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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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The Electric Cell and Battery: An electric cell provides energy to the circuit. It has a positive (+) terminal (long thin line) and a negative (-) terminal (short thick line). 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.

Circuit symbol for a battery showing multiple cells connected in series.
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The Switch (Key): A switch is a control device that can 'make' or 'break' the circuit. When the switch is in the 'ON' position, the circuit is closed, and current flows. When it is in the 'OFF' position, the circuit is open, and current stops.

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Heating Effect of Current: When an electric current flows through a wire, the wire gets hot due to resistance. This is used in appliances like electric heaters and irons. The amount of heat HH depends on the current II, the resistance RR of the wire, and the time tt for which the current flows.

A simple circuit with a battery and a resistive heating element.
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Magnetic Effect of Current: Hans Christian Oersted discovered that when current flows through a wire, it behaves like a magnet. This effect is used to create electromagnets, where a wire is coiled around a soft iron core.

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Electric Fuse: A fuse is a safety device that prevents damage to electrical circuits and possible fires. It contains a wire that melts quickly and breaks the circuit if the current exceeds a safe limit.

The circuit symbol for an electrical fuse.

📐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.

Problem 4:

In the following circuit, if the wire WW is made of a material that melts at low temperatures, what happens to the circuit when the current becomes very high?

A circuit with a battery, a fuse labeled W, and a lamp.

Solution:

The circuit breaks.\text{The circuit breaks.}

Explanation:

The component WW acts as a fuse. When current exceeds the safety limit, the heating effect of the current (H=I2RtH = I^2 R t) causes the temperature to rise. Because WW has a low melting point, it melts and creates an 'open circuit', stopping the flow of electricity to protect other components.

Problem 5:

Identify the components shown in the given circuit diagram and determine the state of the circuit (Open or Closed). If the component labeled 'S' is changed to the 'on' position, what will happen to the component 'L'?

An open circuit containing a battery, an open switch labeled S, and a lamp labeled L connected in a series loop.

Solution:

  1. Components: Battery (two cells), Switch (S), and Lamp (L).
  2. Current State: The switch is in the 'open' position, so the circuit is 'Open'.
  3. Action: When switch 'S' is closed, the circuit becomes complete, and electric current flows from the positive terminal of the battery to the negative terminal. As a result, the lamp 'L' will glow.

Explanation:

In an electric circuit, the switch acts as a control device. When the switch is 'off' (open), there is a gap in the conducting path, preventing the flow of electrons. When the switch is 'on' (closed), the path is continuous, allowing the heating effect of the current to make the filament of the lamp glow.