In unit 2 we will learn about

the physics of electricity and

electronics.

This includes circuits, Ohm’s

law, resistance, electrical
energy and power,
electromagnetism and
electronic components.
 What is
electricity?
Key words: electrons, conductors,
insulators, charge, current

By the end of this lesson you will be able

to:

State that electrons are free to move in a

conductor
Describe the electrical current in terms
of
movement of charges around a circuit
Distinguish between conductors and
insulators
and give examples of each.
Carry out calculations involving Q = It
 Rutherford Bohr
model
Thomson’s Plum pudding model

What is inside an atom?

Quantum model of the nucleus
Charge cloud model Rutherford model
 The atom
An atom is a fundamental unit of
matter
made up of

protons (with a positive charge)

neutrons (neutral – no charge)
electrons (with a negative
charge)
 What is electricity?
Everything is made of atoms which contain
POSITIVE particles called PROTONS and
NEGATIVE particles called ELECTRONS.

Electron (-) Proton (+)

Neutron
An atom will usually have the same
number of positives and negatives
This makes the atom NEUTRAL.

Electron (-) Proton (+)

Neutron
 Electrical Charge
Electric charge is given the symbol

Q
Electrons are the charge carriers
that flow in an electrical
circuit –
from the negative to positive
terminals.
 Electrical Charge
Charge is measured in

Coulombs
which is given the symbol

C
 Electrical Charge
The charge on a proton is

1.6 x 10 C
-19

which is the same size as the

charge on an
electron.
 What is
electricity?
Electrons have a negative
charge
(Q) measured in coulombs (C).

Electrons move round a

circuit from
negative to positive
(remember like
charges repel, opposites
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Conductors & Insulators
What makes something a good
conductor?

Good conductors allow electrons to

move
through them easily. Insulators do
not
allow electrons to move easily.
 What is
electricity?
So electricity is…

movement of charge round a

circuit.

We call this electric

current.
 Charge, Current & Time
Electric current is given the symbol

I
Electric current is the movement
of
negative charges (electrons) in a
circuit
 Charge, Current & Time
Current is the amount of charge
flowing
per second and is given the unit

Amps (A)
 Charge, Current & Time
If
so current isof
a current charge flowing
1 A is 1 C ofper
charge
second then in 1 s.
transferred

Q
I= Charge transferred
in coulombs (C)

t
Current in Amps (A)
time in seconds (s)
 Charge, Current & Time
This can be rearranged as

Q =It
or
Q
t=
I
Key words: series, current, ammeter, voltmeter,
battery, resistor, variable resistor, fuse,
switch, lamp,
voltage

By the end of this lesson you will be able

to:

Draw circuit diagrams to show the correct

positions of
an ammeter in a series circuit.
Draw and identify the circuit symbols for an
ammeter, voltmeter, battery, resistor, variable
resistor, fuse, switch and lamp.
State that in a series circuit, the current is
the same at
all positions.
 Different types of
circuit
There are different ways in which
you can
connect cells and components (such
as
lamps) to create a circuit:

series
parallel
a mixture of both
 Series Circuit

A series circuit has only one

electrical path.

You can trace from one side of

the battery to the other,
through each component, without
lifting your finger from the
page.
 Different types of
circuit
There are different ways in which
you can
connect cells and components (such
as
lamps) to create a circuit:

series
parallel
a mixture of both
 Series Circuit

A series circuit has only one

electrical path.

You can trace from one side of

the battery to the other,
through each component, without
lifting your finger from the
page.
Physics Animations – Series Circuit
 Name that component
Resistor Fuse Battery Ammeter

On the back of p2 carefully draw each symbol

and
label – in pencil!

Voltmeter Switch Lamp

Cell
Variable resistor
 Build a series circuit
On the worksheet you will find
four
building circuit activities.

Follow the instructions carefully!

Answer each question as you go.
Make careful observations.

Lesson 2 build a series circuit.pub

 Build a series circuit
Build a series circuit which contains
a
6V battery pack, three 3.5 V lamps in
lamp holders, and a meter used for
measuring current.

What is the meter called?

Where is it positioned in the circuit?

Activity 1
 Activity 2

Bulbs are much dimmer!

 Activity 3 - Change your
circuit…
Move your ammeter to different
positions
in the series circuit.

Make a note of the positions each time,

and of the current at each position.

What can you say about the current in a

series circuit?
 Successful Circuit
Diagrams
On your worksheet you have drawn a circuit
diagram.

To be successful at circuit diagrams:

• use a ruler and pencil
• draw components carefully
• draw wires as straight lines (with
corners as
• right angles!)
• make sure all components are correctly
draw
• and joined in the circuit.
 Your circuit diagram…
should look like
this:
e in this circuit, current is the same at all point
e in this circuit, current is the same at all point
 Series Circuits and
Current
We are measuring the current I in a series circuit.

What have we observed?

We find that the current is the same
at
all points.

How can this be written

mathematically?
I1 = I2 = I3 = I4 and so on
al Int 2 Physics – Electricity & Electronics – Circuits – Series Cir
 Think…
How could you make use of a series
circuit
to investigate which materials are
conductors and which materials are
insulators?

Which components would you need?

