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Electricity
W Richards
Worthing High School
Basic ideas…
Electric current is when electrons start to flow around a
circuit. We use an _________ to measure it and it is
measured in ____.
Potential difference (also called _______) is how big the
push on the electrons is. We use a ________ to measure
it and it is measured in ______.
Resistance is anything that resists an electric current. It
is measured in _____.”
(Words: volts, amps, ohms, voltage, ammeter, voltmeter)
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
Current in a series circuit
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
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
And the
current here
will be…
The current
here will be…
The current
here will be…
The current
here will be…
Voltage in a series circuit
If the voltage
across the
battery is 6V…
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?
V
V
4V
Voltage in a parallel circuit
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”
An example question:
6V
A3
3A
A1
V1
A2
V2
V3
Advantages of parallel circuits…
There are two main reasons why parallel circuits are used
more commonly than series circuits:
1) Extra appliances (like bulbs) can be added without
affecting the output of the others
2) If one appliance breaks it won’t affect the others
either
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:
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
in this circuit?
Current-voltage graphs
I
I
V
1. Resistor
2. Bulb
I
V
V
3. Diode
Explain the shape of each graph
Three simple components:
1) Diode – only lets current flow in one direction
2) Light dependant resistor – resistance DECREASES when
light intensity INCREASES
3) Thermistor – resistance DECREASES when temperature
INCREASES
Wiring a plug
DC and AC
V
DC stands for “Direct
Current” – the current only
flows in one direction:
Time
1/50th s
AC stands for “Alternating
Current” – the current
changes direction 50 times
every second (frequency =
50Hz)
240V
T
V
Fuses
Fuses are _______ devices. If
there is a fault in an appliance
which causes the ____ and
neutral (or earth) wire to cross
then a ______ current will flow
through the _____ and cause it
to _____. This will break the
_______ and protect the
appliance and user from further
_____.
Words – large, harm, safety, melt, live, circuit, fuse
Circuit breakers
If the current becomes too high the __________ is
activated. This will ______ the iron and the contact will
be _______. This will break the circuit.
Circuit breakers have two main advantages over fuses:
they work ______ and can easily be ______.
Words – electromagnet, broken, attract, reset, quicker
Earth wires
Earth wires are always used if an appliance has a _____
case. If there is a _____ in the appliance, causing the live
wire to ______ the case, the current “_______” down the
earth wire and the ______ blows.
Words – fuse, fault, metal, surges, touch
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
Copy and complete the following table:
Appliance
Power rating
(W)
Voltage (V)
Toaster
720
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)
Charge (Q)
As we said, electricity is when electrons move around a
circuit and carry energy with them. Each electron has a
negative CHARGE. Charge is measured in Coulombs (C).
We can work out how much charge flows in a circuit using
the equation:
Charge = current x time
(in C)
(in A)
Q
(in s)
I
T
Example questions
Charge (C)
Current (A)
Time (s)
5
2
0.4
1
20
0.5
50
250
3
60
1) A circuit is switched on for 30s with a current of 3A. How much
charge flowed?
2) During electrolysis 6A was passed through some copper chloride
and a charge of 1200C flowed. How long was the experiment on
for?
3) A bed lamp is switched on for 10 minutes. It works on a current of
0.5A. How much charge flowed?
Energy and charge
The amount of energy that flows in a circuit will depend on
the amount of charge carried by the electrons and the
voltage pushing the charge around:
Energy transferred = charge x voltage
(in J)
(in C)
(in V)
E
V
Q
Example questions
1) In a radio circuit a voltage of 6V is applied and a charge
of 100C flows. How much energy has been transferred?
2) In this circuit the radio drew a current of 0.5A. How
long was it on for?
3) A motor operates at 6V and draws a current of 3A. The
motor is used for 5 minutes. Calculate: a) The motor’s
resistance, b) the charge flowing through it, c) the
energy supplied to it
4) A lamp is attached to a 12V circuit and a charge of
1200C flows through it. If the lamp is on for 10 minutes
calculate a) the current, b) the resistance, c) the energy
supplied to the bulb.
Static electricity
Static electricity is when charge “build up” on an object
and doesn’t move, e.g. rubbing a rod:
Two different
rods will
attract each
other if they
have different
charge:
Two rods
made of the
same material
will repel each
other due to
having the
same charge:
Van de Graff generators
A charge builds up on
the dome due to
electrons being “______
off” by the belt. If a
big enough ______ is
built up then the voltage
becomes high enough to
_____ the air molecules
and the electrons “___”
down to Earth – this is
an electric _________.
Words – charge, jump,
current, rubbed, ionise
Use of static 1 - Photocopiers
Photocopiers use static
electricity. They work by: 1)
Copying an ______ of the page
onto a ______ plate, 2) Light
then causes the charge to ____
away, leaving an “electrostatic
impression” of the page, 3) The
charges left on the plate
______ small drops of black
powder, 4) The powder is
transferred from the plate onto
the _____, 5) The paper is
_____ to “fix” the powder.
Words – heated, leak, paper, image, charged, attract
Dangers of static
During refuelling the fuel gains electrons from the pipe,
making the pipe positive and the fuel negative. The
resulting voltage may cause a spark – bad news!
Solution: Either earth the fuel tank with a copper rod or
connect the tanker to the plane by a copper conductor.
Electrolysis
Electrolysis is used to extract a HIGHLY REACTIVE metal.
When we electrolysed
copper chloride the
negative chloride ions
moved to the positive
electrode and the positive
copper ions moved to the
negative electrode –
OPPOSITES ATTRACT!!!
= chloride ion
= copper ion
Some example questions
1) A current of 2A flows through some copper chloride for 1 minute
and 0.01g of copper is deposited at the negative electrode.
a) How much would be deposited if the current was increased to
6A?
b) How much would be deposited if the current was kept at 6A and
the experiment was left for another minute?
c) How much charge flowed in question (b) above?
2) A current of 0.05A flows through some copper chloride for 500
seconds and 0.05g of chlorine is released at the positive
electrode.
a) How much would be deposited if the current was increased to
0.1A?
b) How much would be deposited if the current was kept at 0.1A
and the experiment was left for 250 seconds instead?
c) How much charge flowed in question (b) above?