Transcript Today’s lesson

Today’s lesson
• Understand the term resistance
• Resistance = voltage / current
• Recall and use: V = IR
• The gradient of a Voltage current graph
known as the conductance
1
is
𝑅
Electric current flow
Electric current flows from
the POSITIVE terminal of a
power supply around a
circuit to the NEGATIVE
terminal.
The longer thinner line of
the symbol for a cell is the
positive terminal.
In the circuit above the diode is
aligned so that it allows current
to flow through the radio.
Resistance
Resistance is the opposition
that an electrical device has to
the flow of electrical current.
All devices have some
resistance. A resistor is a device
that has a particular resistance.
Now fill in the resistance sheet
a resistor
circuit symbol for a
resistor
Measuring Resistance
The resistance of a
component can be
found by measuring
the current through,
and voltage across,
the component.
Circuit used for measuring the
resistance of an indicator lamp
Current-voltage graphs
These are used to show how the
current through a component varies
with the applied voltage.
The circuit opposite could be used to
obtain a current-voltage graph of a
variable resistor.
Typical results:
Now draw a graph with
the data given
Voltage (V)
0.00 0.30 0.60 0.90
1.20
1.50
Current (A)
0.00 0.04 0.08 0.12
0.16
0.20
The current-voltage graph of a wire or a fixed
resistor at a constant temperature
I
The graph is a straight line
through the origin.
The wire or resistor obeys
Ohm’s law which states
that that the current is
proportional to the voltage
at a constant temperature.
V
Copy and complete:
current
An electric ________
will only flow around a circuit if there
gaps
are no ______
in the circuit.
resistance
All components have __________.
The greater the resistance
smaller
the ________
is the current for the same applied voltage.
ohms
Resistance is measured in ______.
resistor
A current – voltage graph for a ________
is a straight line
origin
through the _______.
This shows that the current through
proportional to the applied voltage.
the resistor is ___________
WORD SELECTION:
smaller
resistor
proportional
gaps
origin
resistance
ohms
current
The irresistible Georg Ohm
Resistance is a measure of
how hard it is for electrons to
move in an electrical circuit.
The connection between
current, voltage and resistance
was discovered in 1827 by
Georg Ohm, a German
physics and maths teacher.
The formula R = V/I is known as
Ohm’s Law. It was such an
important discovery in electricity
that the unit of resistance is
called the ohm. This unit is
represented by the symbol W.
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What is the formula/equation for Ohm’s law?
Ohm’s law is usually written as:
voltage = current x resistance
V = IxR
This formula can also be written as:
resistance =
R =
voltage
current
V
I
What are the units of voltage, current and resistance?
 Voltage is measured in volts (V).
 Current is measured in amps (A).
 Resistance is measured in ohms (W).
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Ohm’s Law
• V = IR
V
I
X
R
What does Ohm’s Law show?
What do the different arrangements of Ohm’s law show
about the links between current, voltage and resistance?
V = IxR
This version of Ohm’s Law shows that as
the voltage increases, the current increases.
The voltage and current are proportional,
while the resistance remains constant.
R =V/I
The voltage and current are proportional,
so the resistance of a material is constant,
as long as the temperature does not change.
I = V/R
For a low resistance material, more current
is allowed to flow for a given voltage.
For a high resistance material, less current
will flow at the same voltage.
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Resistance formula triangle
A formula triangle helps you to rearrange a formula.
The formula triangle for V = IR is shown below.
Cover up the quantity that you have to work out and this
gives the formula needed.
…which gives
the formula…
So to find current (I),
cover up I…


I =
V
R
x
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Calculating the resistance of a bulb
A filament bulb has a current
of 0.2 A running through it,
with a potential difference of
5 V across it.
What is the resistance of
the filament in the bulb?
