Download (A) 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 that the rate of flow
is more rapid.
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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What causes resistance?
Resistance is a measure of
how much a material tries to
stop electricity passing
through it.
Electricity is the flow of electrons along
a wire. As the electrons move along the
wire they collide with the metal ions in
the wire.
These collisions make the atoms vibrate
more, which makes the metal hotter.
All wires and components have some
resistance, so electrical appliances
always waste some energy as heat.
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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.
The heating element in this
kettle is a coil of metal that
resists electricity. As current is
passed through the metal,
resistance causes the metal to
get hot and so boils the water.
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How can resistance be investigated?
Set up this circuit with a resistor and a variable resistor.
voltage
current
(V)
(A)
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
Plot a current-voltage graph of the results.
4.2
A
V
Slowly move the variable resistor
so that the voltage increases by
0.5 V and record the current for
each setting.
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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 graph for a bulb
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Current voltage graph for a diode
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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 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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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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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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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
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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
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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
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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
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Resistors in a circuit
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How can resistance be used?
When a material resists current, some of the electrical energy
is transferred to heat or light. This property of resistance is
used in many electrical appliances.
Hairdryers and electric heaters
contain coils of nichrome wire.
Nichrome, an alloy of nickel
and chromium, resists current
and gets very hot.
Filament light bulbs contain tungsten.
Tungsten resists current and gets so
hot that it glows. This is why bulbs
waste energy as heat.
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Uses of resistors
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Resistor circuit diagram symbols
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Resistor circuit diagram symbols
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Glossary
 diode – A component that allows current to flow in one
direction only.
 light dependent resistor – A component whose






resistance changes in response to light intensity.
ohm – The unit of electrical resistance, named after
Georg Ohm.
Ohm’s law – Formula used to relate current, voltage and
resistance, if the temperature remains constant. R = V/I.
resistance – The opposition to the flow of charge.
resistor – A component that opposes the flow of charge.
thermistor – A component whose resistance changes in
response to temperature.
variable resistor – A component whose resistance can be
adjusted to vary the amount of current.
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Anagrams
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Multiple-choice quiz
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