravity. It can be calculated using the following equation: W = m x g W = weight (Newtons) M = mass (kg) g = gravitational field strength (N/kg)
g has a value of 9.81 on earth
LO: understand the link between force and extension of an object
Changing shape
When a force is applied to an object, it may accelerate. However, a second effect that the force may cause is a change
in shape of the object
LO: understand the link between force and extension of an object
Stretching objects
When an object is stretched, it stores elastic potential energy.
Some objects are better at storing this energy than others. Which of the materials on your pod is better at storing elastic potential energy?
LO: understand the link between force and extension of an object
Material properties
Beyond a point, the material will start to show plastic behaviour.
Beyond the proportional limit, the material shows plastic behaviour. The extension is now much harder to predict
A
small increase in force will give a large increase in extension
. The deformation will be
irreversible
(the material will not go back to the original shape when the force is taken away)
LO: understand the link between force and extension of an object
Real world application
Knowing how materials change shape under force is
essential
to most aspects of university.
The flexing of aircraft wings can dramatically change the lift generated. It also needs to be within limits to make sure the wings don’t break off!
LO: understand the link between force and extension of an object
Hooke’s Law
Hooke’s law states that:
The extension of an object is directly proportional to the force that is applied to it provided that the limit of proportionality is not exceeded
LO: understand the link between force and extension of an object
Hooke’s Law
Hooke’s law can be written as:
F = k x e
• • • F = Force (N) k = spring constant (N/m) e = extension (m)
LO: understand the link between force and extension of an object
Task
1. Calculate the spring constant for the spring that you did the experiment with 2. A spring is loaded with a force of 50N four times. The spring shows extensions of 0.23m, 0.25m, 0.25m and 0.24m. Calculate the spring constant for this spring 3. For the spring in the question above, calculate the force when the extension of the spring is 100cm.
4. A second spring is loaded with 100N. It shows an extension of 60cm. What is the difference between the spring constants of the two springs?
5. What would be the force required to extend the second spring by 0.45m?
LO: understand how energy can be transferred
What is work?
An object is said to have done
WORK
transfers (uses) energy when it
LO: understand how energy can be transferred
Calculating work
The work done by an object is equal to the amount of energy that it transfers
Work done = force x distance W = f x d
• • • W = work done(J) f = force (N) d = distance(m)
LO: understand how energy can be transferred
Example 1
An object of weight 40N is raised by a height of 0.4m. Calculate the work done in raising the object.
LO: understand how energy can be transferred
Example 2
2000J of energy is transferred by a sprinter as he runs a distance of 100m. Calculate the force that is exerted by the sprinter as he is running.
LO: understand how energy can be transferred
Example Questions
1. What is the definition of work done?
2. What is the unit for energy?
3. The engine of a car exerts a force of 750N. How much energy would be transferred by the engine if the car moved a distance of 100m?
4. An object of weight 50N is raised by a height of 200cm. What is the work done in raising the object?
5. 700J of energy is used by a person to move a distance of 10m. What is the force exerted by the person as they walk the distance?
6. Object A has a weight of 200N. Object B has a weight of 350N. If 1000J of energy is used to raise each object, which object will gain the most height?
LO: understand how energy can be transferred
Calculating power
Power is the amount of work done/energy transferred in a given time
Power = work done / time P = W / t
• • • P = power (W) W = work done (J) t = time (s)
LO: understand how energy can be transferred
Example 1
An object of weight 700N is raised by a height of 2m in a time of four seconds. Calculate the work done in raising the object and the power.
LO: understand how energy can be transferred
Example Questions
1. A car engine transfers 3000J in 20 seconds. What is the power generated by the engine?
2. 400J of energy is transferred in raising an object in 1 minute. What is the power?
3. A kettle has a power rating of 2000W. How much work is done by the kettle in boiling water in 40 seconds?
4. A student of weight 500N transfers 2000J whilst running up some stairs. She reaches the top of the stairs in 3 seconds. How high are the stairs and what is her power?