What would you observe?
 …and learn
components and names
formulae and symbols
what is a series circuit?
current in series circuit
drawing a series circuit
diagram
What have I learned?
Key words: series, parallel, ammeter, current,

By the end of this lesson you will be able

to:

Draw circuit diagrams to show the correct positions

of an
ammeter in a parallel circuit.
Draw and identify the circuit symbols for an ammeter,
and
lamp.
State that in a series circuit, the current is the
same at
all positions.
State that in a parallel circuit, the sum of the
current in
the branches adds up to the current drawn from the
supply.
 Quick Quiz
What is a series circuit?
What is the symbol for current?
What are the units of current?
What is the relationship between current
and
time?
What do we know about the current in a
series
circuit?
How do we measure current?
Draw the symbol for this.
Describe how to measure current in a
series
circuit.
 Build another circuit

Build a series circuit which

includes a 6V
battery, a 6V lamp and an
ammeter.

Draw the circuit diagram for

your circuit:
 Build another circuit
We will now take one of your
series
circuits, and “add it” to someone
else’s.
Another ammeter has been added.

What do you notice about the

readings on
the ammeter?
 Build another circuit
We will now “add” another series
circuit.

What do you notice about the

readings on
the ammeter?
What sort of circuit is
this?
We have constructed a parallel
circuit.

What does the circuit diagram look

like?

Try drawing it on Crocodile

Physics.
Draw the circuit diagram
below
 Parallel Circuit
We have constructed a parallel circuit.

This is a circuit with different

branches.

When it reaches a junction, the current

can divide and take different
branches.
 Parallel Circuits and
Current
We are measuring the current I in a
parallel circuit.

What have we observed?

We find that the current in each of the
branches adds up to the total current.
How can this be written mathematically?
IT = I1 + I2 + I3 and so on
 Electric
Circuits
How many ways can you
make two light bulbs
work?
 A SIMPLE CIRCUIT
SWITCH
CELL

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LIGHT BULB
 A SIMPLE CIRCUIT

2A 2A 2A

2A 2A 2A
 A Series
Circuit
1A 1A 1A

Wha
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1A 1A
1A 1A
A
Paralle Wh
at
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Circuit en
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 2A 2A
A
Paralle
l
Circuit
4A 2A 4A 4A

2A 2A

4A 4A 4A
What have you learned
today?
Key words: voltage, potential difference,
voltmeter, series, parallel

By the end of this lesson you will be able

to:

Draw and identify the circuit symbols for a

voltmeter, battery, and lamp
State that the voltage of a supply is a measure
of the energy given to the charges in a circuit.
Draw circuit diagrams to show the correct
positions of a
voltmeter in a circuit.
State that the sum of potential differences
across the
components in series is equal to the voltage of
the
supply.
State that the potential difference across
components
 What is
electricity?

What is a
voltage?

What is a volt?
Discussion
Demonstration
Voltage in series and parallel
What is the energy change
which takes place in a
battery?

Chemical to Electrical
 When a battery is in a
circuit…
The electrical energy is carried by
the
electrons that move round the
circuit.

It is converted into others forms

of
energy.
If there is a bulb in the
circuit, it is
converted from

to

http://www.members.shaw.ca/len92/current_animation.gif
The amount of electrical energy the
electrons have at any point in a circuit
is
known as their “potential”.

As they move the electrons transfer energy

into other forms.

This means at any two points the electron

has
different amounts of energy.
lectrons start with (for example) 6J of energy. They have “potential

As they pass
through the
bulb, some of
the energy is
converted to
light.

Electrons which
have passed
through the bulb
have less
energy. Or less
“potential”.
There is a “potential”
difference in the circuit
 What has “potential
difference” got to do with
voltage?
It is the same thing!

The potential difference (p.d.), or

voltage,
of a battery is a measure of the
electrical
energy given to one coulomb of
charge
passing through the battery.
 Potential Difference or
Voltage (V)

A 9 V battery will give how much

energy
to each coulomb of charge
passing
through the battery?

9 J
 Potential Difference or
Voltage (V)

A 1.5 V battery will give how

much energy
to each coulomb of charge
passing
through the battery?

1.5 J
 Potential Difference or
Voltage (V)

A battery with a p.d. of 6V will

give how
much energy to each coulomb of
charge
passing through the battery?

6 J
 Voltage or p.d.
Voltage (or p.d.) is measured in

volts
and is given the symbol

V
 Summary of Units
Quantity Symbol Units Symbol
charge Q coulombs C

time t seconds s

current I amperes A

voltage V volts V
 How can we measure
voltage?
Voltage (or p.d.) can be measured
using a
voltmeter. V

An ammeter is connected in the

circuit
but a voltmeter must be connected
across
the component.
 You can’t measure
voltage…
in a circuit

through a circuit

through a component

flowing
 Build a series circuit
Build a series circuit which contains
a
6V battery, two 6V lamps, and a meter
used for measuring potential
difference
across each lamp.

What is the meter called?

Where is it positioned in the circuit?

 Drawing a circuit
diagram
Now draw a circuit diagram of the series
circuit which you built.

Remember to use a ruler and pencil, draw

components carefully, draw wires as
straight lines (with corners as right
angles!), and make sure all components
are
correctly draw and joined in the
circuit.
 Series Circuits and
Voltage
We are measuring the potential difference (V) in a
series circuit.

