#### Transcript Physics Unit 2 Revision

P2 revision Motion Forces and their effects Kinetic energy Momentum Electrical circuits Mains electricity Nuclear physics P2 1.1 Distance-time graphs How can we tell from a distance-time graph if an object is stationary or moving at constant speed? How do we calculate speed of a body? The slope on a distance-time graph represents speed. Speed (metre/second, m/s) = distance travelled (m) time taken (s) D S t 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 = Change in y Change in x Gradient = ∆y ∆x The gradient of a distance-time graph represent the speed of an object. P2 1.2/3 Acceleration and Velocity time graphs What is the difference between speed and velocity? What is acceleration and what are its units? How can we tell from a velocity-time graph if an object is accelerating or decelerating? What does the area under a velocity-time graph represent? Velocity is speed in a given direction (units – m/s). Two objects may travel at the same speed but may have different velocities. Acceleration is the change in an object’s velocity per second (m/s2) Acceleration = change in velocity (m/s) . Time taken for the change (seconds) The slope of the line on a velocity-time graph represents acceleration. The area under the line on a velocity-time graph represents distance travelled. LO: understand how to draw and interpret graphs of motion Acceleration Acceleration can be calculated using the following equation: Change in velocity Acceleration = Time taken Final velocity – initial velocity Acceleration = a= v-u t Time taken • • • • a = acceleration (m/s2) v = final velocity (m/s) u = initial velocity (m/s) t = time (s) LO: understand how to draw and interpret graphs of motion Using velocity-time graphs • • How steep the line is (the gradient) on a velocity-time graph tells you the acceleration of that object The steeper the line, the greater the acceleration • acceleration is measured in m/s2 LO: understand how to draw and interpret graphs of motion Measuring the acceleration Gradient = Change in y Change in x Gradient = ∆y ∆x The gradient of a velocity time graph represent the acceleration of an object. . A car is driven along a straight, snow covered, road. The graph shows how the velocity of the car changes from the moment the driver sees a very slow moving queue of traffic ahead. (a) Use the graph to calculate the distance the car travels while it is slowing down. Show clearly how you work out your answer. ........................................................................................................................ ........................................................................................................................ ........................................................................................................................ Distance = ....................................... m (3) (a) 35 (m) allow 1 mark for indicating the correct area allow 1 mark for obtaining correct figures from the graph allow 1 mark for calculating area of triangle (25) but omitting the rectangle underneath (2 x 5) 3 The space shuttle takes 9 minutes to reach its orbital velocity of 8100 m/s. (i) Write down the equation that links acceleration, change in velocity and time taken. ........................................................................................................................... (1) (ii) Calculate, in m/s2, the average acceleration of the space shuttle during the first 9 minutes of its flight. Show clearly how you work out your answer. ........................................................................................................................... ........................................................................................................................... average acceleration = .............................................. m/s2 (2) (iii) How is the velocity of an object different from the speed of an object? ........................................................................................................................... ........................................................................................................................... (1) (b) (i) acceleration = accept a = or a = do not accept velocity for change in velocity do not accept change in speed do not accept a = 1 (ii) 15 allow 1 mark for an answer of 900 or for correct use of 540 seconds 2 (iii) velocity includes direction accept velocity is a vector (quantity) accept converse answer 1 P2 2.1 Forces between objects What is the unit of force? What can we say about the forces acting on two interacting objects? When two objects interact, they always exert equal and opposite forces on each other. The unit of force is Newtons (N) P2 2.2 Resultant force What is a resultant force? What happens if the resultant force on an object is zero? What happens if the resultant force on an object is not zero? We can work out the effect of the forces on an object by replacing them with a single force called the resultant force. When the resultant force is zero, the object: -remains stationary OR - Moves at constant speed in the same direction 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 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 - acceleration • The resultant force on a stationary (not moving) object is zero! • The resultant force on an object travelling at a constant velocity is zero! • If a resultant force is applied to an object, either moving or stationary, it will accelerate in the direction of the force P2 2.3 Force and acceleration How is resultant force, acceleration and mass related to each other? Resultant force (N) = mass (kg) x acceleration (m/s2) F = ma LO: calculate the forces acting on an object Calculating forces F=mxa • F = force (N) • m = mass (kg) • a = acceleration (m/s2) F mxa LO: calculate the forces acting on an object Weight is a force W=mxg • W = weight (N) • m = mass (kg) • g = strength of gravity (m/s2) W mxg . (a) The diagram shows the horizontal forces acting on a car travelling along a straight road. (i) Calculate the size of the resultant force acting on the car. Show clearly how you work out your answer. ............................................................................................................... ............................................................................................................... Resultant force = ......................................... N (2) (ii) Describe the motion of the car when the forces shown in the diagram