Evan's Space

Wonders of Physics


Double Insulation symbol on a hair dryer


The double square symbol represents double insulation. This means that the electrical appliance (a hair dryer in this case) has double layers of insulation. Besides the rubber tubing which cover the wires, the other casing is made of electrical insulator (plastic in this case). This protects the user in the event if the wires become loose and touch the casing. The casing will not become ‘live’.


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Thick Cables for High Power Electrical Appliances


High power electrical appliances are those which draws high current.

The thinner the wire (smaller cross-sectional area), the higher the resistance of the cable which brings the current into the electrical appliances.


More thermal energy will be generated due to this higher resistance in the cable. The cable might be too hot or catch fire.

So it is advisable to have thicker cables for high power electrical appliance as thicker cables have lower resistance.


Double Insulation

Why some electrical appliances do not need earth wire or they only have 2 pins (live and neutral)? For some 3 pins plug, the earth pin is actually plastic (that means no earth wire). Why?


Why make the earth pin with plastic since there is no earth wire?
Having the plastic earth pin is to provide convenience for user to plug in the 3-pins. Without the earth pin the plug wouldn’t go into the socket. The earth pin is needed to open a cover inside the socket that stops people from touching the live parts.

(Thanks Bob Barrett for the suggestion to improve the explanation)

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kWh and J are units of electrical energy

When we pay the electricity bill, we pay for the
A) energy used.
B) power used.
C) voltage used.
D) current used.

Solutions: Option A

When calculating cost of electricity, we have to use E = Pt to find the electrical energy (kWh) used. Then we multiple by the cost of 1 unit of kWh (currently around $0.29).

In normal times, when finding electricity energy in joules (J), all physical quantities in E = Pt must be in SI units. (P in W and t in s).

Hence it is important to know that alternative units for electricity energy is kWh besides J.