What have we observed?

We find that the

How can this be written

mathematically?
 Parallel Circuit
Now use the same components to
construct a parallel circuit.

This is a circuit with different

branches.
 Parallel Circuits and
Voltage
We are measuring the potential
differences in a parallel circuit.

What have we observed?

How can this be written

mathematically?
 Tasks & Homework
Yellow Practice Questions: 2.10,
2.11

Numerical Questions: p33-36 qu 5-

14

Complete for homework for Tuesday

27th
November
What have you learned
today?
 Quick Quiz
What have we learned?
What have you learned
today?
Key words: electrical resistance, voltage,
current, Ohm’s law, ohms, resistor,
variable power supply

By the end of this lesson you will be able

to:
State that V/I for a resistor remains
approximately constant for different
currents.
State that an increase in resistance of a
circuit
leads to a decrease in the current in that
circuit.
draw the symbol or a variable power supply
and
Key words: electrical resistance,
voltage,
current, Ohm’s law, ohms, resistor,
variable power supply

By the end of this lesson you will

have
practised:
building a series circuit
using an ammeter and a voltmeter to
find
current and voltage.
 Resistors

The symbol for a resistor is

 Resistors

Resistors oppose (or resist)

the
flow of electric current. They have
a
property called resistance (R)
which

is measured in ohms (Ω).

 What is the relationship
between current and
voltage in a resistor?
Current is measured using an
ammeter.
Voltage is measured using a
voltmeter.

Investigation: relationship
between
current and voltage in a resistor.
 Relationship between
current and voltage in a
resistor
I / Amps

Straight line through

the origin tells us that
current is

directly proportional to
voltage

The ratio V/I is constant

and is equal to resistance
p.d. / in the circuit.
Volts
 Relationship between
current and voltage in a
resistor
V
is approximatelyconstant
I
The constantis resistanceR
V
=R
I
 Relationship between
current and voltage in a
resistor
V
R= Ohm’s Law
I

V =IR
 Resistors

cell
What do you expect
to happen to the current
A if you increase the value
of the resistor in the
circuit shown?

lamp
resistor Demonstration
 Calculate
For a voltage of 12V, calculate
the
current for a resistant of

(i)1 Ω
(ii)2 Ω
(iii)4 Ω
(iv)24 Ω
(v)1 k Ω
What can you say about current and
resistance for a fixed voltage?
Complete
the sentences.

As resistance increases, the current

As resistance decreases, the current

 Varying Resistance
The opposition to current or
resistance
of a material (measured in Ω)
depends
on several things.

Think and discuss what some of these

might be.
 Varying Resistance
The opposition to current or
resistance of
a material (measured in Ω) depends on
- type of material (the better the
conductor, the lower the resistance)
- length of material (the longer the
material, the higher the resistance)
- thickness of material (the thinner the
material, the higher the resistance)
- temperature of material (the higher
the temperature, the higher the
resistance)
 Varying Resistance
The relationship between length
of the
material and resistance allows
us to make
a
variable resistor (or
rheostat).
 Variable Resistor
A B
Incoming
current

Outgoing
current

Demonstration
 Variable Resistors
In the above diagram, if the
slider is moved in the direction
A→B the resistance will
increase because the length of
wire through which the current
passes increases.
 Uses of Variable
Resistors?
Variable resistors can be used

• as volume or brightness
controls on
televisions
• volume control on MP3 players
• light dimmer switches.
Key words: resistance, series,
parallel,
ohms, ohmmeter
By the end of this lesson you will be
able
to:
State the relationships between total
resistance and individual resistances
in
series and parallel circuits
Carry out calculations involving the
relationships between resistors in
series
Key words: resistance, series,
parallel,
ohms, ohmmeter

By the end of this lesson you will

have
practised:
building a series circuit
building a parallel circuit
drawing circuit diagrams
using an ohmmeter to measure
resistance
 Variation of Resistance
and Current for a Lamp
Filament
Look at the circuit diagram below:

Handout
Name each of the components

Is this a series or parallel circuit?

As the voltage across the lamp

increases, what do you expect to happen
to the current?

Sketch a graph of your prediction of

the relationship between current and
voltage.
In the resistor, current and voltage are
directly proportional.

But in a filament lamp, heat is

generated.
We know that resistance increases as
temperature increases. So we see that as
voltage increases, temperature
increases,
resistance increases and current
increases – but more slowly than we
might
predict.
 Measuring Resistance
We can find the resistance of a
component by measuring

voltage across the component using

a voltmeter

current through the component using

an ammeter
 Measuring Resistance
or we can measure it directly
using an
ohmmeter

Ω
Demonstration & experiment
 Series and Parallel
Circuits
Voltage, Current and
Resistance
V
-
s
+

I1 I3
V1 V2 V3

R1 R2 R3

I2

What type of circuit is this?

 Vs
- +

I1 I3
V1 V2 V3

R1 R2 R3

I2

One electrical path from negative

to positive therefore series.
 Vs
- +

I1 I3
V1 V2 V3
R1 R2 R3

I2

What is the relationship between the three currents?

The current is the same at each point.