act on it. ............................................................................................................... ............................................................................................................... ............................................................................................................... ............................................................................................................... (2) (a) (i) 1500 allow 1 mark for subtraction shown ie 2000 – 500 2 (ii) it accelerates 1 in a forward direction accept gains speed/velocity 1 A car driver sees a dog on the road ahead and has to make an emergency stop. The graph shows how the speed of the car changes with time after the driver first sees the dog. (a) Which part of the graph represents the “reaction time” or “thinking time” of the driver? ..................................................................................................................................... (1) (b) (i) What is the thinking time of the driver? Time ........................ seconds (1) ii) Calculate the distance travelled by the car in this thinking time. ........................................................................................................................... ........................................................................................................................... ........................................................................................................................... Distance ..................................... m (3) (c) Calculate the acceleration of the car after the brakes are applied. ..................................................................................................................................... ..................................................................................................................................... ..................................................................................................................................... ..................................................................................................................................... Acceleration ............................................ (4) M5. (a) AB for 1 mark 1 (b) (i) 0.7 for 1 mark each 1 (ii) 16.8 gains 2 marks 2 but correct working (d = v.t, d = 24 × 0.7, or in terms of area under graph) gains 1 mark 1 (c) a = (v-u)/t = 24/4 =6 m/s2 (see marking of calculations) (can work in terms of graph gradient) 4 (d) Calculate the distance travelled by the car during braking. ..................................................................................................................................... ..................................................................................................................................... ..................................................................................................................................... ..................................................................................................................................... Distance ................................................ m (3) (e) The mass of the car is 800 kg. Calculate the braking force. ..................................................................................................................................... ..................................................................................................................................... ..................................................................................................................................... ..................................................................................................................................... Braking force ........................................ N (3) (d) d = v.t = 24/2 × 4 = 48 (see marking of calculations) (can work in terms of area under graph) 3 (e) F = ma = 800 × 6 = 4800 (see marking of calculations) 3 P2 2.4 On the road What is the resultant force on a vehicle travelling at constant velocity? What does the stopping distance of a vehicle depend on? What factors can increase the stopping distance of a vehicle? For any car travelling at constant velocity, the resultant force on it is zero. The braking force needed to stop a vehicle depends on a) the velocity of the vehicle and b) the mass of the vehicle. Stopping distance = thinking distance + the braking distance Factors affecting stopping distances: Tiredness, alcohol, drugs, how fast the vehicle is travelling, adverse road conditions and poorly maintained vehicles. 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 (a) A car driver makes an emergency stop. The chart shows the ‘thinking distance’ and the ‘braking distance’ needed to stop the car. Calculate the total stopping distance of the car. .................................................................................................................................... Stopping distance = ................................................. m (1 (b) The graph shows how the braking distance of a car driven on a dry road changes with the car’s speed. 15+38 = 53m P2 2.5 Falling objects What is the difference between weight and mass? What is terminal velocity? The weight of an object is the force of gravity on it (Newtons, N) The mass of an object is the quantity of matter (kilograms, kg) Gravitational field strength on Earth = The force of gravity on a 1kg object on Earth. Weight (N) = mass (kg) x gravitational field strength (N/kg) As an object falls, its acceleration decreases as the drag force starts to increase. The object starts to travel at constant velocity – this is called terminal velocity. Terminal Velocity Consider a skydiver: 1) At the start of his jump the air resistance is ZERO so he ACCELERATES downwards. 2) As his speed increases his air resistance will INCREASE 3) Eventually the air resistance will be big enough to EQUAL the skydiver’s weight (force caused by gravity). At this point the forces are balanced so his speed becomes CONSTANT - this is called TERMINAL VELOCITY Terminal Velocity Consider a skydiver: 4) When he opens his parachute the air resistance suddenly INCREASES, causing him to start SLOWING DOWN. 5) Because he is slowing down his air resistance will DECREASE again until it balances his WEIGHT. The skydiver has now reached a new, lower TERMINAL VELOCITY. Velocity-time graph for terminal velocity… Parachute opens – diver slows down Velocity Speed increases… Terminal velocity reached… Time New, lower terminal velocity reached Diver hits the ground P2 3.1 Energy, Work and Power What do we mean by the word ‘work’ and ‘power’ in science? What is the relationship between work and energy? What happens to work done against frictional forces? How do we calculate gravitational potential energy? ‘Work’ is done on an object if it moved by a force = energy transferred Work done (Joules, J) = force (N) X distance moved (m) Work done to overcome friction is mainly transformed into heat energy Gravitational potential energy is a measure of the work done against gravity. GPE (J) = mass (kg) x gravity (N/kg) x height (m) Power is the amount of energy transferred each second. 