Transcript Practical Electricity

Practical Electricity
Recap…

5 important formulae:
1.
Q  It
2.
W  VQ
3.
V  IR
4.
E  Pt
5.
P  VI
Q = Charge (Coulomb)
I = Current (Ampere)
t = time (second)
V = Voltage or potential
difference (volt)
W = Work done (Joule)
R = Resistance (ohm Ω)
E = Energy (Joule)
P = Power (Watt)
Question
The heating element of an electric kettle with a resistance
of 38.4 Ω is connected across a 240 V power supply.
Calculate
(a) the current flowing through the element;
(b) the amount of heat energy produced in one minute.
Solution:
(a) R = 38.4 Ω and V = 240 V
.
V
I  .
R
240
I 
38.4
I  6.25 A
V
I
R
(b) E = P t
P=VI
P = 240 × 6.25
P = 1500 W
E=Pt
E = 1500 × 1 × 60
E = 90 000 J
E = 90 kJ
Measuring Electricity Consumption

The cost of electricity consumed is calculated based on
the number of kilowatt-hours (kWh) of electrical energy
used.

One kilowatt-hour (kWh) is the amount of electrical
energy used by 1 kW device in one hour.

Example of PUB bill
Cost of one kWh
CURRENT MONTH CHARGES
Usage
Rate ($)
Amount ($)
Electricity Services
Reading estimated on 16-01-2010
Electricity
161 kWh
0.2169
34.92
162 kWh
0.2287
37.05
Total ($)
71.97
Example
A 1.5 kW electrical heated is used to heat a large container
of water for 2 hours.
(a)
Calculate the amount of electrical energy used by the
heater in kWh,
(b)
The cost of the electricity if the cost of 1 kWh is 22¢
Solution:
Given P = 1.5 kW, t = 2 hrs
(a) Energy used E (in kWh) = P × t
= 1.5 × 2 = 3.0 kWh
(b) The cost = 3 x 22 = 66 ¢
The figure below shows a utilities bill of a household for one month.
Calculate the total cost of electricity?
The Uses of Electricity:
Heating Elements

The heating elements used in kettles,
irons, ovens and heaters are usually
made of nichrome wire because of its
high resistivity and ability to
withstand high temperatures.

When current flows, the wire heats
up. The thermal energy is spread
evenly over a large metal base.

A thermostat controls the
temperature by turning the current
on or off
Electric kettle
Electric iron
Electric radiator
The Uses of Electricity:
Electrical lighting – Filament Lamp

The filament is made of tungsten
because of its high resistivity and high
melting point.

The filament is very thin, giving it a
higher resistance than the rest of the
circuit (R=l ∕A)

When an electric current flows
through the filament, the tungsten
gets extremely hot (2500 oC) and
generates light.
A filament lamp
The Dangers of Electricity
•
Electric shock
–
–
•
touching wet electrical appliances, and
use of wire with damaged insulation
Fire from overheating of wires due to short circuits and
overloading of extension cords
Safe Use of Electricity at Home
Safety features installed at home for safe use of
electricity:
1. Circuit breakers
2. Three-pin plug
3. Fuses
4. Correct placement of switch in the circuit
5. Earth wire
1. Circuit Breakers

These are safety devices that can switch off the
electrical supply when there is excessive
current in the circuit.
Circuit breaker
2. Three-Pin Plugs



Live wire:
brown insulation
Neutral wire:
blue insulation
Earth wire:
green-and-yellow or
yellow insulation
3. Fuses

A fuse is a safety device that protects
electrical equipment and circuit from
excessive current.

It consists of a short thin piece of
wire which becomes hot and melts
when the current flowing through it
is greater than its rated value.
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Fuse rating = 3 A, 5 A, 13 A
Choose fuse with rating slightly
higher than the maximum allowable
current though an electrical appliance
when it is working properly.


It is connected to the live wire
A glass cartridge fuse contains
a thin metal wire which melts
when excessive current flows
through it.

Calculate the current taken by an 800 W vacuum cleaner
connected to a 240 V supply. Suggest a suitable rating for
a fuse to be used to prevent this vacuum cleaner from
overheating.
P  VI
P
I
V
800
I
240
I  3.33A
Hence, the fuse rating used is 5 A
4. Switches


Switches are used to turn an electrical appliance on or off
Fuse and switches are always insert along ‘live’ wires of
circuits.
Which is a correct circuit connection?
5. Earth Wire


The earth wire (green and yellow) is a low-resistance wire
and is usually connected to the metal casing of the
appliance.
Earthing prevents users from getting an electric shock if the live
wire is not properly connected and touches the metal casing of
the appliance.