5. A sprinter can generate 150W whilst running. If he transfers 450J of energy, how long has he been running for?
LO: understand the nature of gravitational potential energy
Gravitational Potential Energy
Any object that is raised above the ground will have gravitational potential energy
LO: understand the nature of gravitational potential energy
Gravitational Potential Energy
GPE = mass x Gravitational Field strength x height
• • • •
GPE = m x g x h
GPE = gravitational potential energy (J) m = mass (kg) g = gravitational field strength (N/kg) h = height (m)
LO: understand the nature of gravitational potential energy
Example 1
An object of mass 10kg is raised by a height of 20m. What is the gravitational potential energy of the object?
LO: understand the nature of gravitational potential energy
Example 2
An object gains gravitational potential energy of 300J. If the mass of the object is 3kg, what is the height that the object has been raised?
LO: understand the nature of kinetic energy
Kinetic energy
All objects that are moving have kinetic energy!
LO: understand the nature of kinetic energy
Kinetic energy KE = ½ x m x v²
• • • KE = kinetic energy (J) m = mass (kg) v = velocity (m/s)
LO: understand the nature of kinetic energy
Example 1
An object has a mass of 2kg and is moving with a velocity of 5m/s. What is the kinetic energy of the object?
LO: understand the nature of kinetic energy
Example 2
An object of mass 300g has 600J of kinetic energy. How fast is the object moving?
LO: understand the nature of kinetic energy
Example questions
1. What is the equation that is used to calculate the kinetic energy of an object?
2. Calculate the kinetic energy of an object of mass 500g that is moving with a velocity of 20m/s 3. A car of mass 500kg is a moving with a velocity of 10m/s. It accelerates to a velocity of 15m/s. What is the KE of the object before and after it accelerates?
4. A sprinter has kinetic energy of 1000J and a mass of 68kg. How fast is the sprinter running?
5. A ball of mass of 0.5kg is dropped from a height of 2m. Assuming that all of the GPE is transferred to KE, what will be the velocity of the ball when it hits the ground?
LO: understand what is meant by momentum
Momentum ALL MOVING OBJECTS HAVE MOMENTUM!
LO: understand what is meant by momentum
Momentum
P = m x v
• • •
P = momentum (kgm/s) m = mass (kg) v = velocity (m/s)
LO: understand what is meant by momentum
Example question 1
An object of mass 300g is moving with velocity of 5m/s. What is its momentum?
LO: understand what is meant by momentum
Example question 2
An object has momentum of 50kgm/s. If the object has a mass of 25kg, what is its velocity?
LO: understand what is meant by momentum
Example questions
1. What is the momentum of a bullet of mass 50g travelling at 300 m/s?
2. What is the momentum of a dog (mass 12 kg) fired out of a canon at 120 m/s?
3. Calculate the momentum of a 65 kg sprinter when travelling at 9.5 ms -1 .
4. Calculate the velocity of a car of mass 700 kg that has the same momentum as the sprinter in Q3 5. A body has a mass of 2.5 kg. Calculate: • Its momentum when it has a velocity of 3.0 m/s • Its velocity when it has a momentum of 10.0 kgm/s
LO: understand what is meant by momentum
Conservation of momentum
In a closed system, the total momentum before an event and the total momentum after an event are the same. This is called
conservation of momentum .
Events you may be asked about in • • your exams are:
Collisions Explosions
LO: understand what is meant by momentum
Example 1
A railway engine of mass 800kg travelling at 5m/s collides with and becomes attached to a truck of mass 200kg travelling at 2m/s. Calculate the speed of the truck and engine after the collision
LO: understand what is meant by momentum
Example 2
A 0.5kg trolley is pushed at a velocity of 1.2m/s into a stationary trolley of mass 1.5kg. The two trolleys stick to each other after the • • impact. Calculate: The momentum of the 0.5kg trolley before the collision The velocity of the two trolleys straight after the impact
LO: explain how safety features on a car work
Brakes and crumple zones
Brakes and crumple zones are two of the main safety features on a car
LO: explain how safety features on a car work
Brakes and crumple zones
Both features work by transferring kinetic energy into other forms. What energy transfers take place in each of these features?
