Download Document

1 of 40
© Boardworks Ltd 2009
2 of 40
© Boardworks Ltd 2009
Controlling current and voltage
3 of 40
© Boardworks Ltd 2009
Resistance revision
4 of 40
© Boardworks Ltd 2009
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
variable resistor
symbol
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.
5 of 40
© Boardworks Ltd 2009
How do variable resistors work?
6 of 40
© Boardworks Ltd 2009
Ohm’s Law
Ohm’s Law links current, voltage and resistance:
voltage (V) = current (I) × resistance (R)
Volts (V)
Amps (A)
Ohms (Ω)
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.


A formula triangle can be used to
rearrange this equation.
×
7 of 40
© Boardworks Ltd 2009
Ohm’s Law practice questions
A filament lamp 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
100 V
V=I×R R=
=
= 5Ω
I
20 A
Calculate the current flowing
through a 230 V kettle element
which has a resistance of 57.5 Ω.
V
230 V
V=I×R I= =
= 4A
R
57.5 Ω
8 of 40
© Boardworks Ltd 2009
Voltage–current graphs are a
simple plot of voltage, on the x-axis,
against current, on the y-axis.
Ohm’s Law tells us that the
gradient of a V–I graph can
be used to calculate resistance:
change in current
gradient =
change in voltage
current (A)
Voltage–current graphs
voltage (V)
voltage
resistance = current
1
Therefore: resistance = gradient with these axes.
Voltage–current graphs can vary greatly in form depending
on the properties of the substance conducting electricity.
9 of 40
© Boardworks Ltd 2009
Calculating resistance from line graphs
Calculate the resistance of these copper and nichrome wires.
copper
current (A)
4
4
2
2
0
0
0
5
10
15
voltage (V)
1
resistance =
gradient
copper:
20
nichrome
0
10
15
voltage (V)
20
change in current
gradient =
change in voltage
gradient = 2 ÷ 5 = 0.4
nichrome: gradient = 2 ÷ 10 = 0.2
10 of 40
5
R = 1 ÷ 0.4 = 2.5 Ω
R = 1 ÷ 0.2 = 5 Ω
© Boardworks Ltd 2009
Different types of V–I graph
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.
current (A)
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.
voltage (V)
Such components are non-Ohmic – they do not
obey Ohm’s Law.
11 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.
current (A)
What would a V–I graph for a diode look like?
voltage (V)
12 of 40
© Boardworks Ltd 2009
V–I graphs for different components
13 of 40
© Boardworks Ltd 2009
Calculating resistance from curves
14 of 40
© Boardworks Ltd 2009
Ohm’s Law summary
15 of 40
© Boardworks Ltd 2009
16 of 40
© Boardworks Ltd 2009
Understanding voltage
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.
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.
17 of 40
4.0
© 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?
18 of 40
© Boardworks Ltd 2009
Series resistors and potential difference
If two resistors are connected in series with a power supply,
then the voltage is shared out between them.
6V
2.0
4.0
10Ω
20Ω
The voltage is divided between components in proportion to
their resistance. Thus the larger resistor has a larger share of
the power supply voltage.
19 of 40
© Boardworks Ltd 2009
What is a potential divider?
VIN
This principal can be used to
power the alarm clock.
6V
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.
2V
4V
10 Ω
20 Ω
The voltage across the resistor is 2 V, while the voltage across
the clock is 4 V. The clock can now run safely.
A circuit that splits the voltage between components, to
produce a specific output voltage, is a potential divider.
20 of 40
© Boardworks Ltd 2009
Drawing potential dividers
A potential divider uses series resistance to produce an output
voltage (VOUT) that differs to the input voltage (VIN).
Potential dividers are drawn in a slightly different way
to other circuits.
V
IN
The distance between the
horizontal lines represents the
potential difference between
different parts of the circuit.
This arrangement is designed
to visually demonstrate the
change in potential difference
across the resistors.
