Practical Electricity

Contents

1. Uses of Electricity
2. Measuring Electrical Energy
3. Dangers of Electricity
4. Safety Features in Home
Circuitries

2
 Contents

1. Uses of Electricity
2. Measuring Electrical Energy
3. Dangers of Electricity
4. Safety Features in Home
Circuitries

3
1. Uses of Electricity

Learning Outcome
At the end of this section you should be able
to:
● Describe the use of the heating effect of
electricity in appliances such as electric
kettles, ovens and heaters
1. Uses of Electricity
Uses of Electricity - Heating
• Electricity can be converted
into many forms eg. thermal
energy

• Heating Element used in

electric kettles, irons and
electric stoves.

• Heating Element made of

nichrome because of its high
resistivity and its ability to
withstand high temperatures 5
1. Uses of Electricity
Uses of Electricity - Heating
• In an electric kettle,
thermal energy is
generated when an
electric current passes
through the heating
element

• Water surrounding it
gets heated by
conduction and
convection
6
 Lighting effect of Electricity
Filament/ Incandescent Lamps

• Current flows through the

filament which becomes
white hot and gives out
heat and light

• Tungsten is used to make

the filament because of its
high resistivity and high
melting point (3400 ºC)

7
 Contents

1. Uses of Electricity
2. Measuring Electrical Energy
3. Sources of Electrical Energy
4. Dangers of Electricity
5. Safety Features in Home
Circuitries

8
2. Measuring Electrical Energy

Learning Outcome
At the end of this section you should be able
to:
● recall and apply the formulae P = VI and
E = VIt;
● calculate the cost of using electrical
appliances where the energy unit is in kWh
2. Measuring Electrical Energy
Electric Power
We have learnt that power is the rate of work done.
We have also learnt that potential difference is the
work done to drive a unit charge through a component.

10
2. Measuring Electrical Energy

Electric Power
P =VI

where P = power (in watt, W)

I = current (in ampere, A)
V = potential difference (in volt,
V)

11
2. Measuring Electrical Energy

Electric Power

By substituting V=IR, two more electrical power

12
relationships can be established
2. Measuring Electrical Energy

Electric Power & Energy

We can obtain electrical energy (E) equations since
E = Pt

P = VI E = VIt
P = I 2R E = I 2Rt
P =V2⁄R E = (V 2 ⁄ R)t

13
2. Measuring Electrical Energy
The electrical energy consumed when a
600 W hair dryer is turned on for 2 mins is ____ J
2. Measuring Electrical Energy

The electrical energy consumed when a 600 W

hair dryer is turned on for 2 mins is ____ J
2. Measuring Electrical Energy
What is the power dissipated by the lamp of
resistance 6 Ω?

A: 2 W
B: 12 W
C: 24 W
D: 72 W
2. Measuring Electrical Energy
What is the power dissipated by the lamp of
resistance 6 Ω?
P.d of lamp = 12 V
P = V2/R
= (12)2 /6 = 24 W

OR
find current through lamp,
I = V/R = 12/6 = 2.0 A
Then use P = I2R
OR = (2.0)2(6) = 24 W
find current through lamp,
I = V/R = 12/6 = 2.0 A
Then use P = VI
= (12)(2.0) = 24 W
2. Measuring Electrical Energy
The power dissipated by the lamp of resistance
6.0 Ω is ____ W
2. Measuring Electrical Energy
The power dissipated by the lamp of resistance 6.0
Ω is ____ W
OR
Use potential
divider principle:
Current in circuit
I =V/R P.d of lamp
= 12 / (1 +6) = 6/(6+1) x 12
= 1.714 A =10.29 V

Plamp = I2R Plamp = V2/R

= (1.714)2(6) = (10.29)2 / 6
= 17.6 W = 17.6 W
2. Measuring Electrical Energy
The time needed to supply 26 kJ of energy to the
water in the kettle is ………………. s
2. Measuring Electrical Energy
The time needed to supply 26 kJ of energy to the
water in the kettle is ………………. s

E= VIt
26000 = (240)(2.0)t
t = 54.2 s
2. Measuring Electrical Energy
The bulb in the circuit is rated as
A