I1 = I 2 = I 3
 Vs
- +

I1 I3
V1 V2 V3
R1 R2 R3

I2

What is the relationship between the four voltages?

They add to equal the supply voltage.

Vs =V1 + V2 + V3
 Disadvantages of Series
Circuits?
When one component fails the whole
circuit
fails.

The current is the same at all points

and the
voltage is divided between the bulbs.
The
more bulbs added the dimmer each one is.
 Vs
- +

I1 I3
V1 V2 V3
R1 R2 R3

I2

How do you find total resistance in series?

Add each resistance together.

Rtotal =R1 + R2 + R3
 Vs
- +

IT V1 IT

R1 I1

V2

R2
I2

V3

R3
I3

What type of circuit is this?

 Vs
- +

IT V1 IT

R1 I1

V2

R2
I2

V3

R3
I3

More than one electrical path – components connected on different

branches therefore parallel.
 Vs
- +
What is the
relationship
between the
IT V1 IT four
R1 currents?
I1

V2 The four currents

R2 I2 add to give the
total current.
V3
R3 I3

I T = I1 + I 2 + I 3
 Vs
- +
What is the
relationship
between the
IT V1 IT four
R1 voltages?
I1

V2 Each voltage is
R2 I2 equal to the
supply voltage.
V3
R3 I3

VS =V1 =V2 =V3

 Vs
- +

IT V1 IT
R1
I1 The
V2 resistance
R2 I2
in parallel?
V3
R3 I3

1 1 1 1
= + +
R R R R
T 1 2 3
If more resistors are connected in
parallel the total resistance will
always

decrease
This is because there are more
branches through which the
electricity can flow.
Advantages of the Parallel
Circuit?
When one bulb fails the rest of the circuit
continues to work.

The more components, the lower the

resistance. The total current drawn
increases. Voltage in each branch is the same
as
the supply voltage therefore bulbs in
parallel
will each be as bright as a single bulb.What
have you learned today?
 Handout 3
Key words: resistor, resistance, series,
potential, potential divider
By the end of this lesson you will be
able
to:
State that a potential divider circuit
consists of a number of resistors, or a
variable resistor, connected across a
power supply.
Carry out calculations involving
potential
differences and resistance in a
potential
divider.
Name each component.
What type of circuit is this?

V V
he supply voltage is 6V. What is voltage V1? V2?

10Ω 10Ω

V1 V2
he supply voltage is 10V. What is voltage V1? V2?

10Ω 10Ω

V1 V2
he supply voltage is 5V. What is voltage V1? V2?

10Ω 10Ω

V1 V2
he supply voltage is 6V. What is voltage V1? V2?

5Ω 10Ω

V1 V2
A series circuit with two resistor and a
power supply is known as a potential
divider.

Why is it
called a
potential
divider?

V1 V2
The potential difference of the
supply is
divided between the two resistors.

When the two resistors are identical

(i.e.
have the same value of resistance),
the
potential difference is split
equally.
Investigating Potential
Dividers
 Potential Divider
Circuits
A voltage divider consists of
two devices, usually resistors,
connected in series.
R1=100 Ω V1 R1=4.5 kΩ V1

6V 6V

R2=100 Ω V2 R2=9 kΩ V2
The current in each resistor is
calculated
using Ohm’s Law:

V
I=
R
What can we say about the current in a
series circuit?

It stays the same throughout the

circuit.

V I =V
I1 = = 2 R1 2

R 1 2
In a voltage divider
circuit

V V
I1 = = I2 ==
1 2

R 1 R 2
This can also be written

V V
R =
1

R
2

1 2
If the resistance of one resistor
is increased, the voltage across
this
resistor will

This means the other voltage must

 Potential Dividers
R1
V1 = VS What do the symbols mean?
R1 + R2
R2 V1 is the voltage across resistor
V2 = VS
R1 + R2 V2 is the voltage across resistor
R1
V1 = VS VS is the supply voltage
RT
RT is the total resistance
R2
V2 = VS
RT
 Potential Dividers
R1
V1 = VS Look again at the
R1 + R2 worksheet.
R2
V2 = VS Use the formula to
R1 + R2 calculate V1 and V2 for
each circuit.
R1
V1 = VS
RT The answers found
using the formula
R2 match the values
V2 = VS measured using the
RT voltmeter.
 Potentiometer
The potentiometer is a special
type of
voltage divider.

It is a variable resistor with

a sliding
contact.
What range of output is it
possible to
obtain from a potentiometer?

Range of output voltages 0V to

supply
voltage.
Key words: electrical energy, power,
voltage, current, resistance
By the end of this lesson you will be
able
to:
State that when there is an electrical
current
in a component there is an energy
transformation and give some examples.
State the relationship between energy and
power.
Carry out calculations using E = Pt
State that in a lamp electrical energy is
transformed into heat and light.
State that the energy transformation in
an
electrical heater occurs in the
 What is
electricity?

What is a
voltage?

What is a volt?
 What is “potential
difference” ? What is
voltage?
It is the same thing!

The potential difference (p.d.), or

voltage,
of a battery is a measure of the
electrical
energy given to one coulomb of
charge
passing through the battery.
What is the energy change
which takes place in a
battery?