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=fxd • W = work done(J) • f = force (N) • d = distance(m) LO: understand the nature of gravitational potential energy Gravitational Potential Energy GPE = mass x Gravitational x height Field strength 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 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) (a) A chair lift carries two skiers, Greg and Jill, to the top of a ski slope. Greg weighs 700 N and Jill weighs 500 N. (i) Write down the equation that links distance moved, force applied and work done. ........................................................................................................................... (1) (ii) Calculate the work done to lift Greg and Jill through a vertical height of 200 m. Show clearly how you work out your answer and give the unit. ........................................................................................................................... ........................................................................................................................... ........................................................................................................................... work done = .................................................................... (3) (b) The chair takes 5 minutes to move from the bottom to the top of the ski slope. Use the following equation to calculate the power required to lift Greg and Jill to the top of the ski slope. Show clearly how you work out your answer. power = ..................................................................................................................................... ..................................................................................................................................... power = .................................................................. watts (2) (a) (i) work (done) = force (applied) × distance (moved) accept W = F × s or W = F × d accept provided subsequent method is correct 1 (ii) 240 000 allow 1 mark for correct substitution or correct use of 1200 (N) 2 Joules accept J do not accept j / Nm 1 (b) 800 (watts) accept 0.8 kW accept their (a)(ii) ÷ 300 correctly evaluated for 2 marks allow 1 mark for correct substitution (a)(ii) ÷ 5 correctly evaluated for 1 mark 2 P2 3.2 Kinetic and elastic energy What are kinetic energy and elastic potential energy? How does the kinetic energy of an object depend on its speed? How can we calculate kinetic energy? Kinetic energy (J) = ½ x mass (kg) x speed2 (m/s)2 Elastic potential energy is the energy stored in an elastic object when work is done on it to change its shape. 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) (b) The car has a mass of 1200 kg. Calculate the kinetic of the car when it travels at a speed of 12 m/s. Write down the equation you use, and then show clearly how you work out your answer. ........................................................................................................................ ........................................................................................................................ ........................................................................................................................ ........................................................................................................................ ........................................................................................................................ ........................................................................................................................ Kinetic energy = ....................................... J (2) (b) 86 400 allow 1 mark for correct substitution into the correct equation ie 1/2 × 1200 × 122 LO: understand the link between force and extension of an object Stretching objects When you stretch an object, work is done to change its shape. While it remains stretched the object stores elastic potential energy The constant gradient here shows us that the force and extension are proportional Most materials have a range where the force and extension are proportional 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 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=kxe • • • F = Force (N) k = spring constant (N/m) e = extension (m) (b) A student makes a simple spring balance. To make a scale, the student uses a range of weights. Each weight is put onto the spring and the position of the pointer marked The graph below shows how increasing the weight made the pointer move further. The graph below shows how increasing the weight made the pointer move further. (i) Which one of the following is the unit of weight? Draw a ring around your answer. joule (ii) kilogram newton watt (1) What range of weights did the student use? ........................................................................................................................... (1) (iii) How far does the pointer move when 4 units of weight are on the spring? ........................................................................................................................... (1) (iv) The student ties a stone to the spring. The spring stretches 10 cm. What is the weight of the stone? ........................................................................................................................... (1) (b) (i) newton 1 (ii) 0 – 5 (N) or 5 accept1 – 5 (N) do not accept 4 1 (iii) 16 (cm) 1 (iv) 2.5 (N) accept answer between 2.4 and 2.6 inclusive 1 P2 3.3 Momentum and impact How can we calculate momentum? What is its unit? What happens to the total momentum of two objects when they collide? What is the impact force and how can it be reduced? Momentum of a moving object = its mass x velocity Unit is kilogram metre/second (kgm/s) When two objects collide momentum is conserved. The