Magnetism

Question 1

What is the law of attraction/repulsion for magnets?

Answer 1

Like poles repel; unlike poles attract.

Question 2

What are the basic properties of a magnet?

Answer 2

Magnets have a north and a south pole (there are no monopole magnets), are made of iron or steel, and do not affect non-magnetic materials.

Question 3

What are permanent (hard) magnets and what are they made of?

Answer 3

Permanent magnets are made of steel and keep their magnetism for a long time.

Question 4

What are temporary (soft) magnets and what are they made of?

Answer 4

Temporary magnets are made of iron and lose their magnetism after a short time.

Question 5

How can steel be magnetized by stroking?

Answer 5

By moving a magnet on a steel bar in one direction many times.

Question 6

How can steel be magnetized using DC current?

Answer 6

By moving a steel bar inside a coil that is carrying a DC current.

Question 7

What are the methods to demagnetize a magnet?

Answer 7

• By hammering
• By heating B
• By moving a magnet inside a coil carrying AC.

Question 8

What is a magnetic field?

Answer 8

It is the region surrounding a magnet where magnetic force acts.

Question 9

How do magnetic field lines run around a magnet?

Answer 9

They come out of the north pole in all directions and go into the south pole from all directions.

Question 10

How does the spacing of magnetic field lines relate to the field’s strength?

Answer 10

As the spacing between lines increases, the magnetic field strength decreases.

Question 11

Where is the magnetic field strongest and weakest?

Answer 11

It is strongest near the poles and weakest near the center.

Question 12

How can you find the strength of a magnetic field using iron filings?

Answer 12

Place the magnet on paper sprinkled with iron filings; the pattern shows the field strength.

Question 13

How can you determine the direction of a magnetic field using needle compasses?

Answer 13

Place needle compasses at different positions around the magnet to observe the orientation of the needles.

Question 14

How can the magnetic field produced by a current-carrying wire be increased?

Answer 14

• Increase the current/voltage
• Increase the length of the wire
• Move closer to the wire

Question 15

How can the magnetic field from a coil be increased?

Answer 15

• Increase the current/voltage
• Increase the number of turns in the coil
• Use an iron core

Inside the coil the field lines are straight and parallel.

Question 16

What are electromagnets and how do they operate?

Answer 16

• Electromagnets are magnets that can be switched on or off. When switch is closed, current passes through the coil and magnetizes an iron core (producing a strong field)
• When open, no current flows and no field occurs.

Question 17

How can you increase the magnetic field of an electromagnet?

Answer 17

• Increase the current/voltage
• Increase the number of turns in the coil
• Use an iron core

Question 18

What is a relay and how does it use magnetic fields?

Answer 18

• A relay uses a small current circuit to operate a circuit that uses a large current
• Closing the switch sends current through a coil, creating a magnetic field that attracts a free contact to close the circuit and power a motor.

Question 19

What is the function of a circuit breaker in a magnetic field?

Answer 19

It uses the magnetic effect to reduce the current to its normal value when excess current flows through a wire in a magnetic field.

Question 20

How can the magnetic force acting on a moving wire be increased?

Answer 20

• By increasing current/voltage
• By increasing the number of turns of the coil
• By using a stronger magnet

Question 21

What causes a coil to rotate in a magnetic field?

Answer 21

Two equal and opposite forces (one acting downward on AB and one upward on CD) create a moment that rotates the coil.

Question 22

How can you increase the speed of rotation of a coil?

Answer 22

• By increasing current/voltage
• By increasing the number of turns of the coil
• By using a stronger magnet

Question 23

How can the direction of rotation of a coil be reversed?

Answer 23

• Reverse the current
• Reverse the poles of the magnet

Question 24

What is the function of a DC motor?

Answer 24

A DC motor converts electric energy into kinetic energy.

Question 25

What is the role of split ring commutators in a DC motor?

Answer 25

They reverse the direction of current every half cycle, ensuring that the coil rotates continuously in one direction.

Question 26

What is the purpose of carbon brushes in a motor?

Answer 26

Carbon brushes connect the rotating coil with the battery.

Question 27

How is an induced current generated?

Answer 27

When a wire moves inside a magnetic field, the rate at which magnetic lines are cut changes, inducing a current.

Question 28

How can you increase the induced current (emf) in a wire?

Answer 28

* By increasing the length of the wire * By increasing the strength of the magnets * By moving the wire faster

Question 29

What does Faraday’s law of induction state?

