Class 8 General Science Notes: - Unit-10: Electricity and Magnetism: Solved Exercise (Long Questions and Answers)

(National Book Foundation - New Book/New Syllabus Based on National Curriculum Pakistan 2023-2024 and Onwards)

(Class 8 General Science Notes for 8th Centralized Exam 2023-2024 and onward, FDE, Islamabad)

C. Long Questions

Q1. What is resistance and in what units do we measure resistance?

Ans:

Electric Resistance:

Electric resistance is the opposition to the motion of charge through a material. Resistance lowers the flow of current in a circuit. The value of resistance “R” is obtained by dividing the voltage ‘’V” by the current “I” it carries. R = `\ frac {V}{I}`

Unit of Resistance:

The unit of resistance is Ohm and is represented by the Greek letter (omega) Ω. The resistance of a wire is one Ohm if the potential difference of one volt is applied across its ends and causes a current of one Ampere to flow through it. 1Ω = `\ frac {1V}{1A}`

Q.2- State and explain electric power by giving its mathematical description?

Ans:

Electric Power:

Electric power is the rate at which electrical energy is used in a circuit.

Mathematical description of Electric power:

When a charge moves in a circuit, it loses energy. This energy is transformed into useful work, such as the turning of a motor.

The rate at which electrical work is done is electric power(P).

Electric power (P) is the product of total current (I) and voltage (V) across a circuit.

P = `\I` x V

The SI unit of power is Watt (W).

Q.3- What are electromagnets? Explain their uses in daily life.

Ans:

Electromagnet:

It is a coil that has a soft iron core that acts as a magnet when an electric current is in the coil. The electromagnet can be turned ON/OFF by turning the current ON or OFF. When current flows through the coil, it is a powerful magnet. When the current is turned OFF, the magnetic field disappears. The strength of the electromagnet increases with increasing current, and an increasing number of turns in the coil.

Application of Electromagnet:

Stereo Speaker:

Electromagnets are used to produce sound waves. As most stereo speakers work. It consists of a round permanent magnet and an electromagnet attached to a paper cone. The paper cone vibrates with a varying magnitude of current in the coil of the electromagnet and produces sound waves.

Electric Bell:

It consists of a coil of wire wound on an iron piece. The coil acts as an electromagnet. An iron strip with a hammer at one end is kept close to the electromagnet. Also, a contact screw near the iron strip when contact with the iron strip, and current flows through the coil. It then becomes an electromagnet and pulls the iron strip. In this process, the hammer at the end of the strip strikes the gong of the bell to produce the sound.

Unit-10: Electricity and Magnetism: Solved Short Questions and Answers

Class 8 General Science Notes

Source: General Science Book (National Book Foundation, Pakistan)

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