impact force can be reduced by using air bags and crumple zones. LO: understand what is meant by momentum Momentum P=mxv • P = momentum (kgm/s) • m = mass (kg) • v = velocity (m/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: explain how safety features on a car work Brakes and crumple zones Brakes, air bags and crumple zones are the main safety features on a car. They increase the time taken for the impact. As F=ma and acceleration is change in velocity per second, the longer the impact time, the less force is transferred to the occupants of the car. The diagram shows a child on a playground swing. The playground has a rubber safety surface. (a) The child, with a mass of 35 kg, falls off the swing and hits the ground at a speed of 6 m/s. (i) Use the equation in the box to calculate the momentum of the child as it hits the ground. momentum = mass × velocity Show clearly how you work out your answer and give the unit. ........................................................................................................................... ........................................................................................................................... ........................................................................................................................... Momentum = ............................................................ (3) (ii) After hitting the ground, the child slows down and stops in 0.25 s. Use the equation in the box to calculate the force exerted by the ground on the child. force = Show clearly how you work out your answer. ........................................................................................................................... ........................................................................................................................... Force = ............................................................ N (2) (a) (i) 210 allow 1 mark for correct substitution i.e. 35 × 6 2 kg m/s or Ns do not accept n for N accept 210 000g m/s for 3 marks 1 (ii) 840 if answer given is not 840 accept their (a)(i) in kg m/s ÷ 0.25 correctly calculated for both marks allow 1 mark for correct substitution i.e. 210 ÷ 0.25 or their (a)(i) ÷ 0.25 2 (b) The diagram shows the type of rubber tile used to cover the playground surface. Explain how the rubber tiles reduce the risk of children being seriously injured when they fall off the playground equipment. ..................................................................................................................................... ..................................................................................................................................... ..................................................................................................................................... ..................................................................................................................................... ..................................................................................................................................... ..................................................................................................................................... (3) (b) increases the time to stop accept increases impact time do not accept any references to slowing down time 1 decreases rate of change in momentum accept reduces acceleration/deceleration reduces momentum is insufficient 1 reduces the force (on the child) 1 P2 4 Current Electricity. What are charge, current and potential difference and how are they linked? What is Ohm’s Law? What happens to current, potential difference and resistance in series and parallel circuits? Current is the flow of electrical charges around a circuit, measured in Amps (A). Potential difference is the difference in energy per change at two points in a circuit, measured in volts (V). Ohm’s Law states that Potential difference is proportional to current as long as the temperature remains constant. Ohm’s Law: V=IxR Series circuits consist of one ‘loop’ of components through which electrical current can pass. Parallel circuits consist of many branches or paths for the electrical current to take. 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 Circuit Electricity 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 repel 2. Unlike (The opposite) charges attract LO: Understand how to create electrical circuits Calculating current 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. Current = Charge/time I = Q/t • I = Current (Amps, A) • Q = Charge (Coulombs, c) • t = Time (s) LO: Understand how to create electrical circuits Calculating potential difference Potential Difference (Voltage): The potential difference between two points is the work done per unit charge between two points Potential difference = work done/charge V = W/Q • V = P.D. (Volts, V) • W = Work done (Joules, J) • Q = Charge (Coulombs, c) LO: Understand how to create electrical circuits Calculating energy Energy = potential difference x Charge E=VxQ • E = Energy transferred (Joules, J) • V = P.D. (Volts, V) • Q = Charge (Coulombs, c) 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 directly proportional to the potential difference (voltage) provided the temperature is constant LO: Understand the relationship-between current and voltage in a circuit Ohm’s Law Potential difference = Current x Resistance V = IR • V = P.D. (V) • I = Current (A) • R = Resistance (Ohms, Ω) (b) The material, called conducting putty, is rolled into cylinders of different lengths but with equal thicknesses. Graph 1 shows how the resistance changes with length. Graph 1 (i) Why has the data been shown as a line graph rather than a bar chart? ........................................................................................................................... ........................................................................................................................... (1) (ii) The current through a 30 cm length of conducting putty was 0.15 A. Use Graph 1 to find the resistance of a 30 cm length of conducting putty. Resistance = ............................................... ohms (1) (iii) Use your answer to (b)(ii) and the equation in the box to calculate the potential difference across a 30 cm length of conducting putty. potential difference = current × resistance Show clearly how you work out your answer. ........................................................................................................................... ........................................................................................................................... ........................................................................................................................... Potential difference = ............................................... volts (2) (b) (i) data is continuous (variable) 1 (ii) 36 (Ω) correct answer only 1 (iii) 5.4 or their (b)(ii) × 0.15 allow 1 mark for correct substitution 2 Series circuits IN A SERIES CIRCUIT, EVERYTHING IS CONNECTED END TO END. THERE IS NO PLACE FOR THE CURRENT TO SPLIT IN THE CIRCUIT. 