60 W, 240 V. Which solution below 120 V

switch
closed

for the current flowing through the

bulb is correct?
Solution A: Solution C:
Power of the lamp, P = VI Power of the lamp, P =
V2
60 W = 120 V × I R
I = 0.5 A (240 V)2
60 W =
R
Solution B:
Power of the lamp, P = VI R = 960 Ω
60 W = 240 V × I
I = 0.25 A Working voltage V = IR
120 V = I (960 Ω)
I = 0.125 A
2. Measuring Electrical Energy
The bulb in the circuit is rated as 60 W, 240 V. Which
solution below for the current flowing through the bulb is
correct?
Solution C gives the correct value for the current flowing
through the bulb. A

Solution C: switch
V2 120 V
Power of the lamp, P = closed
R
(240 V)2
60 W =
R
Rated 60 W, 240 V
R = 960 Ω

Working voltage V = IR This means that the resistance of

120 V = I (960 Ω) bulb is such that it gives 60 W
I = 0.125 A power when a potential
difference of 240 V is applied
across it.
2. Measuring Electrical Energy

Electricity consumption cost

● The cost of electricity consumed is
based on the number of kilowatt-hours
(kWh) of electrical energy consumed.

● 1kWh is often referred to as 1 unit of

electricity

● 1 kWh = the energy consumed by a 1

kW device in 1 h
= 1 kW × 1 h
= 1000 W × (60 × 60) s
electric meter
= 3.6 × 106 J
24
2. Measuring Electrical Energy
Electricity consumption cost
Electricity cost = no of kWh of usage x cost per kWh

25
2. Measuring Electrical Energy

If the electricity tariff is 21 cents per kWh, how much

would it cost ( in dollars) to use a 2000 W air-con for
8 hours everyday for 30 days.

● 1 kWh is the energy used by 1

kW electrical appliance in an
hour
● Cost of electrical energy
= no of kWh x cost of 1kWh
2. Measuring Electrical Energy

If the electricity tariff is 21 cents per kWh, how much

would it cost ( in dollars) to use a 2000 W air-con in
your room for 8 hours everyday for 30 days.

2000 W =2 kW
No of hours used = 8hr x 30 days =240 hrs

No. of kWh = 2 kW X 240 hrs = 480 kWh

Therefore the cost

= 480 kWh x $0.21 cents =$100.80
2. Measuring Electrical Energy

All of the bulbs in the following circuits are identical.

Each of them is rated at 220 V, 200 W.
The total electrical energy consumed by all the bulbs
in the the two circuits if the bulbs are operated for 5
hours is ___________kWh

● HINTs: 1 kWh is the energy used by 1 kW electrical appliance in an hour

● For circuit X, find resistance of each bulb first since each bulb’s pd is not 220 V
2. Measuring Electrical Energy
All of the bulbs in the following circuits are identical. Each of them is
rated at 220 V, 200 W.
The total electrical energy consumed by all the bulbs in the the two
circuits if the bulbs are operated for 5 hours is _________ kWh
 Contents

1. Uses of Electricity
2. Measuring Electrical Energy
3. Dangers of Electricity
4. Safety Features in Home
Circuitries

30
4. Dangers of Electricity
Learning Outcome
At the end of this section you should be able to:
State the hazards of using electricity in the
following situations:
(i) damaged insulation
(ii) overheating of cables
(iii) damp conditions
4. Dangers of Electricity

Damaged Insulation

exposed wires
• When the insulating material covering the conducting
wires gets worn or damaged, the conducting wires
may be exposed.
• Exposed conducting wires can cause electric shocks
if touched.
4. Dangers of Electricity
Overheating of Cables
• Overheated cables can lead to fires.
• Two common causes of overheated cables:
– Overloaded power sockets
When a power socket is
overloaded, an unusually
large current flows through the
wires.
– Use of inappropriate wires
Appliances that require high
power to function need thicker
wires.
Why it is dangerous to plug in too many appliances
to a wall socket using multi-way adaptors?
A: The flow of current will
be slowed down and the
power to each appliance
will be reduced.
B: Excessive current may
flow through the cables
due to overloading,
causing overheating of the
cables.
C: The voltage across the
live and neutral wires
increases and overheating
occurs.
The live wire is a high voltage wire that carries
current from the supply to the appliance