6V
R1
10 Ω
VOUT
4V
R2
0V
20 Ω
0V
potential divider diagram
21 of 40
© Boardworks Ltd 2009
Fixed output potential dividers
22 of 40
© Boardworks Ltd 2009
The potential divider equation
The output voltage (VOUT) of a potential divider depends on
the size of the resistors, and also the input voltage (VIN).
VOUT can be calculated using the potential divider equation:
VIN
R2
VOUT = VIN × (R + R )
1
2
R1
VOUT
VOUT and VIN are measured in volts (V).
R2
R1 and R2 are measured in ohms (Ω).
0V
23 of 40
0V
© Boardworks Ltd 2009
Potential divider questions
Calculate the output voltage, VOUT, for this potential divider.
R2
VOUT = VIN ×
(R1 + R2)
10 V
R1
60
= 10 ×
15 + 60
VOUT
60
= 10 ×
75
= 10 × 0.8
15Ω
R2
0V
60 Ω
0V
= 8V
24 of 40
© Boardworks Ltd 2009
Potential divider questions
Calculate the output voltage, VOUT, for this potential divider.
VOUT = VIN ×
R2
(R1 + R2)
10V
R1
300
= 10 ×
75 + 300
VOUT
300
= 10 ×
375
= 10 × 0.8
75 Ω
R2
0V
300 Ω
0V
= 8V
25 of 40
© Boardworks Ltd 2009
Variable resistors in potential dividers
26 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…
VIN
R1
VOUT is low when the
resistance of R1 is high.
VOUT
R2
0V
0V
R1 has a high proportion of the
resistance, and thus a high
proportion of the voltage.
27 of 40
© Boardworks Ltd 2009
Potential dividers with a variable output
What happens when R2
is a variable resistor?
VIN
R1
In this arrangement, the
relationship between the
resistance of the variable
resistor and VOUT inverts.
VOUT
R2
0V
0V
VOUT is high when resistance of
R2 is high.
R2 has a high proportion of the
resistance and thus a high
proportion of the voltage is at VOUT.
28 of 40
© Boardworks Ltd 2009
Potential divider summary
29 of 40
© Boardworks Ltd 2009
30 of 40
© Boardworks Ltd 2009
Semiconductors
A semiconductor is a material which has electrical
properties somewhere between an insulator, such as
wood, and a conductor, such as iron.
Semiconductors are usually made from silicon.
Uses for semiconductors include:
computer processor
light dependent resistor
light emitting diode
Some semiconductors are able to vary their conductivity in
response to changes in temperature or light intensity.
31 of 40
© Boardworks Ltd 2009
LDRs: light and resistance
The resistance of a light
dependent resistor (LDR)
is not fixed. It is dependent
on the intensity of incident light.
resistance (k)
An LDR has a high
resistance in the dark but a
low resistance in the light.
light intensity
32 of 40
LDR symbol
The graph shows how the
resistance of an LDR decreases
as the light intensity increases.
This means that LDRs can be
used in light sensing circuits,
because their output is
dependent on the light conditions.
© Boardworks Ltd 2009
Thermistors: temperature and resistance
The resistance of a
thermistor varies
depending on temperature.
resistance (k)
It has a high resistance
when cold but a low
resistance when hot.
temperature (°C)
33 of 40
thermistor
symbol
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.
© Boardworks Ltd 2009
How do semiconductors work?
34 of 40
© Boardworks Ltd 2009
Semiconductors as sensors
10 V
The combination of a potential
divider and a thermistor creates
a temperature sensor.
The thermistor’s resistance will
vary with temperature, resulting
in a VOUT that is temperature
dependent.
R1
VOUT
R2
0V
thermistor
0V
To produce a light sensor, replace the thermistor with an LDR.
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?
35 of 40
© Boardworks Ltd 2009
Comparing conductors and semiconductors
36 of 40
© Boardworks Ltd 2009
37 of 40
© Boardworks Ltd 2009
Glossary
38 of 40
© Boardworks Ltd 2009
Anagrams
39 of 40
© Boardworks Ltd 2009
Controlling current and voltage quiz
40 of 40
© Boardworks Ltd 2009