Chemical to Electrical
 When a battery is in a
circuit…
The electrical energy is carried by
the
electrons that move round the
circuit.

It is converted into others forms

of
energy.
 If there is a bulb in the
circuit, it is
converted from

to

Virtual Int 2 Physics -> Electricity ->Electrical Energy & Power ->Energy Transformation in a Lamp
Filament lamps
Filament of
tungsten wire

Glass

How does it
work?
Filament Lamp
Tungsten (metal) filament becomes
so hot it glows.

Why isn’t oxygen used

inside the bulb?
Filament lamps
Electric current
passes through the
resistance wire
which
is made of tungsten.
Electrical energy is
changed into heat
energy and the
wire glows white
hot.

Filament lamps
produce both heat
and
light.
In an electric fire, energy is
converted
from

to
 Resistance in a wire
We have learned that
when a voltage is
applied across a lamp,
the resistance
increases.

What happens to the

temperature?
 Resistance in a wire
As current passes through a
resistance wire,
the wire gets hot.

This is how electric fires and

filament lights
work.

The filament becomes hot enough to

glow and
emit light. The bar of the electric
fire is a
What are the energy
Electrical changeschange
appliances taking
place in these
electrical appliances?
energy into other
forms.
 Power and Energy
Electrical energy has the symbol

and is measured in
 Power
The power rating of an appliance or
a
component is defined as

the amount of energy used by

the
component / appliance in one
second
 Power
The power rating tells us the
rate at
which energy is transformed,
that is the
energy transformed each second.
 Power

For example, an appliance with a

power
rating of 250 W converts 250
Joules of
electrical energy into another
form each
second.
 Power
How can this be written as a
formula?

E
Power in Watts (W)

P=
Energy in Joules (J)

t time in seconds (s)

Demonstration / experiment
 Investigating Energy and Power
Connect the joule meter to the voltage supply and a ray box
bulb to the joule meter.

Set the supply voltage at 6V and switch on. You’ll see the
counter on the joule meter increasing (note each time the
counter increases by 1, this is 100J of energy).

Record the number of joules used in 50s and 100s. Calculate

the number of joules used per second.

Power is energy used per second, in watts. Write the

formula:

If the supply voltage was increased to 12V, what would you

expect to happen?

Increase supply voltage to 12V and repeat the experiment.

Worksheet / experiment
 Power and Energy
Ray box bulb, 6V supply Ray box bulb, 12V supply
Number of joules used in Number of joules used in
50 s? 50 s?
Number of joules used in Number of joules used in
100 s? 100 s?
Number of joules used Number of joules used
each each
second? second?
Power (W) Power (W)

Were your results as expected?

tt is equivalent to the transfer of 1 joule per sec

Power & Energy Example

If an electric fire
uses 1.8 MJ of energy
in a time of 10
minutes, calculate the
power output of the
fire.
Power & Energy Example

P = ?
E = 1.8 MJ = 1.8x106 J
t=10 minutes = 600 s
 Formula?

E
P=
t
 Power Ratings of
Appliances
Different appliances have
different
power ratings.

What is meant by power?

 Watt’s my power rating?

500 W, 150 W, 1200 W,

100 W, 3000 W, 300 W,
800 W, 1500 W, 30 W, 60 W,
11 W
 Watt’s my power rating?
300 W

500 W
60 W,
1200 W 1500 W

30 W

150 W
100 W 3000 W

11 W

800 W
What have you learned
today?
Key words: electrical energy, power,
voltage, current, resistance
By the end of this lesson you will be
able
to:
State that the electrical energy
transformed each second = VI
Carry out calculations using P=IV and
E=Pt
Explain the equivalence between VI,
I2R
and V2/R.
Carry out calculations involving the
relationships between power, current,
 Watt’s my power rating?

500 W, 150 W, 1200 W,

100 W, 3000 W, 300 W,
800 W, 1500 W, 30 W, 60 W,
11 W
 Watt’s my power rating?
300 W

500 W
60 W,
1200 W 1500 W

30 W

150 W
100 W 3000 W

11 W

800 W
 Current through
Appliances
Different appliances have different
power ratings.

P = IV
For appliances which use the mains
supply
V =
 Current through
Appliances
As power increases for a fixed
voltage,
what happens to the current?

As power increases the current

increases
Red flag
indicates
9V.

Live Neutral
 Even with
the switch
open and
zero current
the lamp is
still at 9V.

Live Neutral
 This time,
when the
switch is
open, the
lamp is at
0V and is
safe to
touch.
Live Neutral
The red flags indicate
that voltage at these
points is 9V.
Closing the third switch
results in a current greater
than 1A, blowing the fuse.
Inserting a voltmeter across a bulb
shows that the bulbs are at zero
volts.

If you touch them, you won’t receive

an electric shock as they are
isolated from the voltage supply.
The red flags indicate
that voltage at these
points is 9V. The fuse
is now in the neutral
Closing the third switch results in a
current greater than 1A, blowing the
fuse.
The red flags show that at these
points the voltage is still at 9V.
If you touch this now, you’ll complete
the circuit and receive an electric
shock – you become the “neutral wire”
and allow electricity to flow through
Why can a bird
sit safely on this
high voltage
power line?

