Electromagnetism

Question 1

How does an electric current produce a magnetic field?

Answer 1

• A current-carrying conductor generates a magnetic field around it.
• The field forms concentric circles around a straight wire, with direction given by the right-hand grip rule (thumb = current, fingers = field direction).
• The strength of the magnetic field increases with current and decreases with distance from the wire.
• Used in electric circuits, electromagnets, and transformers.

Question 2

How is an electromagnet constructed?

Answer 2

• An electromagnet consists of a coil of wire (solenoid) wrapped around a magnetically soft iron core.
• When current flows, the core becomes magnetised, producing a strong magnetic field.
• The magnetism is temporary—it disappears when the current stops.
• Increasing the number of coils, using a higher current, or adding an iron core makes the electromagnet stronger.
• Used in relays, scrapyard cranes, electric bells, and MRI machines.

Question 3

What are the magnetic field patterns for a straight wire, a flat circular coil, and a solenoid?

Answer 3

• Straight wire: Concentric circles around the wire, strongest near the wire.
• Flat circular coil: Field is circular around each loop, forming a complex pattern in the center.
• Solenoid: Strong, uniform field inside, resembling a bar magnet with north and south poles.
• Outside the solenoid, the field is weaker and spreads out like a bar magnet.
• Used in electromagnets and transformers due to the strong, controllable field.

Question 4

Why does a charged particle moving in a magnetic field experience a force?

Answer 4

• A moving charge cuts through magnetic field lines, experiencing a force.
• The force is perpendicular to both the velocity and magnetic field direction.
• If the motion is parallel to the field, no force is experienced.
• Used in cathode ray tubes and particle accelerators.

Question 5

Why does a current-carrying wire experience a force in a magnetic field?

Answer 5

• The moving charges in the wire experience a force due to the external magnetic field.
• This causes the wire to move, demonstrating the motor effect.
• The force is strongest when the wire is perpendicular to the field and zero when parallel.
• Used in electric motors, loudspeakers, and electromagnetic relays.

Question 6

How does a simple d.c. motor work?

Answer 6

• A loop of wire carrying current is placed in a magnetic field.
• The two sides of the loop experience opposite forces, causing rotation.
• A split-ring commutator reverses the current every half-turn, maintaining continuous rotation.
• Brushes allow electrical contact while ensuring uninterrupted movement.
• Used in electric fans, drills, and conveyor belts.
• The speed and direction of rotation can be controlled by changing the current or magnetic field strength.

Question 7

How does a loudspeaker use electromagnetism?

Answer 7

• A coil of wire is placed in a permanent magnet’s field.
• A varying current passes through the coil, producing a changing magnetic force.
• This force moves the diaphragm, pushing air and creating sound waves.
• The frequency of the alternating current (A.C.) determines the pitch of the sound.
• Used in speakers, headphones, and PA systems.

Question 8

What is Fleming’s Left-Hand Rule, and how is it used?

Answer 8

• Thumb: Direction of force/motion of the conductor.
• First finger: Direction of magnetic field (N to S).
• Second finger: Direction of current (conventional, + to -).
• Used to determine the motion of a conductor in motors and speakers.
• Ensures correct wiring and functionality of electromagnetic devices.

Question 9

How does the force on a current-carrying wire change with field and current?

Answer 9

• Increasing the current increases the force.
• Stronger magnetic fields result in a greater force.
• Reversing the current or the magnetic field reverses the force direction.
• Used in motor control and electromagnetic machinery.