V = IR
R = V
I
= 5V
0.2 A
= 25 W
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Question 1
Calculate the resistance of a lamp if a voltage of
12V causes a current of 3A to flow through the
lamp.
resistance = voltage
current
= 12V / 3A
resistance = 4 ohms (4Ω)
Question 2
Calculate the resistance of a heater if a voltage of 230V
causes a current of 200mA to flow through the heater.
resistance = voltage
current
= 230V / 200mA
= 230V / 0.200A
resistance = 1150 Ω
Question 3
Calculate the voltage across a resistance of 40Ω
when a current of 5A is flowing.
V=IxR
= 5A x 40Ω
voltage = 200V
Question 4
Calculate the current flowing through a wire of resistance
of 8Ω when a voltage of 12V is connected to the wire.
current = voltage
resistance
= 12V / 8Ω
current = 1.5A
Now complete the Ohm’s Law questions
Resistance calculations
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What affects resistance?
The resistance of a wire depends on several factors:
 material
 thickness
 length
 temperature
For example, a copper wire has a lower resistance than a
nichrome wire of the same size. Copper’s excellent ability
to conduct electricity means it is often used in wiring.
What experiments could be used to test how length
and thickness affect resistance?
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Investigating resistance and length
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How does length affect resistance?
The table shows the resistance of different lengths of wire.
What general pattern do these results show?
length
(cm)
voltage
(V)
current
(A)
resistance
(W)
5
2.6
5.0
0.52
10
2.9
4.3
0.67
20
3.0
2.1
1.48
When the length of the wire increases, resistance increases.
Why would increasing the length of the wire result in a
increase in resistance?
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Why does length affect resistance?
The effect of length of a wire on resistance can be
understood by looking at the atomic structure.
Resistance is caused by electrons colliding with metal ions.
When the length of the wire is increased, the electrons
have to travel further. So the chance of collisions will
increase, causing the resistance to increase.
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Investigating resistance and thickness
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How does thickness affect resistance?
The table shows the resistance of different thicknesses of
wire.
What general pattern do these results show?
thickness
(mm)
voltage
(V)
current
(A)
resistance
(W)
1
3.0
2.1
1.48
2
2.9
4.3
0.67
4
2.6
5.0
0.52
When the thickness of the wire increases, resistance
decreases.
Why would increasing the thickness of the wire result in a
decrease in resistance?
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Why does thickness affect resistance?
Increasing the thickness
of a wire increases the
surface area that the
electrons can flow
through.
This decreases the
chance of collisions
with metal ions.
In thick materials the
charge carrying
particles are able to
move through the
conductor more easily,
reducing resistance.
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How does temperature affect resistance?
If a component obeys Ohm’s law, then decreasing the
temperature decreases the resistance of the material.
This is because the positively charged ions in the metal do
not vibrate as fast, and the electrons can flow more easily.
A superconductor is a
material that conducts
electricity with very little
resistance at very low
temperatures.
Superconductors can be
used to make very fast
circuits and to make
magnets levitate.
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Do Now
• Question paper 3 November
2006
• Question paper 6 June 2004
• Stick in notes on factors
affecting resistance.
November 2006 paper 3
June 2004 paper 6
Today’s lesson
• Understand the term resistance
• Know how the four factors affect resistance
and their mathematical relationships needed
for paper 6 questions – question November
2004
• How to calculate resistance in series
• How to calculate resistance in parallel
November 2004
Resistance – true or false?
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How is resistance affected in a series circuit?
When two (or more) resistors are
connected in series, the combined
resistance is higher than the
individual resistors.
There is only one path for the
current to travel, which means
that it flows through the resistors
one after the other.
This has the same effect as using a
longer piece of wire.
total resistance in series
= R1 + R2
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4W
2W
Total resistance
= R1 + R2
= 4W + 2W
= 6W
© Boardworks Ltd 2007
How is resistance affected in a series circuit?
When two (or more) resistors are
connected in series, the combined
resistance is higher than the
individual resistors.
There is only one path for the
current to travel, which means
that it flows through the resistors
one after the other.