LO: understand static electricity
Structure of an atom
All matter is made up of atoms However, an atoms is
NOT
the smallest unit of matter like you might have been previously taught Atoms of themselves made of smaller particles
LO: understand static electricity
What is an atom made up of?
Protons –
Positively charged
particles found inside the nucleus Neutrons –
Neutral
particles found inside the nucleus Electrons –
Negatively charged
particles that orbit the nucleus
LO: understand static electricity
Static electricity by friction
When you rub one of the rods with the cloths, you create static electricity. This happens in one of two
ways. For the polythene rod, the dry cloth transfers electrons TO the surface of the rod and gives it a negative charge
LO: understand static electricity
Static electricity by friction
When you rub one of the rods with the cloths, you create static electricity. This happens in one of two
ways. For the perspex rod, the dry cloth transfers electrons away from the surface of the rod. This gives it a positive charge
LO: understand static electricity
Static electricity rules 1. Like (The same) charges attract 2. Unlike (The opposite) charges repel
LO: Understand how to create electrical circuits
Key definitions
When considering electricity, we will usually use three key terms: 1) Current: This is the flow of electric charges around a circuit. The size of the current is dependent on the rate of flow of electric charges 2) Potential Difference (Voltage): The potential difference between two points is the work done per unit charge between two points 3) Resistance: This is the resistance to the flow of electrons around a circuit
LO: Understand how to create electrical circuits
Calculating current I = Q/t
• • •
I = Current (Amps, A) Q = Charge (Coulombs, c) t = Time (s)
LO: Understand how to create electrical circuits
Example question 1
Calculate the current when 4C passes a point in 8 seconds
LO: Understand how to create electrical circuits
Example question 2
An ammeter is records a current of 8A. Calculate how much charge is passing through the ammeter in 10 seconds.
LO: Understand how to create electrical circuits
Task
1. What is the current when 20C of charge pass through an ammeter in 2minutes?
2. A battery can produce 20A of current. How much charge does it discharge in 30s?
3. Another battery can produce a charge of 30A. How long will this battery be running before it has discharge the same amount of charge as the battery in Q2?
4. A car engine requires a battery that can produce a current of 40A to start. A mechanic places a battery that can discharge 100C in 30s into a car. Will this battery be good enough to start the car? Why?
5. For the question above, how much charge would the battery have to discharge in 30s to start the engine?
LO: Understand how to create electrical circuits
Calculating voltage V = W/Q
• • •
V = Voltage (Volts, V) W = Work done (Joules, J) Q = Charge (Coulombs, c)
LO: Understand how to create electrical circuits
Example question 1
A battery transfers 30J for every coulomb of charge that passes through the battery. What is the potential difference of the battery?
LO: Understand how to create electrical circuits
Example question 2
A battery has a voltage rating of 40V. How much energy is transferred by the battery if 20C of charge pass through the battery?
LO: Understand how to create electrical circuits
Task
1. What is the voltage of a battery if it transfers 40J of energy for every 10C that pass through it?
2. A builder requires a 400V battery to power his pneumatic drill. He is told that a battery transfers 1000J for every 3C of charge that pass through it. Will this battery be good enough? Why?
3. How much energy would the battery in the question above need to transfer for every 3C to have a voltage of 400V?
4. Battery A has a rating of 300V. Battery B has a rating of 500V. What is the difference in the amount of work done by the two batteries if 20C of charge pass through both batteries?
LO: Understand how to create electrical circuits
Circuit symbols
LO: Understand the relationship-between current and voltage in a circuit
Ohm’s Law
Ohm’s Law states that the current through a resistor is proportional to the potential difference
provided the temperature is constant
LO: Understand the relationship-between current and voltage in a circuit
Ohm’s Law V = IR
• • •
V = Voltage (V) I = Current (A) R = Resistance (Ohms, Ω)
LO: Understand the relationship-between current and voltage in a circuit
Ohm’s Law 1
Calculate the potential difference across a 4Ω resistor when the current through it is 10A.
