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Electricity: Magnetic and Heating Effects - Electric Circuit Components and Symbols

Grade 7CBSE

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

🔑Concepts

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An electric circuit is a closed path through which current flows. Common components include the cell, bulb, switch, and connecting wires.

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The symbol for an electric cell consists of a longer, thinner line representing the positive (++) terminal and a shorter, thicker line representing the negative (−-) terminal.

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A battery is a combination of two or more cells connected in series, where the positive (++) terminal of one cell is connected to the negative (−-) terminal of the next.

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The Heating Effect of electric current occurs when a wire gets hot as current passes through it. This is used in appliances like electric heaters, irons, and geysers which contain a heating element (e.g., Nichrome).

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The amount of heat HH produced depends on the material of the wire, its length ll, its thickness, and the duration of current flow tt.

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An electric fuse is a safety device based on the heating effect. It contains a wire with a low melting point that melts and breaks the circuit if the current II exceeds a safe limit.

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The Magnetic Effect of electric current was discovered by Hans Christian Oersted. It states that a current-carrying wire behaves like a magnet and can deflect a compass needle.

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An electromagnet is a coil of insulated wire wound around a soft iron core. It acts as a magnet only when current II flows through it. Its strength can be increased by increasing the number of turns nn in the coil or increasing the current II.

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Electric bells use electromagnets to pull an iron strip attached to a hammer, which strikes a gong to produce sound.

📐Formulae

H=I2RtH = I^2 R t

Power (P)=V×I\text{Power (P)} = V \times I

B∝n×IB \propto n \times I

💡Examples

Problem 1:

Calculate the change in heat produced HH in a circuit if the electric current II passing through it is doubled, keeping resistance RR and time tt constant.

Solution:

The heat produced becomes four times the original heat (4H4H).

Explanation:

According to Joule's law of heating, H∝I2H \propto I^2. If the current is doubled (2I2I), the new heat H′H' will be (2I)2×R×t=4×I2Rt=4H(2I)^2 \times R \times t = 4 \times I^2 R t = 4H.

Problem 2:

In a battery of four cells, how are the terminals connected?

Solution:

(+)−[Cell1]−(−)→(+)−[Cell2]−(−)→(+)−[Cell3]−(−)→(+)−[Cell4]−(−)(+)-[Cell 1]-(-) \rightarrow (+)-[Cell 2]-(-) \rightarrow (+)-[Cell 3]-(-) \rightarrow (+)-[Cell 4]-(-)

Explanation:

To form a battery, the positive terminal of one cell must be connected to the negative terminal of the adjacent cell to allow the flow of electrons through the circuit.

Problem 3:

An electromagnet has n=50n = 50 turns and a current of I=2 AI = 2\text{ A}. If the number of turns is increased to n=150n = 150, how does the magnetic strength change?

Solution:

The magnetic strength increases by 33 times.

Explanation:

The magnetic field strength BB of an electromagnet is directly proportional to the number of turns nn (B∝nB \propto n). Since nn increased from 5050 to 150150 (a factor of 33), the strength also increases three-fold.