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Controlling current and voltage. Resistance revision. Variable resistors. A variable resistor, also known as a rheostat, allows the resistance of a circuit to be varied. slider. thick bar. coil. variable resistor symbol. variable resistor. - PowerPoint PPT Presentation
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5 of 40 © Boardworks Ltd 2009
Variable resistors
A variable resistor has two potential paths for current: one along a short, thick bar; another along a thin long coil.
The slider is a mobile point of contact between these two routes, and its position determines the path of the current.
A variable resistor, also known as a rheostat, allows the resistance of a circuit to be varied.
variable resistorvariable resistor
symbol
slider thick bar
coil
7 of 40 © Boardworks Ltd 2009
Ohm’s Law links current, voltage and resistance:
×
Ohm’s Law
voltage (V) = current (I) × resistance (R) Volts (V) Amps (A) Ohms (Ω)
A formula triangle can be used to rearrange this equation.
Ohm’s Law explains why resistance helps to control the current and voltage in a circuit. Any changes in resistance will have a knock-on effect on both the current and voltage.
8 of 40 © Boardworks Ltd 2009
A filament lamp has a current of 20 A running through it, with a potential difference of 100 V across it.
V = I × R
Ohm’s Law practice questions
R =
100 V = 5 Ω
What is the resistance of the filamentin the bulb?
V
57.5 Ω= 4 A
Calculate the current flowing through a 230 V kettle element which has a resistance of 57.5 Ω.
V = I × R
V
I=
20 A
I =R
230 V=
9 of 40 © Boardworks Ltd 2009
Voltage–current graphs
Ohm’s Law tells us that the gradient of a V–I graph can be used to calculate resistance:
cu
rre
nt
(A)
voltage (V)
Voltage–current graphs are a simple plot of voltage, on the x-axis, against current, on the y-axis.
change in current change in voltage
voltagecurrent
1gradientTherefore:
Voltage–current graphs can vary greatly in form depending on the properties of the substance conducting electricity.
with these axes.
gradient = resistance =
resistance =
10 of 40 © Boardworks Ltd 2009
Calculating resistance from line graphs
nichrome
voltage (V)
2
0
4copper
cu
rre
nt
(A)
2
0
4
0 5 10 15 20
Calculate the resistance of these copper and nichrome wires.
1gradient
change in current change in voltage
copper:
nichrome:
gradient = 2 ÷ 5
gradient = 2 ÷ 10
= 0.4
= 0.2
R = 1 ÷ 0.4 = 2.5 Ω
R = 1 ÷ 0.2 = 5 Ω
voltage (V)0 5 10 15 20
resistance = gradient =
11 of 40 © Boardworks Ltd 2009
Different types of V–I graph
While a resistor produces a constant resistance, and thus a straight line graph, other components show a variation in resistance at different levels of current and voltage.
Such variation in resistance leads to a curved V–I graph.
A light bulb has a curved graph: it warms up as more electricity passes through it, increasing resistance.
cu
rre
nt
(A)
voltage (V)
Such components are non-Ohmic – they do not obey Ohm’s Law.
12 of 40 © Boardworks Ltd 2009
V–I graphs for diodes
A diode is a component that stops current flowing in one direction, but allows it to flow readily in the other, providing it is over a certain voltage.
What would a V–I graph for a diode look like?c
urr
en
t (A
)
voltage (V)
17 of 40 © Boardworks Ltd 2009
Understanding voltage
When voltage is measured across a component it records the difference in electrical potential energy between the two sides of the component. This is also known as the potential difference.
Thus the voltmeter reading of 4 V tells us that there is 4 V more electrical potential energy on one side of the resistor than the other.
4.0
Voltage is an electrical pushing force.
The voltage of a cell describes how much electrical potential energy it gives the electrons: this pushes them around a circuit.
18 of 40 © Boardworks Ltd 2009
Controlling voltage
Imagine your alarm clock’s battery is flat.
It requires 4 V to run successfully, but you only have a 6 V battery. This will overload its circuitry.
However, you do have a selection of fixed resistors.
How can these resistors help you to run the alarm clock from the battery without damaging it?
19 of 40 © Boardworks Ltd 2009
If two resistors are connected in series with a power supply, then the voltage is shared out between them.
10Ω 20Ω
Series resistors and potential difference
The voltage is divided between components in proportion to their resistance. Thus the larger resistor has a larger share of the power supply voltage.
2.0 4.0
6 V
20 of 40 © Boardworks Ltd 2009
What is a potential divider?
10 Ω 20 Ω
2 V 4 V
6 V
VINThis principal can be used to power the alarm clock.