LO: Understand the relationship-between current and voltage in a circuit
Ohm’s Law 2
The potential difference across a 30Ω is 20V. What is the current through the resistor?
LO: Understand the relationship-between current and voltage in a circuit
Task
Calculate:
1. The resistance of a bulb if the current is 0.5 A and the potential difference across the bulb is 2 V.
2. The potential difference across a bulb if the resistance of the bulb is 3 and the current flowing is 2 A 3. The potential difference across a resistor of 5 with a current of 1.5 A.
4. The total resistance of a circuit if the potential difference across the cell is 12V and the current is 3 A.
5. The current flowing in a circuit with a total resistance of 5 and a potential difference across the cell of 6V.
LO: Understand the relationship-between current and voltage in a circuit
Non-Ohmic Components1
An LED does not follow Ohm’s law and is designed to only allow current to flow through in one direction
LO: Understand the relationship-between current and voltage in a circuit
Non-Ohmic Components2
An LED does not follow Ohm’s law and will only light up when current to flows through in the right direction.
If current tries to flow in the other direction it encounters a MAHOOSIVE resistance!
LO: Understand the relationship-between current and voltage in a circuit
Non-Ohmic Components2
An LDR is a component whose resistance decreases as the light intensity that falls on it increases Where would this be useful?
LO: Understand the relationship-between current and voltage in a circuit
Non-Ohmic Components3
A thermistor is a component whose resistance decreases when the temperature increases . Where would this be useful?
LO: describe features of mains electricity
AC vs DC
If you turn on any battery powered device the electricity will only ever flow in one direction. This is called
DIRECT CURRRENT
(d.c.) as the electricity goes around in just one direction.
LO: describe features of mains electricity
AC vs DC
However, the same isn’t true for mains electricity. Mains electricity uses
ALTERNATING CURRENT (a.c.)
which repeatedly flows in one direction and then reverses its flow. The frequency is how many times it changes direction in one second
LO: describe features of mains electricity
Key points
1. Mains electricity uses a.c.
2. Mains electricity is at 230V 3. Mains electricity has a frequency of 50Hz. This means it changes direction 50 times in one second
LO: describe features of mains electricity
Cables and Plugs
Cables and wires are designed to allow people to use them without risk of hurting themselves. Most appliances are supplied with three-core cable. This means the cable is made up of three separate wires.
LO: describe features of mains electricity
Components of a plug and cable
1) Live wire (brown)
– This carries the current to the appliance. Touching this can be deadly!
2) Neutral wire (blue) – This completes the circuit and is usually at 0V
3) Earth wire (green/ yellow
) – This ‘earths’ the appliance in case one of the wires touches the casing
4) Fuse
– This stops the flow of current if it gets too high
LO: describe features of mains electricity
Earthing
Components are earthed to make sure you don’t get an electric shock if the live wire accidentally touches the casing. The electricity will flow harmlessly through the earth wire instead of through you when you touch the casing.
However, appliances with plastic cases (hairdryers etc.) don’t have earth wires. Why is this?
LO: describe features of mains electricity
Earthing
Plastic is an insulator, so there is no danger if the live wire touches the casing. Therefore, these appliances are supplied with two core cables instead of three-core cables i.e. they don’t have earth wires because they don’t need them
LO: describe features of mains electricity
Fuses
A fuse is a component that has a wire running through it made of a different material/thickness than the rest of the circuit. It is designed to stop current that is too high flowing through it.
LO: describe features of mains electricity
Fuses
Fuses have a rating based on the amount of current they will allow through. For example, a 13A fuse will allow a maximum of 13 amps of current to flow through. If MORE than this tries to flow through, the wire heats up and melts, breaking the circuit and protecting the appliance
LO: describe features of mains electricity
Circuit Breakers
Circuit breakers are fitted in newer homes. They measure the difference in current in the live and neutral wires. If the difference is too great, an electromagnetic switch opens (‘trips’) which stops the flow of current. They work a lot faster than fuses and can be reset easily
LO: describe features of mains electricity
Circuit Breakers
Use the textbook spread to create a poster on fuses and circuit breakers. Your poster to include details of how they work and advantages and disadvantages of both
LO: describe features of mains electricity
Knowledge check
Copy the true sentences and change the false sentences to make them true:
1. The earth wire in a three-core cable is usually brown 2. Appliances with metal casings are supplied with three core cables 3. Fuses stop current flowing through a circuit by melting when the current flowing through them is above a certain value 4. A circuit breaker works by monitoring the difference in current between the live and earth wire 5. Mains electricity uses direct current at 100Hz.
