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Physics - Household Circuits

Grade 10ICSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Power is supplied to houses through the main board at a voltage of 220 V220 \text{ V} and a frequency of 50 Hz50 \text{ Hz}.

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The Ring System of wiring is used in modern houses, where all appliances are connected in parallel. This ensures that every appliance receives the same 220 V220 \text{ V} and can be operated independently.

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The Live wire (LL) is at a potential of 220 V220 \text{ V}, the Neutral wire (NN) is at 0 V0 \text{ V}, and the Earth wire (EE) is connected to a copper plate buried deep in the ground.

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A Fuse is a safety device made of an alloy of lead and tin (low melting point and high resistance). It must always be connected in the Live wire to protect the circuit from excessive current due to short-circuiting or overloading.

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Earthing is a safety measure for appliances with metallic bodies (like irons or refrigerators). If the live wire touches the metallic body, the current flows to the earth through the low-resistance earth wire, blowing the fuse and preventing electric shock.

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The Three-pin plug has an Earth pin that is longer and thicker than the Live and Neutral pins. This ensures that the appliance is earthed before the live connection is made and prevents the plug from being inserted incorrectly.

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A Switch must be connected in the Live wire. This ensures that when the switch is OFF, the appliance is disconnected from the high-potential wire and is safe to touch (0 V0 \text{ V}).

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Current color coding (International Convention): Live is Brown, Neutral is Light Blue, and Earth is Green or Yellow.

📐Formulae

P=V×IP = V \times I

I=PVI = \frac{P}{V}

E=P×tE = P \times t

H=I2RtH = I^2 R t

Cost of Electricity=Energy in kWh×Rate per unit\text{Cost of Electricity} = \text{Energy in kWh} \times \text{Rate per unit}

💡Examples

Problem 1:

An electric heater is rated 2.2 kW,220 V2.2 \text{ kW}, 220 \text{ V}. Calculate the current drawn by the heater and the appropriate fuse rating.

Solution:

Given: P=2.2 kW=2200 WP = 2.2 \text{ kW} = 2200 \text{ W}, V=220 VV = 220 \text{ V}.\Using the formula I=PVI = \frac{P}{V}:\I=2200220=10 AI = \frac{2200}{220} = 10 \text{ A}.\The fuse rating should be slightly higher than the operating current, so a 13 A13 \text{ A} or 15 A15 \text{ A} fuse is appropriate.

Explanation:

The heater draws a steady current of 10 A10 \text{ A} during normal operation. A fuse rating of 10 A10 \text{ A} might melt due to minor fluctuations, so a slightly higher capacity is chosen for safety.

Problem 2:

Why is the Earth pin in a three-pin plug made longer and thicker than the other two pins?

Solution:

The Earth pin is made longer so that it makes contact with the earth socket first, before the live and neutral pins connect. It is made thicker so that it cannot be inserted into the live or neutral holes of the socket.

Explanation:

The longer pin ensures the safety of the user by grounding the appliance body before power is supplied. The thickness acts as a safety 'key' to prevent incorrect electrical connections.

Problem 3:

If a circuit has a total power load of 4400 W4400 \text{ W} on a 220 V220 \text{ V} line, will a 15 A15 \text{ A} fuse blow?

Solution:

Total current I=PV=4400220=20 AI = \frac{P}{V} = \frac{4400}{220} = 20 \text{ A}.\Since the current drawn (20 A20 \text{ A}) is greater than the fuse rating (15 A15 \text{ A}), the fuse will blow.

Explanation:

A fuse melts when the current passing through it exceeds its rated capacity. In this case, the load requires more current than the fuse can safely handle, indicating an overload.