Recap!Electrical Energy

Example 1A 24 ohm resistor is to be connected in series

with a 12V battery. What is the power loss in the resistor?

Hint : Solve using V=IR & P=IV

Example 2A Lamp has a power rating of 100w and was

used for 5 hrs. A cooker has a power rating of 3kW and was used of 0.5 hrs. What was the cost for the total usage of both the appliances if the cost of a unit (kWH) of electricity is $0.10?

Practical ElectricityDangers of Electricity

Types of DangersOverheating of Cables Damp Environments Damaged Insulation

• Overloaded power sockets cause large current flow

• Use of inappropriate wires may result in overheating

• Insulating materials can become worn with time exposing the conducting wires.

• Conducting wires can cause shocks if touched

• Water is a good conductor of electricity and it can act as a conducting path for currents if in contact with uninsulated parts of electrical wires

Safety Features at Home

• Circuit Breakers

• Fuses

• Switches

• Earthing

• Three-pin Plugs

• Double Insulation

Circuit Breakers

•circuit when a current that flows

through them is too large

• Live wires are connected to circuit

breakers to ensure that the circuit

breaker will trip andcut off supply

to the applianceSwitch off electrical supply in a

Switches

•

• Switches should to connected to live wires to prevent electric shock in case of electrical faults

Switches break or complete a circuit

• Live Wire (Brown) is connected

to high voltage & delivers current

• Neutral Wire (Blue) completes the

circuit by providing a return path to

the supply for the current

• Earth Wire (Green & Yellow) is of

low resistance so that current can

flow through without causing electric

shocks to the users

Earthing

Earthing

Fuses

• Acts similarly to circuit breakers,

preventing excessive current

flow

• However, instead of tripping,

fuses blow and have to replaced

once the blow

• Fuses have a thin wire, which

overheats and melts creating an

open circuit.

All fuses come with a rating which indicates maximum current that is allowed to flow through before the fuse blows

Calculation involving FusesExample Qn

A iron is rated at 1740W & 240V. What would be the current required for the iron to operate? What would be a suitable fuse rating to protect the iron from overheating?

Using P=VI, I= P/V = 1740/240 = 7.25A

Therefore, a fuse slightly larger than the current flowing should be use. In this case, 8.0A Fuse would be ideal

Three-Pin Plug

• Fuse protects the appliance

when there is an electrical

fault

• When excessive current

flows through, the fuse

blows, breaking the circuit

so that it does not overheat

the appliance and damage it

Double Insulation• Electric cables are

insulated from the internal components

• Internal components are insulated from external casing

• Usually cased in non-metallic materials

Homework!