This has the same effect as using a
longer piece of wire.
total resistance in series
= R1 + R2
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4W
2W
Total resistance
= R1 + R2
= 4W + 2W
= 6W
© Boardworks Ltd 2007
Calculating resistance in series
What is the total resistance for each of these circuits?
Total resistance = R1 + R2
6W
34 W
= 6 W + 34 W
= 40 W
Total resistance = R1 + R2
= 15 W + 5 W
15 W
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5W
= 20 W
© Boardworks Ltd 2007
How is resistance affected in a parallel circuit?
When two (or more) resistors
are connected in parallel, the
current splits at the branches
and does not go through each
resistor.
This means the total combined resistance
is less than any of the individual resistors.
This has the same effect as using a
thicker piece of wire.
1
1
1
=
+
total resistance
R1
R2
in parallel
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4W
2W
Total resistance
= R1 x R2
R1 + R2
= 4W x 2W
4W + 2W
= 1.33W
© Boardworks Ltd 2007
Calculating resistance in parallel
What is the total resistance for each of these circuits?
8W
6W
5W
5W
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Total resistance = R1 x R2
R1 + R2
= 8 W x 6W
8W + 6W
= 3.4 W
Total resistance = R1 x R2
R1 + R2
= 5W x 5W
5W + 5W
= 2.5 W
© Boardworks Ltd 2007
Resistors in a circuit
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Complete use page 181
symbol
component
A
ammeter
diode
filament bulb
cell
symbol
component
heater
resistor
thermistor
LDR
Resistance of a lamp
• As the current in a lamp increases, it gets
hotter, and its resistance increases. Why?
Resistance of a lamp
• As the current in a lamp increases, it gets
hotter, and its resistance increases.
Copy please.
Resistance of a lamp
Vary the voltage and current using a variable resistor
(rheostat). Plot a graph of resistance against current
V
Resistance = voltage/current
A
R = V/I
Filament lamp
The resistance of a filament lamp
increases as the temperature of
the filament increases.
I
V
Reversing the voltage (negative
values on the graph) reverses the
direction of the electric current
but does not change the shape
of the curve.
The lamp does not obey Ohm’s
law
Diode
I
Current only flows through a diode in
one direction. (Shown by the arrow
on its symbol)
The diode has a very high resistance
in the reverse direction.
V
A light emitting diode (LED) emits
light when electric current flows.
Thermistor
The resistance of a
thermistor decreases as
the temperature increases.
The higher temperature
line therefore has a greater
slope than the lower
temperature case.
I
HOT
COLD
V
Light dependent
resistor (LDR)
The resistance of a lightdependant resistor
decreases as light
intensity increases.
The bright light line
therefore has a greater
slope than the dim light
case.
I
BRIGHT
DIM
V
Choose appropriate words to fill in the gaps below:
increases
The resistance of a filament lamp _________
when the lamp
temperature
comes on and the filament rises in ___________.
diode only allows electric current to flow one way. The
A ______
arrow on its circuit
allowed direction is shown by the _______
symbol.
resistance of a thermistor decreases if its temperature is
The ________
decreases if the
increased. The resistance of a LDR _________
light
_______level
is increased.
WORD SELECTION:
temperature
arrow decreases
increases
diode
resistance
light
Online Simulations
Simple demonstration of the effect of a variable
resistor - Freezeway.com
Battery-Resistor Circuit - PhET - Look inside a
resistor to see how it works. Increase the battery
voltage to make more electrons flow though the
resistor. Increase the resistance to block the flow of
electrons. Watch the current and resistor
temperature change
Light bulb being controlled by a variable resistor
Electric circuits with resistors - series & parallel
with meters - netfirms
Variable resistor with an ammeter & a voltmeter
Resistance measurement demo - Molecular
Expressions
Ohm's Law - PhET - See how the equation form of
Ohm's law relates to a simple circuit. Adjust the
voltage and resistance, and see the current change
according to Ohm's law. The sizes of the symbols in
the equation change to match the circuit diagram.