What will
happen if the
bird spreads its
wings and
touches the
pylon?
 Which fuse to use?
How would you calculate which fuse
is required for an appliance?

An appliance operating from the

mains supply has a supply voltage of
230V.

The rating plate gives you

information on the power of the
appliance.
The formula which links
voltage, power and current:

P = VI
 The general rule for fuses

The fuse value needs to be just above the

normal operating current

If the Fuse value

appliance has should be:
a power rating
of:
Less than 700W 3A
More than 700W 13A
 Example
What is the appropriate choice of
fuse for a
mains appliance with a power rating
of
330 W? P =IV V = 230V
P =330W
P
I = I =?
V
 Example
What is the appropriate choice of
fuse for a
mains appliance with a power rating
of
330 W? 330 V = 230V
I = P =330W
230 I =?
I =1.44A
 Power Ratings of
Appliances
Which type of appliances tend to
have the
highest power ratings?

Generally, appliances which

produce heat.
 Power Ratings of
Appliances
Which type of appliances draw
the
highest current?

Generally, appliances which

produce heat.
 Power Ratings of
Appliances
Which type of appliances need
the largest
value of fuse?

Generally, appliances which

produce heat.
Examples of
rating
plates
What have you learned
today?
Key words: electrical energy, power,
voltage, current, resistance
By the end of this lesson you will be
able
to:
State that the electrical energy
transformed each second = VI
Carry out calculations using P=IV and
E=Pt
Explain the equivalence between VI,
I2R
and V2/R.
Carry out calculations involving the
relationships between power, current,
Investigating…
power, voltage, current
and
resistance.
What do you notice about
V 2

IV , I R ,
2

R
Worksheet / experiment
Power can be calculated from
the voltage across the
appliance and the current
flowing through it. Written as
an equation:

P = IV
 Relationship between
power, current, voltage
and resistance
Our experiments showed that

2
V 2
IV =I R =
R
 Relationship between
power, current, voltage
and resistance

2
2 V
P =IV =I R =
R
Equations for Power

P =VI and V =IR

Substituting
P =IxRxI
2
P =I R
Equations for Power
P =VI and V =IR
V
I=
R
Substituting
V
P =Vx
R
2
V
P=
R
What have you learned
today?
Key words: alternating current, direct
current, mains supply, frequency
By the end of this lesson you will be
able
to:

Explain in terms of current the terms a.c. and d.c.

State that the frequency of the mains supply is
50Hz.
State that the quoted value of an alternating
voltage is
less than its peak value.
State that a d.c. supply and an a.c. supply of the
same quoted value will supply the same power to
a given resistor.
 Direct Current (d.c.)
•The voltage drives a
steady or direct current.
•The electrons move in one
direction.
•The current (or voltage)
does not change with
time.
Direct Current
 Alternating Current
(AC)
•An alternating
current is
continually changing
direction
•The alternating
voltage and current
has a distinctive
waveform
 Alternating Current
Using the oscilloscope, we can
measure the peak voltage of the
a.c. supply.

The declared, quoted or

“effective”, voltage is
always less than the peak
voltage.
 Calculating Declared
Voltage
The declared (or effective) voltage can
be calculated from the peak voltage.

The quoted voltage is ~

0.7 x peak voltage.
The declared voltage is the value of
a.c. voltage which gives the same
heating or lighting effect as d.c.
voltage.
 Mains Supply
What is the frequency of the
mains supply?

50 Hz
 Mains Supply
What is meant by the frequency
of the supply?
Alternating current flows one way then
the other. It is continually changing
direction. The rate of the changing
direction is called the frequency and it is
measured in Hertz (Hz) which is the
number of forward-backward cycles in
one second.
 Mains Supply
Why does the current change
direction?
Voltage pushes the current. The voltage
changes polarity causing the current to
change direction.
 Mains Supply
What is the declared value of
the mains supply voltage?
230V
What is meant by the voltage of
the supply?
The voltage of a power supply or battery
is a measure of how much “push” it can
provide and how much energy it can give
to the electrical charge.
 Measuring effective
voltage / current in an
a.c. circuit

The effective voltage or current

in an a.c.
circuit can be measured using
a.c.
voltmeter or ammeter.
 Measuring peak a.c.
voltage using an
oscilloscope
1. Adjust the position so the
trace is central on the screen.
2. Adjust the volts/div so the
trace fills the screen.
3. Count the number of boxes from
the axis to the peak.
4. Multiply the number of boxes by
the volts / div.
What have you learned
today?
Key words: electromagnetism, induced
voltage, field strength, turns.
By the end of this lesson you will be able to:
State that a magnetic field exists
around
a current carrying wire.
Identify circumstances in which a
voltage
will be induced in a conductor.
State the factors which affect the size
of the induced voltage i.e. field
strength,
number of turns on a coil, relative
movement.
 Permanent Magnets

A magnetic field is the region

around a
magnet in which a magnetic force
can be
detected.
 Magnetic Field Around a
Current Carry Wire

What happens when the direction

of the
current is reversed?

The direction of the magnetic

field is
reversed.
 Electromagnets
When an electric current passes through a
wire which is coiled around an iron core,
the
core becomes magnetised and an
electromagnet is produced.

