13/04/2015
Electricity
W Richards
Worthing High School
Basic ideas…
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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
______, a unit named after Volta.
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
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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
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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
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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
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If the voltage
across the
battery is 6V…
…what will the
voltage across
two bulbs be?
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
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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
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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
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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
in this circuit?
More examples…
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3A
6V
12V
3A
2A
4V
2V
1A
What is the
resistance of
these bulbs?
Current-voltage graphs
I
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I
V
1. Resistor
2. Bulb
I
V
V
3. Diode
Explain the shape of each graph
Three simple components:
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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
1. Earth
wire
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4. Live
wire
5. Fuse
2. Neutral
wire
3. Insulation
6. Cable
grip
DC and AC
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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
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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
Earth wires
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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
Circuit breakers
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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
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
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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)
Charge (Q)
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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)
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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
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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
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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.
Comparing magnets and solenoids
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Magnet:
Solenoid:
N
S
Electromagnets
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The strength of an electromagnet can be increased by
doing three things:
1) Increasing the voltage
2) Increasing the number of coils
3) Inserting an iron core
(The poles can be reversed by
reversing the direction of the current)
Examples of electromagnets
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1. Speaker
2. Relay switch
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Electromagnetic
induction
The direction of the induced
current is reversed if…
1) The magnet is moved in the
opposite direction
2) The other pole is inserted
first
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Electromagnetic
induction
The size of the induced current
can be increased by:
1) Increasing the speed of
movement
2) Increasing the magnet
strength
3) Increasing the number of
turns on the coil
AC Generator
Induced current can be increased in 4 ways:
1) Increasing the speed of movement
2) Increasing the magnetic field strength
3) Increasing the number of turns on the coil
4) Increasing the area of the coil
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The National Grid
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Electricity reaches our homes from power stations through the
National Grid:
Power station
Step up
transformer
Step down
transformer
Homes
If electricity companies transmitted electricity at 240 volts through
overhead power lines there would be too much ______ loss by the time
electricity reaches our homes. This is because the current is ___. To
overcome this they use devices called transformers to “step up” the
voltage onto the power lines. They then “____ ____” the voltage at
the end of the power lines before it reaches our homes. This way the
voltage is _____ and the current and power loss are both ____.
Words – step down, high, power, low, high
Transformers
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Transformers are used to _____ __ or step down
_______. They only work on AC because an ________
current in the primary coil causes a constantly alternating
_______ ______. This will “_____” an alternating
current in the secondary coil.
Words – alternating, magnetic field, induce, step up, voltage
We can work out how much a transformer will step up or
step down a voltage:
Voltage across primary (Vp)
No. of turns on primary (Np)
Voltage across secondary (Vs)
No. of turns on secondary (Ns)
Some transformer questions
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Primary
voltage
Vp
Secondary
voltage
Vs
No. of turns
on primary
Np
No. of turns
on secondary
Ns
Step up or
step down?
12V
24V
100
?
?
400V
200V
20
?
?
25,000V
50,000V
1,000
?
?
23V
230V
150
?
?
Some example questions
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Primary
voltage
Vp
Secondary
voltage
Vs
No. of turns
on primary
Np
No. of turns
on secondary
Ns
Step up or
step down?
6V
24V
100
?
?
400,000V
200V
?
1,000
?
25,000V
?
20,000
20
?
?
230V
150
1,500
?
1) A transformer increases voltage from 10V to 30V. What is the ratio
of the number of turns on the primary coil to the number of turns on
the secondary coil?
2) A step-down transformer has twice as many turns on the primary coil
than on the secondary coil. What will be the output (secondary)
voltage if the input voltage is 50V?
Static electricity
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Static electricity is when charge “build up” on an object
and doesn’t move, e.g. rubbing a rod:
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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
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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
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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
Use of static 2 - Printers
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Inkjet printers work by spraying charged drops of ink
onto a page. The droplets can be directed using two
oppositely charged plates. The voltage on these plates
can be easily swapped or varied. The inkjet cartridge
can also moved across the page by the printer
Dangers of static
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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
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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
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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?
Numerical quiz
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1) What is the resistance of a bulb with a voltage of 12V
and a current of 2A through it?
2) This bulb transfers 100C of electrical charge into light
energy. If the bulb is 100% efficient how much light
energy was given out?
3) A step up transformer changes 24V into 240V. It has
100 turns on the primary coil. How many does it have on
the secondary coil?
4) 10g of copper is formed at the negative electrode
during electrolysis. It took 5 minutes to do this. How
much would you have after half an hour?
5) In this electrolysis experiment the current during the 5
minutes was 2A. How much charge flowed?
Numerical quiz
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6) A thermistor has a resistance of 200 when 20V is
applied across it. What is the current through the
thermistor?
7) The same thermistor is put in a warm water bath. The
resistance drops to 120. What is the current through
it now?
8) A kettle runs on a power rating of 2.4kW. If it is
plugged into a mains supply what current will it draw?
9) What fuse should this kettle have?
10)A transformer has 5 times as many turns on the
secondary coil as it does on the primary coil. What will
the voltage across the primary coil be if the secondary
coil voltage is 250V?
Numerical quiz
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11) During electrolysis 0.1g of chlorine is released at the
positive electrode. How much would have been released
if the current was doubled?
12)A 150 resistor is placed across a 3V battery. What
current will it draw?
13)How much power will this resistor use?
14) An electric fire uses 1200C of charge over 2 minutes.
What current did it draw?
15)A laptop runs on 12V. A step down transformer is used
so that it can be plugged in to a 240V mains supply. If
the transformer has 200 turns on the primary coil how
many will it have on the secondary coil?