The clock itself has a resistance of 20 Ω.
When placed in series with a 10 Ω resistor, the battery’s voltage is split between the resistor and clock in a 2:1 ratio.
A circuit that splits the voltage between components, to produce a specific output voltage, is a potential divider.
The voltage across the resistor is 2 V, while the voltage across the clock is 4 V. The clock can now run safely.
21 of 40 © Boardworks Ltd 2009
Potential dividers are drawn in a slightly different way to other circuits.
6 V
4 V
0 V 0 V
10 Ω
20 Ω
Drawing potential dividers
VIN
VOUT
R1
R2
potential divider diagram
This arrangement is designed to visually demonstrate the change in potential difference across the resistors.
The distance between the horizontal lines represents the potential difference between different parts of the circuit.
A potential divider uses series resistance to produce an output voltage (VOUT) that differs to the input voltage (VIN).
23 of 40 © Boardworks Ltd 2009
The output voltage (VOUT) of a potential divider depends on the size of the resistors, and also the input voltage (VIN).
VOUT and VIN are measured in volts (V).
The potential divider equation
R1 and R2 are measured in ohms (Ω).
VOUT can be calculated using the potential divider equation:
VOUT = VIN × R2 (R1 + R2)
VIN
VOUT
0 V 0 V
R1
R2
24 of 40 © Boardworks Ltd 2009
10 V
VOUT
0 V 0 V
15 Ω
60 Ω
Calculate the output voltage, VOUT, for this potential divider.
R2
(R1 + R2)
60
15 + 60
60
75
= 8 V
Potential divider questions
= 10 × 0.8
R1
R2
VOUT = VIN ×
= 10 ×
= 10 ×
25 of 40 © Boardworks Ltd 2009
10V
VOUT
0 V 0 V
75 Ω
300 Ω
Calculate the output voltage, VOUT, for this potential divider.
R2
(R1 + R2)
300
75 + 300
300
375
= 8 V
Potential divider questions
= 10 × 0.8
R1
R2
VOUT = VIN ×
= 10 ×
= 10 ×
27 of 40 © Boardworks Ltd 2009
Potential dividers with a variable output
If a variable resistor is used in a potential divider,VOUT becomes variable.
If R1 is a variable resistor…
VOUT is low when the resistance of R1 is high.
VIN
VOUT
0 V 0 V
R1
R2
R1 has a high proportion of the resistance, and thus a high proportion of the voltage.
28 of 40 © Boardworks Ltd 2009
Potential dividers with a variable output
What happens when R2
is a variable resistor?
VOUT is high when resistance of R2 is high.
VIN
VOUT
0 V 0 V
R1
R2
R2 has a high proportion of the resistance and thus a high proportion of the voltage is at VOUT.
In this arrangement, the relationship between the resistance of the variable resistor and VOUT inverts.
31 of 40 © Boardworks Ltd 2009
A semiconductor is a material which has electrical properties somewhere between an insulator, such as wood, and a conductor, such as iron.
Some semiconductors are able to vary their conductivity in response to changes in temperature or light intensity.
Semiconductors
Semiconductors are usually made from silicon.
computer processor light dependent resistor light emitting diode
Uses for semiconductors include:
32 of 40 © Boardworks Ltd 2009
The resistance of a light dependent resistor (LDR) is not fixed. It is dependent on the intensity of incident light.
res
ista
nce
(k
)
light intensity
LDRs: light and resistance
An LDR has a high resistance in the dark but a low resistance in the light.
This means that LDRs can be used in light sensing circuits, because their output is dependent on the light conditions.
The graph shows how the resistance of an LDR decreases as the light intensity increases.
LDR symbol
33 of 40 © Boardworks Ltd 2009
The resistance of a thermistor varies depending on temperature.
Thermistors: temperature and resistance
This is unusual, as resistance normally increases with increasing temperature.
Thermistors are useful in the sensor circuits of a thermostat, as their output varies with temperature fluctuations.
It has a high resistance when cold but a low resistance when hot.
res
ista
nce
(k
)
temperature (°C)
thermistor symbol
35 of 40 © Boardworks Ltd 2009
10 V
VOUT
0 V 0 V
R1
thermistor
The combination of a potential divider and a thermistor creates a temperature sensor.
Semiconductors as sensors
The thermistor’s resistance will vary with temperature, resulting in a VOUT that is temperature dependent.
If the thermistor is in the R2 position, VOUT will be high at low temperatures, as the thermistor’s resistance will be high relative to R1.
How will VOUT change if the thermistor is in the R1 position?
To produce a light sensor, replace the thermistor with an LDR.
R2