Download Using Electricity - The Student Room

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Sensing devices
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A Light Dependent Resistor (LDR)
is an input transducer, converting
light energy to a change in electrical
properties. Its resistance
decreases as light intensity
increases.
cadmium
sulfide track
LDR symbol
resistance (Ω)
Light dependent resistors
light intensity (lux)
As photons of light hit a
cadmium sulfide track, they
give bound electrons enough
energy to jump into the
conduction band.
The resistance can fall from 1 MΩ in darkness to 500 Ω in light.
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Thermistors
resistance (Ω)
Negative temperature coefficient
(NTC) thermistors are input
transducers that have a
decreasing resistance as
temperature is increased.
temperature (°C)
As the surrounding temperature
increases, the electrons in the
metal oxide of the thermistor
gain energy. This increases
thermistor
the number of charge carriers,
symbol
decreasing resistance.
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Sensors summary
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Sharing voltage
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Potential dividers
Potential dividers reduce voltage. Varying the ratio of a pair
of resistors changes the output voltage of a circuit.
VIN
R1
VOUT
VOUT = VIN ×
R2
R1 + R2
R2
0V
0V
VOUT will be a fraction of VIN. The magnitude of VOUT is
dependent upon the ratio of the two resistors R1 and R2.
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Using the potential divider equation
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Sensors and potential dividers
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Potential divider questions
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Potential dividers summary
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Electrical power
The power, or rate of energy transfer, of a device is a product
of the voltage and current passing through the component.
power (W) = voltage (V) × current (A)
What is the power of a bulb which uses a 230 V mains supply
and has a current of 0.44 A passing through it?
P = V × I = 230 × 0.44 = 101.2 W
What is the voltage across a microchip if it has a normal
operating power of 0.5 W and draws a current of 0.1 A?
V = P ÷ I = 0.5 ÷ 0.1 = 5 V
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Different forms of the power equation
Electrical power can also be calculated using resistance.
The equations linking power to resistance are found by
substituting the equation V = I × R into the power equation:
P=V×I
P=V×I
and…
V=I×R
and…
I=V÷R
Therefore, using substitution:
Therefore, using substitution:
P=I×R×I
P=V×V÷R
P = I2 × R
P = V2 ÷ R
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Energy in circuits
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Efficiency
Efficiency is a measure of how well a device transforms
energy into useful forms.
A light bulb converts electrical energy to useful light and
wasted heat.
light
electrical
heat
What is the efficiency
of the bulb if it
useful energy out
converts 50 J of
efficiency =
× 100
total energy in
electrical energy into
45 J of heat energy?
5
=
× 100 = 10 %
50
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Efficiency of a motor
6V
2A
pulley
motor
1.5 m
What is the efficiency of this
system, if the motor takes
5 seconds to lift the weight?
(take gravity to be 9.81 N/kg)
energy into system:
electrical energy = I × t × V = 2 × 5 × 6 = 60.0 J
1.4 kg
energy used:
gravitational potential energy = m × g × h
= 1.4 × 9.81 × 1.5 = 20.6 J
useful energy out
efficiency =
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total energy in
× 100 =
20.6
60.0
× 100 = 34.3 %
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Electricity in the home
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Current and drift velocity
Current is a flow of charge. Electrical
devices activate almost instantly once
they are supplied with power, however
the electrons actually move around a
circuit quite slowly. Their velocity is
called drift velocity.
Current and drift velocity are linked by the following equation:
I = current (amps)
I = nAve
n = charged particles per unit volume
A = cross-sectional area (m2)
v = drift velocity (m/s)
e = charge on an electron (1.6 x 10-19 C)
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Understanding I = nAve
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Alternating current and direct current
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RMS voltage
The voltage of AC can be viewed using an oscilloscope.
There are three common voltage measures, namely peak,
peak-to-peak and RMS (root mean squared) voltage.
peak
voltage
RMS
voltage
peak-to-peak
voltage
zero
volts
RMS is a measure of the average
magnitude of the voltage.
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VRMS =
VPEAK
√2
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RMS current and RMS power
To investigate voltage we use an oscilloscope connected
across a resistor. As V  I, the equation for calculating RMS
current is similar to the equation for RMS voltage:
IRMS =
IPEAK
√2
The equation for RMS power is a little different:
PPEAK = IPEAK × VPEAK
PRMS = IRMS × VRMS =
PRMS =
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IPEAK
√2
×
VPEAK
√2
PPEAK
2
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AC calculations
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AC/DC summary
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Glossary
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What’s the keyword?
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Multiple-choice quiz
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