Ohm's Law - Fendt
Simple V-I circuit with a light bulb - Freezeway.com
Resistance in a Wire - PhET - Learn about the
physics of resistance in a wire. Change its
resistivity, length, and area to see how they affect
the wire's resistance. The sizes of the symbols in
the equation change along with the diagram of a
wire.
Resistance Wire Simulation - by KT - Designed for
the GCSE Investigation but can also be used to
show the affect of source resistance and to show
power supply maximum power. Introduction .
Worksheets for GCSE investigation
Conductivity - PhET - Experiment with conductivity
in metals, plastics and photoconductors. See why
metals conduct and plastics don't, and why some
materials conduct only when you shine a flashlight
on them.
BBC AQA GCSE Bitesize Revision:
Calculating resistance
Changing resistance
Filament lamp with IV curve
Thermistors & LDRs
Complete:
Answers
V
I
R
20 V
4A
5Ω
200 V
5A
40 Ω
300 V
0.20
6A
50 Ω
8V
500 mA
16 Ω
3 kV
20 A
150 Ω
120 V
4 mA
30 kΩ
Resistance
Measures how difficult it is for current to flow.
Measured in Ohms (Ω)
V
Resistance = voltage/current
A
R = V/I
Resistance
Measures how difficult it is for current to flow.
Measured in Ohms (Ω)
V
☺
A
Can you copy
this please?
Resistance = voltage/current
R = V/I
Question
Calculate the voltage of a battery if it supplies
300 joules of energy to 50C of charge.
voltage = energy ÷ charge
= 300 J
50 C
battery voltage = 6V
Complete:
Answers
Voltage
Energy
Charge
12V
480J
40C
20V
500J
25C
6V
120J
20C
230V
69kJ
300C
Filament lamp
Can you copy
this please?
I
The resistance of a filament lamp
increases as the temperature of
the filament increases.
V
The lamp does not obey Ohm’s
law
Diode
I
Current only flows through a diode in
one direction. (Shown by the arrow
on its symbol)
The diode has a very high resistance
in the reverse direction.
V
Can you copy
this please?
A light emitting diode (LED) emits
light when electric current flows.
Today’s lesson
• Understand the term power
• Recall and use: P =VI
• Recall and use E = VIt
Power
The amount of energy used by a device per
second, measured in Watts (Joules per
second)
A
V
Power = voltage x current
P = VI
Power
The amount of energy used by a device per
second, measured in Watts (Joules per
second)
A
V
☺
Can you copy
this please?
Power = voltage x current
P = VI
Power of a lamp
Measure the power of the lamp at it’s operating voltage.
What is the electrical energy being turned into?
A
V
Power = voltage x current
P = VI
Example
• A 200 W television is plugged into the 110V
mains. What is the current in the television?
P
V I
X
Example
• A 200 W television is plugged into the 110V
mains. What is the current in the television?
• I = P/V = 200/110 = 1.8A
P
V I
X
Example
• A kettle uses 240V and 8A. What is its power?
P
V I
X
Example
• A kettle uses 240V and 8A. What is its power?
• P = VI = 240x8 = 1920W (=1.9kW)
P
V I
X
Remember
Power is the amount of energy used by a
device per second, measured in Watts (Joules
per second)
A
V
Power = voltage x current
P = VI
Total energy
So the total energy transformed by a lamp is
the power (J/s) times the time the lamp is on
for in seconds,
E = VIt
E = energy transformed (J)
V = Voltage (also called p.d.)
I = current (A)
t = time (s)
Example
• A kettle uses 240V and 8A. What is its power?
• P = VI = 240x8 = 1920W (=1.9kW)
• How much energy does the kettle use in 5
minutes?
P
V I
X
Example
• A kettle uses 240V and 8A. What is its power?
• P = VI = 240x8 = 1920W (=1.9kW)
• How much energy does the kettle use in 5
minutes?
• E = VIt = 240x8x300 = 576000 J
P
V I
X