Download Current, Voltage and Resistance

Current, Voltage and Resistance
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What obstacles affect the flow of current?
How do different components affect the flow of current?
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What affects the size of the current?
Current is a measure of the rate of flow of electric charge
through a circuit. A large current means a faster rate of flow.
Current can be changed by increasing or decreasing
the voltage of the circuit.
Components in a circuit may
reduce the size of the current.
This is called resistance.
All wires and components
resist current.
Resistance is a measure of
how hard it is for electrons to
move in an electrical circuit.
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Resistance simulation
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What is a resistor?
A resistor is a component designed to reduce the current.
 A variable resistor has a resistance that can be changed.
 A fixed resistor has a resistance that remains the same.
Many domestic appliances use
resistance to transfer electrical
energy to heat and light energy.
As current is passed through
the metal element in this kettle,
resistance causes the metal to
get hot and so boils the water.
Resistors are also used to control
the current in electric circuits.
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How can resistance be investigated?
Set up this circuit with a resistor and a variable resistor.
voltage
current
(V)
(A)
A
V
Slowly adjust the variable resistor,
increasing the voltage by 0.5 V at a
time and record the current for
each setting.
0.0
0.0
0.5
0.6
1.0
1.1
1.5
1.8
2.0
2.5
2.5
3.0
3.0
3.5
3.5
4.2
Plot a current-voltage graph of the results.
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Investigating current and voltage
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Current–voltage graphs
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Plotting current–voltage
results for nichrome and
copper wires on the same
graph gives straight lines
with different gradients.
At the same voltage, a
copper wire allows a larger
current than a nichrome
wire of the same size.
current (A)
Is resistance the same in all wires?
copper
nichrome
voltage (V)
This means that the copper wire has a lower resistance
than the nichrome wire. What does the gradient tell you
about resistance?
The steeper the gradient of a current–voltage
graph, the lower the resistance of the wire.
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Current–voltage graphs
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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 V = I × R is known as
Ohm’s Law. It was such an
important discovery that the unit
of resistance is called the ohm.
It is represented by the symbol W.
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What is Ohm’s law?
Ohm’s law is usually written as:
voltage = current × resistance
V = I×R
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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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 = I×R
As voltage increases, the current increases.
Voltage and current are proportional, while
the resistance remains constant.
R =V/I
Voltage and current are proportional, so
the resistance of a material is constant, as
long as the temperature does not change.
I = V/R
If voltage is constant, then current
decreases as resistance increases.
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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.
This gives the formula needed.
…which gives
the formula…
So to find current (I),
cover up I…


I =
V
R
×
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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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In calculations like this one,
the resistance of the
connecting wires is usually
small enough to be ignored.
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Resistance calculations
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Finding the resistance of a component
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What affects resistance?
The resistance of a wire depends on several factors:
 material
 thickness
 length
 temperature
What experiments could be used to test how length
and thickness affect resistance?
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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.
Resistance is caused by electrons colliding with metal ions.
When the length of the wire is increased, the electrons
have to travel further, so more collisions will occur.
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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 wire increases, resistance decreases.
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.
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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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Resistance – true or false?
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Glossary
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Multiple-choice quiz
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