Magnetic Effects of Electric Current — Previous Year Questions (Class 10 Science)
Electric currents produce magnetic fields. Understanding electromagnets, motors, and generators is fundamental to modern technology.
TL;DR: Electric currents produce magnetic fields. Understanding electromagnets, motors, and generators is fundamental to modern technology.
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Electric currents produce magnetic fields. Understanding electromagnets, motors, and generators is fundamental to modern technology.
Magnetic Effects of Electric Current — Previous Year Questions with Solutions
Q (2023, 2 marks): State the right-hand rule for determining the direction of the magnetic field around a current-carrying conductor.
Answer: Right-hand rule:
When the thumb of the right hand points in the direction of current flow, the fingers curl in the direction of the magnetic field lines around the conductor.
Alternatively: If you grasp the conductor with your right hand such that your thumb points in the direction of current, your fingers point in the direction of the magnetic field.
This rule helps determine the direction of the magnetic field without needing to perform measurements.
Q (2022, 3 marks): What is an electromagnet? Describe how it is constructed and explain its advantages over a permanent magnet.
Answer: An electromagnet is a temporary magnet created by passing electric current through a coil of wire wrapped around an iron core.
Construction:
1. Insulated wire wound tightly around an iron or soft iron core.
2. When current flows, the iron core becomes magnetized.
3. The magnetic field strength depends on the number of turns, current magnitude, and core material.
Advantages over permanent magnets:
1. Magnetic strength can be controlled by adjusting the current.
2. Polarity can be reversed by reversing current direction.
3. Magnet can be switched on and off.
4. Stronger magnetic fields can be produced with less material.
Q (2023, 3 marks): A rectangular coil carrying current is placed in a uniform magnetic field. Explain the forces acting on the coil and describe the motion that results.
Answer: When a rectangular coil carries current in a uniform magnetic field:
Forces: Using F = BIL (force = magnetic field × current × length):
1. Two sides perpendicular to the magnetic field experience forces in opposite directions.
2. Two sides parallel to the magnetic field experience no net force (or forces cancel).
Result:
- The opposite forces create a torque (turning effect) on the coil.
- The coil rotates about its axis.
- This is the principle of an electric motor.
Direction: Using Fleming's left-hand rule, the force direction can be determined.
F = BIL sin(θ), where θ is the angle between the conductor and magnetic field.
Q (2022, 3 marks): Explain the principle of an electric motor and describe its main components.
Answer: Principle: A current-carrying coil in a magnetic field experiences a torque that causes rotation.
Main components:
1. Stator (permanent magnet or electromagnet): Creates a uniform magnetic field.
2. Rotor (armature coil): Current-carrying coil that rotates about its axis.
3. Commutator: Split ring that reverses current direction every half rotation to maintain continuous rotation.
4. Brushes: Carbon contacts that supply current to the commutator.
5. Axle/shaft: Mechanical part that transmits rotational motion.
Working: As the coil rotates in the magnetic field, the commutator periodically reverses the current. The alternating forces keep the coil rotating in one direction.
Q (2023, 2 marks): What is electromagnetic induction? State Faraday's law of electromagnetic induction.
Answer: Electromagnetic induction is the phenomenon where a changing magnetic field induces an electric current in a conductor.
Faraday's law: The magnitude of the induced electromotive force (EMF) is directly proportional to the rate of change of magnetic flux through the conductor.
Mathematically: ε = -N(dΦ/dt)
Where:
ε = induced EMF
N = number of turns
dΦ/dt = rate of change of magnetic flux
The negative sign (Lenz's law) indicates that the induced EMF opposes the change in magnetic flux.
Q (2021, 3 marks): Describe the construction and working of a simple alternating current generator.
Answer: Construction:
1. A rectangular coil rotates in a uniform magnetic field provided by two permanent magnets.
2. Two slip rings (sliprings) are connected to the ends of the coil.
3. Brushes contact the sliprings to collect the generated current.
4. The coil is rotated by an external mechanical source.
Working:
1. As the coil rotates, the magnetic flux through it changes continuously.
2. By Faraday's law, this changing flux induces an EMF: ε = NBSω sin(ωt)
3. The induced EMF varies sinusoidally with time, producing an alternating current.
4. The sliprings allow the rotating coil to maintain electrical contact with external circuits.
Frequently Asked Questions
What is Lenz's law and why is it important?
Lenz's law states that the direction of an induced EMF (and current) is such that it opposes the change in magnetic flux that caused it. This is represented by the negative sign in Faraday's law. It's important because it explains why energy must be supplied to maintain motion against the magnetic force, and it ensures energy conservation in electromagnetic systems.
What is the difference between a direct current (DC) motor and an alternating current (AC) generator?
A DC motor uses a split-ring commutator that reverses current direction to maintain continuous rotation. An AC generator uses slip rings that allow the rotating coil to connect to external circuits, producing alternating current. Conversely, a DC generator uses a commutator to produce DC, while an AC motor uses slip rings to run on AC.
More Class 10 Science PYQs
- Chemical Reactions and Equations
- Light (Reflection and Refraction)
- Electricity
- Acids, Bases and Salts
- Metals and Non-metals
- Life Processes
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