The neutral wire is at zero volts and completes

the circuit by forming a path for the current
back to the supply
Explain what will happen if the live wire touches the
neutral wire due to damaged wire insulation
Explain what will happen if the live wire touches the
neutral wire due to damaged wire insulation

A short circuit will happen, the resistance will drop

and excessive current will travel from the live wire
to neutral wire overheating the wire and causing a
fire.
4. Dangers of Electricity

Damp Conditions
• Water in contact with exposed electrical wires
provides a conducting path for current.
• This can lead to electric shocks.
• Electrical appliances should be kept in dry places
and handled with dry hands.
 Effects of Current Through Body
Approximate Effect
Current / mA
1 Threshold- no pain below this point

5 Frightening but harmless shock

10-20 Uncontrolled muscular contractions

50 Pain and exhaustion, breathing

affected
> 70 for more Uncoordinated contraction
than 1 s of heart, leading to death

39
 Contents

1. Uses of Electricity
2. Measuring Electrical Energy
3. Sources of Electrical Energy
4. Dangers of Electricity
5. Safety Features in Home
Circuitries

40
5. Safety Features in Home Circuitries
Learning Outcomes
At the end of this section you should be able to:
● describe the function of fuses and circuit
breakers;
● explain why metal cases should be earthed and
double insulation is important;
● define the terms live, neutral and earth;
● describe the wiring in a mains plug, and the
safety mechanisms in place;
● explain why switches, fuses and circuit breakers
are fitted to the live wire.
5. Safety Features in Home Circuitries

Safety features that can be found in

our homes:
1. Circuit breakers
2. Fuses
3. Switches
4. Earthing
5. Three-pin plugs
6. Double insulation
5. Safety Features in Home Circuitries

Use your PLD to Complete SLS lesson on

“Wiring Switches, Fuses and Circuit Breakers
[Sec Phy]”

43
5. Safety Features in Home Circuitries
Circuit Breakers
• Circuit breakers are safety devices that can
switch off the electrical supply in a circuit when
large currents flow through them.
• Without circuit breakers, a surge of current can
damage home appliances or even cause a fire.
• Circuit breakers are connected to live (high
voltage) wires.
5. Safety Features in Home Circuitries
Fuses
• A safety device that protects electrical appliances from
damage when excessive current flows through.
• However, unlike a circuit breaker that can be reset
after it trips, a fuse must be replaced after it blows.
• Fuses are connected to live wire (high voltage wire)

– A fuse consists of short, thin piece of

wire.
– If the current flowing through it is too
large, the wire heats up and melts.
– This causes the circuit to be opened
(current no longer flows in the circuit).
5. Safety Features in Home Circuitries
Fuses
• It is always installed on the live wire so that the high voltage
wire is disconnected from the appliance.

• fuse ratings: choose a fuse which can take a current slightly

larger than what the appliance will draw in normal
circumstances. Standard fuse ratings are 1A,2A,5A,10A,13A

Fuse

Circuit symbol
for fuse
5. Safety Features in Home Circuitries
Fuses

An electric kettle is A: 1 A
rated 2200 W 240V. B: 2 A
Suggest a suitable C: 5 A
rating for a fuse to be D: 9 A
used in the electric E: 10 A
kettle plug. F: 13 A
5. Safety Features in Home Circuitries
Fuses

A hot water heater is rated 2200 W

240V. Suggest a suitable rating for a
fuse to be used in the water heater
plug.
I = P/V = 2200/240 = 9.17 A
A suitable rating will be 10A since this is
slightly higher than the current
5. Safety Features in Home Circuitries
What is the most suitable fuse rating for fuse X in
the circuit ?

