Download Electricity – Resistance and Power

KS4 Physics
Resistance and
Power
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Contents
Resistance and Power
Resistance
Calculating resistance
Resistance components
Electrical power
Summary activities
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Electron flow in a wire
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Electron flow and resistance
Electricity is the flow of electrons along a wire.
As the electrons move along the wire they collide with
the metal atoms.
These collisions make the atoms vibrate more…
…which makes the metal hotter.
Resistance is a measure of how much a material tries
to stop electricity passing through it.
All wires and devices have some resistance, which is why
electrical appliances always waste some energy as heat.
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Factors that affect resistance
The resistance of a wire depends on several factors:
 Material – Different materials have different resistances
because some materials are better conductors. Nichrome
wire has a higher resistance than a copper wire of the
same size.
 Length – The longer a wire is the higher its resistance.
When electrons travel down a long wire they can collide
with more metal ions than in a short wire.
 Thickness – A thin wire has a higher resistance than a
thick wire.
 Temperature – The higher the temperature of a wire the
higher its resistance. Metal ions vibrate more at higher
temperatures and so collisions with electrons are more
likely to happen.
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Investigating current and voltage
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How are current and voltage related for a resistor?
Set up this circuit with a resistor and a variable resistor.
A
V
Slowly move the variable resistor
so that the voltage increases by
0.5 V and record the current for
each setting.
voltage
(V)
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
current
(A)
0
0.6
1.1
1.8
2.5
3.0
3.5
4.2
Plot a current-voltage graph of the results.
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The graph is a straight line so
the two quantities current and
voltage are proportional.
So if voltage doubles then the
current doubles and so on.
x
x
x
x
x
Plot the current and voltage
readings for a resistor on a
graph and draw a line of best fit.
Current / A
Current-voltage graph for a resistor
x
x
x
Potential difference / V
This is called Ohm’s Law after the scientist Georg Ohm.
Ohm’s Law
The current flowing through a wire is proportional
to the potential difference (voltage) across it
provided the temperature remains constant.
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Current-voltage graphs for different wires
The copper wire has a steeper
gradient and so has a lower
resistance than the nichrome.
x
Current / A
The points produce straight
lines with different gradients.
copper
x
x
x
x
Plot the current and voltage
readings for nichrome and
copper wires of the same size.
x
x
x nichrome
x
x x
x
Potential difference / V
At the same potential difference, a copper wire lets a
larger current flow than a nichrome wire of the same
length and thickness.
The steeper the gradient of a current-voltage graph,
the lower the resistance of the wire.
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Current-voltage graphs for a bulb
Increasing the voltage across
the filament in the bulb causes
this wire to get very hot and
give out light.
x
Current / A
The graph produced is not a
straight line but a curved line.
x
x
x
Plot the current and voltage
readings for a filament bulb.
x x
x
x
Potential difference / V
As the wire gets hotter, its resistance gets higher, which
means the current flow is less. So as the temperature
rises the current is not proportional to the voltage.
The higher the temperature of a wire,
the higher its resistance.
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Current-voltage graphs and Ohm’s Law
Which of the components obeys Ohm’s Law?
I
1
I
3
V
V
V
1. A wire or
resistor
2. A filament
lamp
3. Wires of
different
materials

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I
2


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Resistance – true or false?
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Contents
Resistance and Power
Resistance
Calculating resistance
Resistance components
Electrical power
Summary activities
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The irresistable 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 equation 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, which is
represented by the symbol W.
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Resistance formula
The resistance of a conductor can be calculated using:
resistance =
R =
voltage
current
V
I
This equation can also be written as:
voltage = current x resistance
V = IxR
What are the units of voltage, current and resistance?
 Potential difference is measured in volts (V).
 Current is measured in amps (A).
 Resistance is measured in ohms (W).
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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.
Whatever quantity you are trying to find cover it up
and it will leave you with the calculation required.
So if you are
trying to find
current (I)...
…cover up I…
…which gives
the formula…

