Secondary 3 Combined Science Physics

1. Recap!
Electrical Energy

2. Example 1
A 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

3. Example 2
A 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?

4. Practical
Electricity
Dangers of Electricity

5. Types of Dangers
Overheating 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

6. Safety Features at Home
• Circuit Breakers
• Fuses
• Switches
• Earthing
• Three-pin Plugs
• Double Insulation

7. 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 appliance
Switch off electrical supply in a

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

9. • 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
Earthing

10. Earthing

11. 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

12. Calculation involving
Fuses
Example 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

13. 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

14. Double Insulation
• Electric cables are
insulated from the internal
components
• Internal components are
insulated from external
casing
• Usually cased in non-metallic
materials

15. Homework!