When an a.c. current is used, the current

changed direction and so the magnetic
field
changes direction.

e-m demo
 Electromagnets
Strength of electromagnet with/without
iron
core?

Effect of increasing current through the

coil?

Effect of increasing number of turns in

the
coil (while keeping current constant)?
 How is an electromagnet
constructed?

A current through a wire can be

used to
create an electromagnet.
http://micro.magnet.fsu.edu/electromag/java/compass/index.html
 How is an electromagnet
constructed?

A conducting wire is wound round an iron

core.

When a current passes through the

conductor there is a magnetic field around
the conductor. By wrapping it round a soft
iron core, the magnetic field is
concentrated.
How can the strength of an
electromagnet be
increased?
By increasing the current
through the
coil.

By increasing the number of

turns on the
coil of wire.
What are the advantages of
an electromagnet over a
permanent magnet?
The electromagnet can be switched off.

The magnetic field strength can be varied

(how?)

The electromagnet provides a much stronger

magnet field for the same size than a
permanent magnet.
 Electromagnetic
Induction
What happens when a wire is moved
in a
magnetic field?

A voltage is created – or induced.

For this
reason we call this electromagnetic
induction.
 Electromagnetic
Induction
http://micro.magnet.fsu.edu/electromag/java/faraday2
/

What happens when a permanent

magnet
is moved towards or away from a
coil of
wire?
What do we know so far?
When a current passes through a coil
of
wire, there is a magnetic field
around the
wire.

Changing direction of the current

changes
the direction of the magnetic field.
What do we know so far?
When we move a wire in a magnetic
field,
voltage is induced.

When we move a magnet in a coil of

wire,
a voltage is induced.

What do we have in common? Changing

magnetic field leading to electricity!
FARADAY’S EXPERIMENT 1832

B
A
FARADAY’S EXPERIMENT 1832

A current in B is
only present when
the current in A is
changing.

I
B
A
I HAVE DISCOVERED
ELECTROMAGNETIC INDUCTION
 Now I understand!
VOLTAGE IS ONLY
INDUCED WHEN
THERE IS RELATIVE
MOTION BETWEEN A
CONDUCTOR AND A
MAGNETIC FIELD
S N
S N
STRONGER FIELD (B)

S N
STRONGER FIELD (B)

S N
FASTER

S N
FASTER

S N
 THE INDUCED VOLTAGE IS
DIRECTLY PROPORTIONAL TO
THE RATE OF CHANGE OF
MAGNETIC FIELD
 What is observed when…
the magnet is stationary next to the
coil?
Nothing! No voltage is induced.

The magnet is moved in the opposite

direction (towards the coil instead
of
away from it)?
The voltage produced has opposite
polarity.
 What is observed when…
the magnet is moved backwards and
forwards?
Voltage induced which has a
changing polarity.

What does this mean for the

current?
The current will change direction
– it is
a.c.!
 Generating Electricity
A voltage can be induced in a coil
of wire
if a magnet is moved towards (or
away
from the coil).

This effect is known as induction.

What does the induced voltage depend

on?
 Generating Electricity
Induced voltage depends on:
strength of the magnetic field (the
stronger the greater the induced
voltage)
speed of movement (the faster the
greater the induced voltage).
number of turns in the coil (the more
turns of wire on the coil the
greater the induced voltage).

Virtual Int 2 Physics – Electricity & Electronics – em induction

 Generating Electricity
To summarise:

A voltage is induced across the ends

of a wire
coil is the coil experiences a
changing magnetic
field.
I HAVE DISCOVERED
ELECTROMAGNETIC INDUCTION
 Now I understand!
VOLTAGE IS ONLY
INDUCED WHEN
THERE IS RELATIVE
MOTION BETWEEN A
CONDUCTOR AND A
MAGNETIC FIELD
 THE INDUCED VOLTAGE IS
DIRECTLY PROPORTIONAL TO
THE RATE OF CHANGE OF
MAGNETIC FIELD
Generating Electricity

How do we “create”
electricity?
 A Simple Generator
A current can be passed through
a wire to
result in movement (a motor!).

Electrical energy was changed to

kinetic
energy.
 A Simple Generator
The motor can work “in reverse”.

Kinetic energy can be used to

create
electricity in a dynamo or
simple
generator.
 Transformers

What is a
transformer?

Demonstration.
 Transformers
A transformer consists of two separate coils of
wire wound on the same iron core.

The first coil, the primary, is connected to an

a.c.
voltage supply. There is therefore a changing
magnetic field around the core.

This changing field induces a voltage across the

other coil, the secondary. A current flows as a
result of the induced voltage.
 Transformer Terms
We talk about

Primary coil (the first one –

connected to
a.c. voltage)
Secondary coil (the second one –
voltage
is induced)
Number of turns – number of “loops” of
wire in coil
 Transformer Terms
We talk about

Np – the number of turns on the primary coil

Ns – the number of turns on the secondary
coil
Vp – the voltage applied to the primary coil
Vs – the voltage induced across secondary
coil
Ip – the current in the primary coil
Is – the current in the secondary coil
 THE TRANSFORMER
Laminated soft
iron core

PRIMARY Np Turns SECONDARY NS Turns

AC input VP Volts AC output VS Volts

VS NS
=
VP NP
 Equipment
2 coils
1 x a.c. voltmeter
Four wires
A variable power supply.