X
A: 4 A
B: 5 A
C: 8 A
D: 10 A
5. Safety Features in Home Circuitries
What is the most suitable fuse rating for fuse X in
the circuit ?
X
5. Safety Features in Home Circuitries
Switches
What will happen when a user touches contacts of
the lamp. Switch is OPEN.
A: User will be
electrocuted since
he is touching the
live wire.
B: User will be safe
as the switch is off
hence there is no
current in the
circuit.
Even though the switch is off, the high
voltage live wire is still connected to the lamp.
Touching the contacts of the lamp will cause
a complete circuit and cause current from the
live wire to flow through the user’s hand and
body and to the ground, electrocuting him

For safety, the fuse and switch must be on

the LIVE wire
5. Safety Features in Home Circuitries
Switches
• Switches are designed to break or complete an electric
circuit.
• They should be fitted to the live wire of the appliance.
Due to an electrical fault,
the live wire touches the
metal casing of the
appliance.

1• However, the switch fitted on the live No current flows

2
wire disconnects the metal casing through the person
from the live wire when it is opened. touching the casing.
• The metal casing is at low voltage
(almost 0 V).
5. Safety Features in Home Circuitries
Earthing
There are usually three wires in a home
circuit:
• Live wire (colour: brown) earth neutral
– connected to a high voltage and
delivers current to the appliance
– is the wire to which circuit breakers,
fuses and switches are fitted
• Neutral wire (colour: blue)
– completes the circuit by providing a
live
returning path to the supply
– at 0 V
• Earth wire (colour: yellow and green)
– low resistance wire
– connected to the metal casing of
appliances
5. Safety Features in Home Circuitries
5. Safety Features in Home Circuitries
5. Safety Features in Home Circuitries
Explain what happens when the user touches
the metal kettle.
5. Safety Features in Home Circuitries
5. Safety Features in Home Circuitries
Earthing
5. Safety Features in Home Circuitries

How does the earth wire prevent an

electric shock in a metal kettle?
Choose the correct statement
A: It allows a current to flow to earth, so that
the appliance continues working.
B: It allows a large current to flow to earth,
blowing the fuse.
C: It prevents the fuse from blowing.
D: It reduces the current to a safe level.
5. Safety Features in Home Circuitries

How does the earth wire prevent an

electric shock in a metal kettle?

A: It allows a current to flow to earth, so that the appliance

continues working.,
B: It allows a large current to flow to earth, blowing the fuse.,
C: It prevents the fuse from blowing.,
D: It reduces the current to a safe level.,
5. Safety Features in Home Circuitries
Three-pin plug

Live wire

Neutral Wire

Earth Wire

Draw arrows to match the correct wire

5. Safety Features in Home Circuitries
Three-pin plug

Live - Brown,
Neutral - Blue,
Earth -Yellow with green stripes
5. Safety Features in Home Circuitries

Three-pin plug
Suggest why the earth pin in the 3 pin plug is
the longest compared to the other 2 pins

Earth pin
5. Safety Features in Home Circuitries
Three-pin plug

Earth pin

The earth pin is longer because it is the first pin to make

contact when the plug is inserted into a socket. Hence in a
faulty appliance, the current will be able to flow through the
earth wire and trip a circuit breaker or blow the fuse instead
of electrocuting the user.
A student swapped the live wire and earth wire when
wiring the plug and connected it to the socket to power a
metal kettle. Is this dangerous? Explain.
A student swapped the live wire and earth wire when
wiring the plug and connected it to the socket to power a
metal kettle. Is this dangerous? Explain.

kettle
PLUG

Live
Neutral
Earth

PLUG

Earth (swapped)
Neutral
Live (swapped)

kettle
A student swapped the live wire and earth wire when
wiring the plug and connected it to the socket to power a
metal kettle. Is this dangerous? Explain.

PLUG

Earth (swapped)
Neutral
Live (swapped)

Yes it is dangerous. The casing of the kettle will become

live as it is connected to a high voltage. A person
touching the metal casing will be electrocuted as the
current will flow through him
5. Safety Features in Home Circuitries

Double Insulation
● Some electrical appliances (e.g.hair dryers,
electric shavers and television sets)
connected to power circuits are not earthed
and have a 2-pin plug (live and neutral wires).

● These appliances do not have an earth wire

5. Safety Features in Home Circuitries
Double Insulation
● These appliances have double insulation to
protect users from electric shocks.
○ The electric cables are insulated from the internal
components of the appliance.
○ The internal components are insulated from the
outer casing. ( casing usually made of plastic)

The symbol for

double insulation
is shown at the
back of the
appliance