LO: describe features of mains electricity
QWC Practice
Using as much detail as possible, explain how fuses and circuit breakers work to protect people and appliances. Which, in your opinion, is the better choice for installation
• • •
into a home and why?
• 5-6 marks criteria: Knowledge of accurate information appropriately contextualised Detailed understanding , supported by relevant evidence and examples Answer is coherent and in an organised, logical sequence, containing a wide range of appropriate or relevant specialist terms used accurately The answer shows almost faultless spelling, punctuation and grammar .
LO: describe features of mains electricity
Calculating power P = V x I
• • •
P = Power (w) V = Voltage (V) I = Current (A)
LO: describe features of mains electricity
Example 1
Calculate the power of a bulb if it is supplied with a potential difference of 230V and the current flowing through it is 0.4A
LO: describe features of mains electricity
Example 2
A kettle has a power rating of 1000W. If it is supplied with a potential difference of 230V, what is the current flowing through it?
LO: describe features of mains electricity
Example questions
1. A light bulb is connected to a 2V supply and experiences a current of 6.4A. What is the power rating of the bulb?
2. A kettle has a power rating of 1500w. What is the potential difference that it must be supplied with to have a current flowing through it of 30A?
3. A student attaches a 10V supply to a bulb with a power rating of 100w. What is the current running through the bulb?
4. The student now connect a 25w bulb to the same supply. What is the difference between the current going through this bulb compared to the 100w bulb?
5. Bulb A transfers 1000J in 10seconds. Bulb B transfers 1500J in 3 seconds. Which bulb will have a higher current running through it when connected to a 12V supply?
LO: Understand how to create electrical circuits
Calculating energy E = V x Q
• • •
E = Energy transferred (Joules, J) V = Voltage (Volts, V) Q = Charge (Coulombs, c)
LO: Understand how to create electrical circuits
Example question 1
A battery transfers 30J for every coulomb of charge that passes through the battery. What is the potential difference of the battery?
LO: Understand how to create electrical circuits
Example question 2
A battery has a voltage rating of 40V. How much energy is transferred by the battery if 20C of charge pass through the battery?
LO: Understand how to create electrical circuits
Task
1. What is the voltage of a battery if it transfers 40J of energy for every 10C that pass through it?
2. A builder requires a 400V battery to power his pneumatic drill. He is told that a battery transfers 1000J for every 3C of charge that pass through it. Will this battery be good enough? Why?
3. How much energy would the battery in the question above need to transfer for every 3C to have a voltage of 400V?
4. Battery A has a rating of 300V. Battery B has a rating of 500V. What is the difference in the amount of work done by the two batteries if 20C of charge pass through both batteries?
LO: understand the nature of radioactive decay
What is an atom made up of?
Protons –
Positively charged
particles found inside the nucleus Neutrons –
Neutral
particles found inside the nucleus Electrons –
Negatively charged
particles that orbit the nucleus
LO: understand the nature of radioactive decay
Protons, neutrons and electrons
Particle
Proton Neutron Electron
Relative charge
+1 0 -1
Relative mass
1 1 1/2000
LO: understand the nature of radioactive decay
Relative sizes
LO: understand the nature of radioactive decay
Atomic and Mass number
Atomic number:
This is the number of protons inside the nucleus of an atom WARNING: Even though the number of protons and electrons in a neutral atom are the same, make sure you say the correct definitions if you are asked in an exam!
