24/09/08
OCR Additional Science
Electric Circuits
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Static Electricity
Static electricity is when charge “builds up” on an object and
then stays “static”. How the charge builds up depends on what
materials are used:
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Static Electricity
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Circuit Symbols
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Switch
Bulb
A
V
Ammeter
Voltmeter
Cell
Fuse
Battery
Electric Current
Electric current is a flow
of negatively charged
particles (i.e. electrons).
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e-
Note that
electrons go
from negative
to positive
By definition, current is “the
rate of flow of charge”
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More basic ideas…
If a battery is
added the current
will ________
because there is a
greater _____ on
the electrons
If a bulb is added
the current will
_______ because
there is greater
________ in the
circuit
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Current in a series circuit
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If the current
here is 2
amps…
The
current
here will
be…
The current
here will
be…
And the
current
here will
be…
In other words, the current in a series
circuit is THE SAME at any point
Current in a parallel circuit
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A PARALLEL circuit is one where the current has a “choice
of routes”
Here comes the current…
Half of the current
will go down here
(assuming the bulbs
are the same)…
And the rest will
go down here…
Current in a parallel circuit
If the
current
here is 6
amps
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And the
current here
will be…
The current
here will be…
The current
here will be…
The current
here will be…
Some example questions…
3A
6A
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Voltage in a series circuit
If the voltage
across the
battery is 6V…
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V
…and these
bulbs are all
identical…
…what will the
voltage across
each bulb be?
V
V
2V
Voltage in a series circuit
If the voltage
across the
battery is 6V…
…what will the
voltage across
two bulbs be?
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V
V
4V
Voltage in a parallel circuit
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If the voltage across
the batteries is 4V…
What is the
voltage here?
4V
V
And here?
V
4V
Summary
In a SERIES circuit:
Current is THE SAME at any point
Voltage SPLITS UP over each component
In a PARALLEL circuit:
Current SPLITS UP down each “strand”
Voltage is THE SAME across each”strand”
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An example question:
6V
A3
3A
A1
V1
A2
V2
V3
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Another example question:
10V
A3
3A
A1
V1
A2
V2
V3
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Resistance
Resistance is anything that will
RESIST a current. It is measured
in Ohms, a unit named after me.
Georg Simon Ohm
1789-1854
The resistance of a component can be
calculated using Ohm’s Law:
Resistance
(in )
=
V
Voltage (in V)
Current (in A)
I
R
An example question:
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Ammeter
reads 2A
A
V
Voltmeter
reads 10V
1) What is the resistance across
this bulb?
2) Assuming all the bulbs are the
same what is the total resistance
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More examples…
3A
6V
12V
3A
2A
4V
2V
1A
What is the
resistance of
these bulbs?
Resistance
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Resistance is anything that opposes an electric current.
Resistance (Ohms, ) =
Potential Difference (volts, V)
Current (amps, A)
What is the resistance of the following:
2) A bulb with a voltage of 3V and a current of 1A.
3) A resistor with a voltage of 12V and a current of 3A
4) A diode with a voltage of 240V and a current of 40A
5) A thermistor with a current of 0.5A and a voltage of
10V
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Electromagnetic
induction
The direction of the induced current is
reversed if…
2) The magnet is moved in the opposite
direction
3) The other pole is inserted first
The size of the induced current can be
increased by:
2) Increasing the speed of movement
3) Increasing the magnet strength
4) Increasing the number of turns on
the coil
Generators (dynamos)
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Induced current can be
increased in 4 ways:
2) Increasing the speed of
movement
3) Increasing the magnetic
field strength
4) Increasing the number of
turns on the coil
5) Increasing the area of the
coil
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Power and fuses
Power is “the rate of doing work”.
The amount of power being used in
an electrical circuit is given by:
Power = voltage x current
in W
in V
in A
P
V
I
Using this equation we can work out the fuse rating for any
appliance. For example, a 3kW (3000W) fire plugged into a
240V supply would need a current of _______ A, so a
_______ amp fuse would be used (fuse values are usually
3, 5 or 13A).
Power and fuses
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Copy and complete the following table:
Appliance
Power rating
(W)
Voltage (V)
Toaster
960
240
Fire
2000
240
Hairdryer
300
240
Hoover
1000
240
Computer
100
240
Stereo
80
240
Current
needed (A)
Fuse needed
(3, 5 or 13A)
Energy and Power
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The POWER RATING of an appliance is simply how much
energy it uses every second.
In other words, 1 Watt = 1 Joule per second
E = Energy (in joules)
E
P
P = Power (in watts)
T
T = Time (in seconds)
Some example questions
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1) What is the power rating of a light bulb that transfers 120
joules of energy in 2 seconds?
2) What is the power of an electric fire that transfers
10,000J of energy in 5 seconds?
3) Farhun runs up the stairs in 5 seconds. If he transfers
1,000,000J of energy in this time what is his power rating?
4) How much energy does a 150W light bulb transfer in a) one
second, b) one minute?
5) Shaun’s brain needs energy supplied to it at a rate of 40W.
How much energy does it need during a physics lesson?
6) Damien’s brain, being more intelligent, only needs energy at
The Cost of Electricity
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Electricity is measured in units called “kilowatt hours” (kWh).
For example…
A 3kW fire left on for 1 hour uses 3kWh of energy
A 1kW toaster left on for 2 hours uses 2kWh
A 0.5kW hoover left on for 4 hours uses __kWh
A 200W TV left on for 5 hours uses __kWh
A 2kW kettle left on for 15 minutes uses __kWh
The Cost of Electricity
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To work out how much a device costs we do the following:
Cost of electricity = Power (kW) x time (h) x cost per kWh (p)
For example, if electricity costs 8p per unit calculate the cost
of the following…
1) A 2kW fire left on for 3 hours
48p
2) A 0.2kW TV left on for 5 hours
8p
3) A 0.1kW light bulb left on for 10 hours
8p
4) A 0.5kW hoover left on for 1 hour
4p
Efficiency
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Efficiency is a measure of how much USEFUL energy you
get out of an object from the energy you put INTO it.
For example, consider a TV:
)
J
0
8
(
ight
L
Electrical
Energy (200J)
Sound (40J)
Heat (
?)
Efficiency = Useful energy out
Energy in
x100%
Some examples of efficiency…
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1) 5000J of electrical energy are put into a motor. The
motor converts this into 100J of movement energy. How
efficient is it?
4) A laptop can convert 400J of electrical energy into 240J
of light and sound. What is its efficiency? Where does
the rest of the energy go?
7) A steam engine is 50% efficient. If it delivers 20,000J of