V
I

R
V
I =
R
x
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Using the resistance formula triangle
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Calculating the resistance of a bulb
A filament bulb has a current of
20 A running through it, with a
potential difference of 100 V
across it.
What is the resistance of the
filament in the bulb?
V = IR
R = V
I
= 100 V
20 A
= 5W
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Resistance calculations
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Resistors in series
When resistors are connected in series, the total resistance
can be calculated using:
Total resistance = R1 + R2
What is the total resistance for this circuit?
Total resistance
= R1 + R2
4W
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2W
= 4W + 2W
= 6W
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Resistors in series
What is the total resistance for this circuit?
6W
34 W
Total resistance = R1 + R2
= 6 W + 34 W
= 40 W
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Resistors in parallel
When resistors are connected in parallel, the total resistance
can be calculated using:
Total resistance = R1 x R2
R1 + R2
What is the total resistance for this circuit?
4W
2W
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Total resistance
= R1 x R2
R1 + R2
= 4W x 2W
4W + 2W
= 1.33W
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Resistors in parallel
What is the total resistance for this circuit?
8W
6W
Total resistance = R1 x R2
R1 + R2
= 8W x 6W
8W + 6W
= 3.4 W
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Contents
Resistance and Power
Resistance
Calculating resistance
Resistance components
Electrical power
Summary activities
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Different types of resistors
Component
Circuit symbol
resistor
variable resistor
thermistor
light dependent
resistor
diode
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Thermistors
A thermistor is a component
that has a high resistance
when cold but a low resistance
when hot.
This property makes
thermistors useful in circuits
that control and monitor
temperatures.
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Resistance / W
This is the reverse of the
normal trend for resistance.
x
x
x
x
x
x
The graph shows how the
resistance of a thermistor
decreases as its temperature
increases.
x x x
Temperature / C
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Light dependent resistors
The resistance of a light
dependent resistor (LDR)
is not fixed and depends on
the light intensity.
This makes LDRs useful in
circuits that are controlled by
light intensity.
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Resistance / kW
The graph shows how the
resistance of an LDR
decreases as the light
intensity increases.
x
x
x
x
x
x
A LDR has a high resistance
in the dark but a low
resistance in the light.
x x x
Light intensity
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Diodes
A diode is a component that
allows a current to flow in one
direction only.
It has a low resistance in one
direction and a very high
resistance in the other.
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x
x
x
x
x
x
No current flows
x
x
If the voltage is reversed or
the diode is connected the
other way around, the high
resistance of the diode
‘blocks’ the flow of current.
x
Current / A
Current flows in the direction
with low resistance but is not
proportional to the voltage.
Voltage / V
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Calculating the current through a diode
A diode has a current of 5 A
running through it and a
resistance of 5 W.
What is the potential
difference across the diode?
V = IR
= 5A x 5W
= 25 V
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Contents
Resistance and Power
Resistance
Calculating resistance
Resistance components
Electrical power
Summary activities
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Electrical power
The relationship between power, current and voltage is
shown by the equation:
power = current x voltage
P = IxV
What are the units of power, current and voltage?
 Power is measured in watts (W).
 Current is measured in amps (A).
 Voltage is measured in volts (V).
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Power formula triangle
A formula triangle helps you to rearrange a formula.
The formula triangle for P = IV is shown below.
Whatever quantity you are trying to find cover it up
and it will leave you with the calculation required.
So if you are
trying to find
current (I)...
…cover up I…
…which gives
the formula…

P
I

V
P
I =
V
x
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Calculating power
A filament bulb has a potential
difference of 200 V across it
and a current of 0.2 A running
through it.
What power is the filament
bulb operating at?
P = IV
= 0.2 A x 200 V
= 40 W
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Calculating current
A filament bulb operates at a
power of 60 W and has a
potential difference of 240 V
across it.
What is the current running
through the bulb?
P = IV
I = P
V
= 60 W
240 V
= 0.25 A
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Converting units
1 kV = 1000 V
1 kJ = 1000 J
1 kW = 1000 W
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How many volts in 6 kV?
6 000 V
_________
How many joules in 12.3 kJ?
12 300 J
_________
How many watts in 0.6 kW?
600 W
_________
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Converting units
1 kV = 1000 V
1 kJ = 1000 J
1 kW = 1000 W
How many kilovolts in 9 000 V?
9.0
_________
kV
23.5
How many kilojoules in 23 500 J? _________
kJ
How many kilowatts in 325 W?
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0.325 kW
_________
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Power calculations
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Contents
Resistance and Power
Resistance
Calculating resistance
Resistance components
Electrical power
Summary activities
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Glossary
 diode – A device that allows current to flow in one direction
only.
 ohm – The unit of electrical resistance, named after
Georg Ohm.
 power – The rate at which energy is transferred.
 resistance – The reduction in the flow of electrons
through a piece of wire or a device.
 resistor – A component that opposes the flow of electrons.
 thermistor – The resistance of this device decreases
as its temperature increases.
 variable resistor – The resistance of this device can
be changed.
 watt – The unit of power. 1 watt = 1 joule of energy
transferred every second.
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Anagrams
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Multiple-choice quiz
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