Mass number:
This is the number of protons + neutrons in the nucleus of an atom
LO: understand the nature of radioactive decay
Atomic and Mass number
Atomic number:
This is the number of protons inside the nucleus of an atom
Mass number:
This is the number of protons + neutrons in the nucleus of an atom
LO: understand the nature of radioactive decay
Example 1
Calculate the following quantities for the element below (i) Atomic number (ii) Mass number (iii) Number of protons (iv) Number of electrons (v) Number of neutrons
LO: understand the nature of radioactive decay
Example 2
Calculate the following quantities for the element below (i) Atomic number (ii) Mass number (iii) Number of protons
LO: understand the nature of radioactive decay
Task
Use your periodic table to find the following quantities for:
nitrogen, oxygen, iron, platinum, gold, lead, mercury, potassium, calcium, phosphorus, argon, xenon
(i) Atomic number (ii) Mass number (iii) Number of protons (iv) Number of electrons (v) Number of neutrons
If the numbers are decimals, round them to the nearest whole number
LO: understand the nature of radioactive decay
The Plum Pudding Model - 1897
LO: understand the nature of radioactive decay
Enter Rutherford
Ernest Rutherford fired alpha particles at gold foil. Alpha particles have a positive charge and he expected them to go through the particle, with a small amount of deviation from their path
LO: understand the nature of radioactive decay
Gold Foil Experiment - 1911
The results are very different!
Most alpha particles go straight through with no deviation! Some, however, are diverted through very large angles! The physics community is flummoxed by this finding!
LO: understand the nature of radioactive decay
Gold Foil Experiment - 1911
The results are very different!
Most alpha particles go straight through with no deviation! Some, however, are diverted through very large angles! The physics community is flummoxed by this finding!
LO: understand the nature of radioactive decay
Conclusions
Most of the fast, highly charged alpha particles went whizzing straight through undeflected. SUGGESTS THAT MOST OF THE ATOM IS EMPTY SPACE!!
LO: understand the nature of radioactive decay
Conclusions Some of the alpha particles were deflected back through large angles. A very small number of alpha particles were deflected backwards! SUGGESTS THAT THERE IS A CONCENTRATED POSITIVE MASS SOMEWHERE IN THE ATOM.
LO: understand the nature of radioactive decay
Conclusions A very small number of alpha particles were deflected backwards! SUGGESTS THAT THE CONCENTRATED MASS IS MINISCULE COMPARED TO THE SIZE OF THE REST OF THE ATOM, BUT CONTAINS MOST OF THE MASS
LO: understand the nature of radioactive decay
Types of Radiation
There are three different kinds of radiation. Each one has a unique nature and penetration
Alpha radiation:
This particle is made up of two protons and two neutrons (i.e. a Helium nucleus). It has a charge of +2 and moves slowly because of it’s large mass. It can be stopped by a few cm of air or by a piece of paper
LO: understand the nature of radioactive decay
Types of Radiation
There are three different kinds of radiation. Each one has a unique nature and penetration
Beta radiation:
During beta radiation, a neutron turns into a proton inside the nucleus and gives off an electron, which is fired from the nucleus. The electron is small and light and so moves very fast! Beta particles can be stopped by a thin sheet of aluminium
LO: understand the nature of radioactive decay
Types of Radiation
There are three different kinds of radiation. Each one has a unique nature and penetration
Gamma radiation:
Gamma radiation usually follows alpha or beta decay. It is
NOT
a particle like the other two. It is a high energy EM wave that travels at the speed of light (the fastest that anything can travel Joel). It can only be stopped by a very thick piece of lead or concrete.
LO: understand the nature of fusion and fission
Isotopes
The diagram below shows three
isotopes
of hydrogen. What is the same and different for each
isotope
of hydrogen?
LO: understand the nature of fusion and fission
Isotopes
An isotope of an element has the same number of protons and neutrons as the original, but a different number of neutrons.
LO: understand the nature of fusion and fission
Radioactivity of a substance
LO: understand the nature of fusion and fission
Radioactivity of a substance
As a radioactive substance decays, the number of particles left in it will start to reduce. Therefore the radioactivity of the substance will begin to decrease. It will continue to decrease, until the radioactivity has reached zero!