1) The current through each component in a series circuit is the same 2) The potential difference of the source is shared out between the components in a series circuit Parallel circuits IN A PARALLEL CIRCUIT, THERE ARE BRANCHES THAT SEPERATE THE CIRCUIT INTO SMALLER CIRCUITS. THERE IS MORE THAN ONE PATH FOR THE CURRENT TO TAKE. 1) The potential difference across each component is the same in a parallel circuit 2) The total current in the circuit is the sum of the currents through the individual components in the circuit 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! P2 5 Mains Electricity. What is the difference between A.C. and D.C? The features of a Plug. How fuses and circuit breakers (RCCBs) work to make electrical appliances safe. Alternating current (A.C.) oscillated forwards and backwards. Direct Current (D.C.) only flows in one direction. A 3 pin plug contains a live, neutral and earth wire, along with a fuse, and cable grip. Fuses melt when too much current passes through them, breaking the circuit. Circuit Breakers measure the difference between the live and neutral wires and ‘trip’ when this is too big. 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. Voltage / V Direct current is what comes out of a battery. It only flows in one direction and should remain at a constant value throughout the life of the battery. If you connected this sort of supply to an oscilloscope it would look like this. time © Boardworks Ltd 2001 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 Voltage / V +320 0 One cycle (1/50th second) -320 Time Frequency (Hz) = 1 ÷ time taken for one cycle (s) © Boardworks Ltd 2001 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. 2) Neutral wire (blue) – This completes the circuit and is usually at 0V 3) Earth wire (green/yellow) – This ‘earths’ the appliance in case of a fault 4) Fuse – This stops the flow of current if it gets too high . (4) (a) The diagram shows a three-pin plug and electrical cable. Name a suitable material to make: the plug casing ................................................................. the inner cores of the cable. .............................................. Give the reason for your choice of each material. plug casing ..................................................................................................... ........................................................................................................................ inner core of the cable ................................................................................... ........................................................................................................................ . (a) plastic accept rubber 1 as it is a good electrical insulator accept as it is a poor electrical conductor any mention of heat negates this mark 1 copper 1 as it is a good electrical conductor any mention of heat negates this mark 1 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. 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 The Circuit breaker What happens if the current is too large? Too much current causes the electromagnet to produce a magnetic field strong enough to open the switch. switch electromagnet (The circuit break is said to ‘trip’). This switches off the current. © Boardworks Ltd 2003 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 Calculating electrical power Power = Potential difference x Current P=VxI • P = Power (w) • V = P.D. (V) • I = Current (A) 2 (a) (iii) The bulb is at full brightness when the potential difference across the bulb is 12 V. The current through the bulb is then 3 A. Calculate the power of the bulb when it is at full brightness and give the unit. Use the correct equation from the Physics Equations Sheet. ............................................................................................................................................ ............................................................................................................................................ ............................................................................................................................................ Power = .................................................. (3 marks) P2 6 Radioactivity. The discovery of the nucleus. The different types of ionising radiation. What happens during nuclear fission and nuclear fusion. How to calculate half life. Rutherford discovered the presence of the nucleus after preforming his alpha scattering experiment. Ionising radiation occurs in the form of alpha particles, beta particles and gamma waves. In nuclear fission, heavy unstable nuclei split to release ionising radiation and lots of energy. In nuclear fusion, helium nuclei fuse to become hydrogen nuclei, creating LOTS of energy. Half life is the time taken for half of a quantity of radioactive isotope to decay, It is useful in dating materials. Radioactivity 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 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 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 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. Neutron Uranium nucleus 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 fusion In nuclear fusion, two nuclei are fused together to release energy. It is the opposite of nuclear fission. 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 P2 7 Stars. The features of the life cycle of a main sequence star like the sun. What could happen in the life of a star larger than then sun. Main sequence stars go through the following stages: Nebulae – protostar – main sequence – red giant – white dwarf – black dwarf. Larger stars could form super red giants, these will then turn in to a supernova – neutron star/black hole. 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 and brighter 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 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’.