Set your power supply to 2V.

YOU
MUST NOT EXCEED 2V as the
primary voltage.
Measure the output voltage for a
2V input
for each of the combinations of
number
of turns in the primary and
secondary.

Record your results in your

table.
 V
Calculate S
Nand
S

Vp Np

and record your results in your

table.
 Investigating
Transformers
Vs Ns
VP N P Vs Ns Vp Np

2V 125 125
2V 125 500
2V 125 625
2V 500 125
2V 500 500
2V 500 625
 Transformers
Vs Ns
VP N P Vs Ns Vp Np
2 V 125
2 V 125 1 1
2 V 125
8 V 500 4 4
2 V 125
10 V 625 5 5
2 V 500
0.5 125 0.2 0.2
V 5 5
2 V 500 2 V 500 1 1
2 V 500 2.5 625 1.2 1.2
V 5 5
 Transformers
A step-up transformer is one in
which the secondary voltage is
greater than the primary.

A step-up transformer has more turns

on the secondary coil than the
primary coil.

Which of the transformers are step-

up?
 Transformers
A step-down transformer is one in
which the secondary voltage is less
than the primary.

A step-down transformer has fewer

turns on the secondary coil than the
primary coil.

Which are step-down transformers?

 What would happen if a d.c.
supply was connected to a
transformer?
At the moment of switching on, there is a
changing magnetic field which would induce
a voltage in the secondary coil. The same
at
the moment of switching off.

Once switched on, no changing magnetic

field (since steady current) and therefore
no
induced voltage.
 Step-Up Transformer
What is a step up transformer?

What can you say about the relationship

between the number of turns in the
secondary
and primary? N s > N P

and the voltage in the secondary and

primary? V > V
s P
 Step-Down Transformer
What is a step down transformer?

What can you say about the relationship

between the number of turns in the
secondary
and primary?
N s < NP

and the voltage in the secondary and

primary? Vs < VP
 Energy Losses in
Transformers
For calculations, we often assume
that
the transformer is 100% efficient.
however in reality they are about 95%
efficient.

What causes the energy losses?

 Energy Losses in
Transformers
- Heating effect of current in
coils (coils are long length of
wire with resistance hence
electrical energy changed to
heat)
- Iron core being magnetised and
demagnetised
- Transformer vibrating -> sound
- Magnetic field “leakage”
 Voltage and Current in
Transformers
Assuming an ideal transformer with no
energy losses total energy input
must
equal total energy output.

Since rate of energy input is power:

power input = power output

 Voltage and Current in
Transformers
Power is given as

P = V I

so

V p I p =VS I S
V I
which can be rearranged as s
= P
VP I S
 Voltage and Current in
Transformers

Vs I P
=
VP I S
In a step-up
transformer, the voltage
in the secondary is
greater than the
primary. What happens to
 Voltage and Current in
Transformers

Vs I P
=
VP I S
The current in the coils
is in the reverse ratio
to the voltage therefore
as voltage increases,
current decreases.
 Voltage and Current in
Transformers

Vs I P
=
VP I S
In a step-down
transformer, the voltage
in the secondary is less
than the primary. What
happens to the current?
 Voltage and Current in
Transformers

Vs I P
=
VP I S
The current in the coils
is in the reverse ratio
to the voltage therefore
as voltage decreases,
current increases.
 Transformers

np Is Vp
= =
ns Vs I p
np = number of turns on primary coil
ns = number of turns on secondary co
Vp = voltage across primary coil
Vs = voltage across secondary coil
Ip = current in primary coil
Is = current in secondary coil
 Type of
Turns Effect on Effect on
transforme
ratio? VOLTAGE? CURRENT?
r

Step-up

Step-down
What have you learned
today?
Key words: electromagnetism, induced
voltage, field strength, turns.
By the end of this lesson you will be able
to:
State that high voltages are used in
the
transmission of electricity to reduce
power loss.

Carry out calculations involving

power loss
in transmission lines.
 Transmitting Electrical
Energy

Transformers are used by the

National
Grid system through which
electrical
energy is transmitted.

Demonstration
 Electricity Transmission
trical energy is transferred from the power station
consumer via the National Grid.
ectricity is sent for many kilometres along transmi
nes on pylons.
Transformers in Electrical
Transmission

What happens as current flows

through
the wires?

The length of the wires means large

resistance and hence heating in the
wires.
 Transformers in Electrical
Transmission
Energy is changed from electrical to
heat resulting in large power losses
in the wires.

Relationship between power, current

and resistance?
2
P =I R
Transformers in Electrical
Transmission
At the power station, a step-up
transformer is used to increase the
voltage.

Why?
Transformers in Electrical
Transmission
Vs I P
=
VP I S

As voltage stepped up, current

2
=I R And
stepped down by the same Pfactor.
since by
reducing current the power losses
due to heating are reduced.
 Transformers in Electrical
Transmission

This stepping up of the

voltage and hence
stepping down of the
current makes the
transfer much more
efficient. The losses due
to heating are reduced.
Transformers in Electrical
Transmission
At the consumer end,
a step-down
transformer reduces
the voltage to 230V,
increasing the
current.