Answer 29

The induced emf is directly proportional to the rate at which magnetic field lines are cut by the wire.

Question 30

What does Lenz’s rule state about the direction of induced current?

Answer 30

The induced current is opposite to the normal current.

Question 31

Is the induced current produced by a moving wire AC or DC?

Answer 31

It is alternating current (AC).

Question 32

How can the induced current (emf) in a coil be increased?

Answer 32

* By increasing the number of turns in the coil * By increasing the strength of the magnets * By rotating the coil faster

Question 33

What is an AC generator and how does it operate?

Answer 33

* An AC generator converts kinetic energy into electrical energy * The current is zero when the coil is vertical and maximum when the coil is horizontal

Question 34

What is mutual induction in electromagnetic induction?

Answer 34

It is the process by which a changing magnetic field in one circuit induces a current in a nearby circuit.

Question 35

How does the movement of a magnet relative to a stationary coil affect induced current?

Answer 35

* When the magnet moves toward the coil, current is induced * When it moves away, induced current occurs in the opposite direction * If both move together at the same speed in same direction, no current is induced.

Question 36

How does induced current occur when the coil moves relative to a stationary magnet?

Answer 36

* The coil moving toward the magnet induces a current, and moving away induces current in the opposite direction * If both remain at rest, no induced current occurs.

Question 37

How do transformers work?

Answer 37

Transformers use mutual induction with AC. An alternating current in the primary coil creates an alternating magnetic field (via an iron core), which induces a current in the secondary coil.

Question 38

What is the relationship between primary and secondary coils in a transformer?

Answer 38

* Power is constant: 𝐼(𝑃) x 𝑉(𝑃) = 𝐼(𝑆) x 𝑉(𝑆) * In step-up transformers: 𝑁(𝑃) < 𝑁(𝑆) so 𝑉(𝑃) < 𝑉(𝑆) and 𝐼(𝑃) > 𝐼(𝑆) * In step-down transformers: 𝑁(𝑃) > 𝑁(𝑆) so 𝑉(𝑃) > 𝑉(𝑆) and 𝐼(𝑃) < 𝐼(𝑆)

Question 39

Why is high alternating voltage used in the national grid instead of high alternating current?

Answer 39

1. To reduce current 2. To reduce heating effect 3. To reduce energy lost 4. Use thinner wires 5. Cheaper 6. Less metal used 7. Less insulating material used

Question 40

What are the key methods to increase magnetic effects in various devices?

Answer 40

* Increase current/voltage * Increase the number of turns in coils * Use stronger or iron cores * Adjust proximity or movement speed * When needed, reverse current or magnetic poles

Question 41

What does the Right‑Hand Rule for Magnetic Fields around a straight wire state, and how do you use it?

Answer 41

Extend your **right thumb in the direction of the conventional current** (from positive to negative). **Your curled fingers then show the circular magnetic field lines around the wire**. Use this rule to determine the field direction around any straight current‑carrying conductor.

Question 42

How is the Right‑Hand Rule for Magnetic Fields in Coils (Solenoids) applied, and what does it reveal?

Answer 42

**Curl the fingers of your right hand in the direction of the conventional current flowing through the coil loops.** **Your thumb will point in the direction of the magnetic field inside the coil**, indicating the north pole of the electromagnet. Use this rule to determine the polarity and field direction inside coils or solenoids.

Question 43

What is Fleming’s Left‑Hand Rule (Motor Rule), and when should you apply it?

Answer 43

Hold your left hand so that **the thumb, forefinger, and middle finger are perpendicular. The index finger shows the direction of the magnetic field, the middle finger the direction of the current, and the thumb the direction of the force (motion) on the conductor.** Use this rule to predict the force direction on a current‑carrying conductor in electric motors.

Question 44

Describe Fleming’s Right‑Hand Rule (Generator Rule) and its use in determining induced current direction.

Answer 44

Extend your right hand so that **your thumb points in the direction of the conductor’s motion relative to the magnetic field, your index finger points in the direction of the magnetic field, and your middle finger then indicates the direction of the induced current.** Use this rule when a conductor moves within a magnetic field, as in generators, to find the direction of the induced current.

Question 45

Describe the magnetic field inside a solenoid.

Answer 45

* The magnetic field inside a solenoid is strong, uniform, and consists of straight, parallel lines. * It resembles the field of a bar magnet, with distinct north and south poles at the ends.