LO: understand the nature of fusion and fission
Half-life
The half-life of a substance is the time it takes for HALF of the particles in a sample to decay or for the radioactivity of a substance to decrease by HALF .
LO: understand the nature of fusion and fission
Half-life
What is the half life of this substance?
LO: understand the nature of fusion and fission
Recap
Particle
Proton Neutron Electron
Charge
+1 0 -1
In each atom, the number of protons will ALWAYS be the same as the number of electrons. This makes sure that the overall charge is zero.
LO: understand the nature of fusion and fission
Nuclear Fission Nuclear fission is a process that uses atoms to generate VAST amounts of energy.
LO: understand the nature of fusion and fission
Nuclear Fission
Neutron To begin with, we have a simple Uranium nucleus. Uranium is used because it is already unstable. A slow moving neutron is fired at the Uranium. Uranium nucleus
LO: understand the nature of fusion and fission
Nuclear Fission
Neutron Unstable nucleus The neutron attaches itself to the uranium and makes it even more unstable! Uranium nucleus
LO: understand the nature of fusion and fission
Nuclear Fission
The unstable Uranium splits into two smaller nuclei, releasing energy in the process Neutron Uranium nucleus Unstable nucleus 2 smaller nuclei (e.g. barium and krypton)
LO: understand the nature of fusion and fission
Nuclear Fission
Along with the energy, some more neutrons are also released!
Neutron Uranium nucleus Unstable nucleus 2 smaller nuclei (e.g. barium and krypton) More neutrons
LO: understand the nature of fusion and fission
Chain reactions
These fission reactions produce a lot of energy and are used in nuclear generators. However, each fission reaction produces more and more neutrons. More neutrons
LO: understand the nature of fusion and fission
Chain reactions
Each of the neutrons can cause more fission reactions, releasing more energy and more neutrons. The reaction can soon become an
uncontrollable chain reaction,
and when that happens… More neutrons
LO: understand the nature of fusion and fission
Using fission
LO: understand the nature of fusion and fission
True or False?
Copy the true sentences and change the false sentences to make them true
1) Nuclear fission uses fast-moving electrons 2) The most common fuel used in a nuclear reactor is uranium 3) Nuclear fission involves one nucleus splitting into smaller nuclei and releasing energy in the process 4) An advantage of nuclear fission is that it doesn’t produce any harmful radioactive waste 5) A chain reaction occurs when too many neutrons cause fission reactions and the process can no longer be controlled
LO: understand the nature of fusion and fission
Nuclear fusion
Although the names sound very similar, fission and fusion are
VERY DIFFERENT PROCESSES
.
LO: understand the nature of fusion and fission
Nuclear Fission
In fission, one nuclei is split into smaller nuclei to release energy!
Neutron Uranium nucleus Unstable nucleus 2 smaller nuclei (e.g. barium and krypton) More neutrons
LO: understand the nature of fusion and fission
Nuclear fusion
In nuclear
fus
ion, two nuclei are
fuse
d together to release energy.
It is the opposite of nuclear fission
.
LO: understand the nature of fusion and fission
Where does this happen?
Contrary to popular belief, our sun is not a massive fireball. It is actually a massive
fusion reactor
!
LO: understand the nature of fusion and fission
Where does this happen?
The sun is made up of mainly
hydrogen
. The high temperature on the sun allows the hydrogen to fuse together and make helium, releasing
massive amounts of energy
in the process
LO: understand the nature of fusion and fission
Why don’t we use fusion?
Fusion seems like a great process! We only need hydrogen to do it (which we can get from water) and make helium, which is not a greenhouse gas…so why are we not using it?
LO: understand the nature of fusion and fission
Fusion future
Although fusion isn’t economically viable right now, it will (probably!) be one of the main ways we generate energy in the future. Lots and lots of research is currently being done into it currently….
LO: understand the lifecycle of a star
Nebula
All stars start their lives as part of a
nebula
. Nebulae are large clouds of dust and gas (mainly hydrogen).
LO: understand the lifecycle of a star
Protostar
Over millions of years, gravity will cause the dust and gas in the nebula to come together. As it does this, the temperature increases until hydrogen can fuse. When this happens, a
protostar
is born. This is kind of like a ‘baby’ star.
LO: understand the lifecycle of a star
Main sequence star
The
main sequence star
is the next stage after a protostar. Hydrogen fusion is now in full flow and the star is much
hotter brighter
and than the protostar.
LO: understand the lifecycle of a star
Red Giant star
When a star runs out of hydrogen, it begins to fuse other, heavier elements. This releases more energy, causing the star to expand. It also gives off red light, giving it the name ‘
Red Giant
’.
LO: understand the lifecycle of a star
White dwarf
When the red giant has run out of all fuel and can fuse nothing more, it will lose its outer layers. This leaves just the core, which is still extremely hot. It is so hot it glows white hot, giving the name to this stage – the ‘
white dwarf
’.
LO: understand the lifecycle of a star
Black dwarf
After a long enough time, the white dwarf will cool down enough so that it stops glowing white hot. It is now called a ‘
black dwarf
’.
LO: understand the lifecycle of a star
Task
The lifecycle that you have just covered is for stars about the same mass as our sun…Heavier stars, however, lead a slightly different life
LO: understand the lifecycle of a star
Red Super Giant star
Following the main sequence, the star begins to fuse together heavier elements. However, as it has far more fuel, it expands to a much larger size and gives off much more energy.
LO: understand the lifecycle of a star
Supernova
For very heavy stars, once they have run out of fuel, the star begins to collapse in on itself. It continues to collapse until it reaches a critical point when it can’t collapse any more. This causes a
MASSIVE
shockwave!
LO: understand the lifecycle of a star
Supernova
The shockwave is so large that the outer layers
EXPLODE
outwards! The explosion only lasts seconds, but can release as much energy in those seconds as the star has released up to that point! It can be as bright as the light from 10billion stars.
LO: understand the lifecycle of a star
Neutron star
After a supernova, only the star’s core is left behind. During the collapsing process, this core is turned into just neutrons.
The resulting ‘neutron star’ is
very very dense
. One spoonful of a neutron star would weigh more than the Earth!
LO: understand the lifecycle of a star
Black hole
In some very very rare cases, the core of a star left over after a supernova will continue to collapse. It will keep getting smaller and smaller until the whole star has collapsed into an infinitely small point.
LO: understand the lifecycle of a star
Black hole
This ‘singularity’ has an immense gravitational force. It’s attraction is so strong that not even light can escape from it. Hence the name ‘
black hole
’.
LO: understand the lifecycle of a star
How elements are formed
Stars are the perfect place for elements to be made! They are like massive ovens that have the energy needed to make new elements
LO: understand the lifecycle of a star
Stage 1
When a star is ‘young’ it has plenty of hydrogen. It fuses the hydrogen together to form helium. This releases massive amounts of energy in the form of light and heat.
But what happens when the hydrogen runs out?
LO: understand the lifecycle of a star
Stage 2
When a star runs out of hydrogen, it has massive amounts of helium left. It has no choice but to start fusing helium, instead of hydrogen.
Fusing helium makes heavier elements, like lithium and beryllium.
LO: understand the lifecycle of a star
Stage 2
Fusing helium releases more energy than fusing hydrogen. This makes the star bigger and it enters the red giant phase.
But what does it do when it runs out of helium?
LO: understand the lifecycle of a star
Stage 3
When the star has run out of helium, it will start fusing the heavier elements that it has created. This will make even heavier elements, such as Boron and Carbon.
LO: understand the lifecycle of a star
Stage 4
The star will keep on going through this process of running out of fuel and fusing heavy elements that it has created. This will make heavier and heavier elements.
The heaviest element that stars can make is Iron.
LO: understand the lifecycle of a star
What about elements heavier than Iron?
LO: understand the lifecycle of a star
Supernova
The temperature in stars is not hot enough to make elements heavier than Iron. For these temperatures, a supernova is
required!