Magnetism and Electromagnetism

Question 1

What are the ends of a magnet called?

Answer 1

Poles

Question 2

How many poles does a magnet have, and what are they called?

Answer 2

Magnets have two poles: a north pole and a south pole.

Question 3

State the law of magnetism.

Answer 3

Like poles (S and S or N and N) repel each other, and unlike poles (S and N) attract each other.

Question 4

Define magnetically soft and hard materials.

Answer 4

Soft materials (e.g., iron) are easily magnetized and lose magnetism easily.

Hard materials (e.g., steel) are difficult to magnetize and retain magnetism.

Question 5

What is a magnetic field line?

Answer 5

A magnetic field line is the region around a magnet where a force acts on another magnet or on a magnetic material.

Question 6

How can permanent magnets be used to produce a uniform magnetic field pattern?

Answer 6

Two bar magnets with opposite poles facing each other create a uniform magnetic field in the gaps between the poles.

Question 7

What produces a magnetic field around a conductor with an electric current?

Answer 7

When a current flows through a conducting wire, a magnetic field is produced around the wire.

Question 8

Describe the construction of electromagnets.

Answer 8

Electromagnets consist of a coil of current-carrying wire wrapped around a magnetically soft core and can be turned on and off.

Question 9

Explain the force on a charged particle moving in a magnetic field

Answer 9

The charged particle’s movement, creating a current, produces a force perpendicular to the direction of motion and the magnetic field.

Question 10

How does a simple DC electric motor work?

Answer 10

A coil of wire, positioned in a uniform magnetic field, rotates when a current flows through it, producing a force due to the interaction of magnetic fields.

Question 11

How does the left-hand rule predict the force direction on a current-carrying wire in a magnetic field?

Answer 11

The force direction is determined by the left-hand rule, where the current, magnetic field and force are mutually perpendicular:

• the thumb points towards the direction of the force experienced by the conductor
• the forefinger points toward the direction of the magnetic field
• the middle finger points toward the direction of the electric current

Question 12

What factors affect the force on a current-carrying conductor in a magnetic field?

Answer 12

• The force increases with the magnitude of the current or the size of the magnet.
• Changing the current direction or reversing the poles changes the force direction.

Question 13

What induces a voltage in a conductor, and what factors affect its size?

Answer 13

• Relative movement between a conductor and a magnetic field induces a voltage.

Factors affecting size:

• speed, number of turns, size of coils, and magnetic field strength.

Question 14

How is electricity generated by rotating a magnet within a coil?

Answer 14

• Rotating a magnet or a coil in a magnetic field induces a voltage and current in the coil.

Voltage can be increased by:
- increasing wire length
- using a stronger magnet
- increasing rotation speed.

Question 15

Describe the structure and working of a transformer

Answer 15

A transformer consists of:

• primary and secondary coils
• an iron core
• and changes the size of an alternating voltage by having different turns on the input and output sides.

Question 16

Explain the use of step-up and step-down transformers in electrical energy transmission.

Answer 16

Step-up transformers increase voltage for transmission, while step-down transformers lower voltage for local use.

Question 17

What affects the size of the induced potential difference in a transformer?

Answer 17

• The speed of movement
• number of turns on coils
• size of coils
• strength of the magnetic field

Question 18

What is the relationship between input and output voltages for a transformer?

Answer 18

Input (primary) voltage / Output (secondary) voltage = Primary turns / Secondary turns.

Question 19

What is induced magnetism, and how does it occur?

Answer 19

• Induced magnetism occurs when a magnetic material is placed in a magnetic field
• temporarily turning it into a magnet
• with one end becoming a North Pole and the other end becoming a South Pole.

Question 20

Explain what happens to the metal rod AB. (4)

Answer 20

• Current on the rod
• Producing a magnetic field around the rod
• Magnetic fields interact
• producing a force on the rod

Question 21

Describe how the alternating current generates a sound wave. (4)

Answer 21

• Alternating current changes direction continuously
• Current in the coil produces alternating magnetic field
• Producing force on the coil
• Reversing the direction of current reverses the direction of the force, hence coil vibrates.

Question 22

Describe two ways that the student can make the pointer move to the right

Answer 22

• Reverse the magnet
• Move the magnet out of the coil

Question 23

Apart from changing the speed of the friction wheel, suggest how the output voltage of the dynamo can be increased.

Answer 23

• More turns on the coil

Question 24

Suggest why the current-carrying wire used in this investigation must be thick

Answer 24

Thin wire would melt

Question 25

Explain why the current-carrying wire experiences a force when there is a current in the circuit

Answer 25

• Magnetic field of wire interacts with magnetic field of 2 magnets

Question 26

State two ways in which this force can be reduced

Answer 26

• Reduce the current
• Decrease the voltage

Question 27

Describe how to investigate the shape and direction of the magnetic field between the magnets

Answer 27

• Place the plotting compass near the magnet on a piece of paper
• Mark the direction the compass needle points
• Move the plotting compass to many different positions in the magnetic field, marking the needle direction each time
• Join the points to show the field lines

Question 28

Suggest how the curved magnets and the piece of iron improve the performance of the electric motor

Answer 28

• Coil remains in the field for a longer time
• Force is increased by a stronger field

Question 29

Suggest how to make the coil spin in the opposite direction

Answer 29

Reversing the direction of current

Question 29

Describe two ways to increase the speed of rotation of the coil in this motor

Answer 29

• Increase the number of turns
• Increase magnetic field

Question 30

Describe an experiment to investigate the shape of the magnetic field pattern of a bar magnet.

Answer 30

• Place the magnet under the paper
• Sprinkle iron fillings over
• Tap paper gently to reveal pattern

Question 30

Explain how to produce a uniform magnetic field

Answer 30

• Two bar magnets
• opposite poles facing, e.g. North facing South
• Magnets should be close together, but not touching

Question 31

State why the metal plate is made of iron

Answer 31

Iron is (soft) magnetic

Question 32

Describe the changes that allow the electromagnet to release the door when the fire alarm sounds

Answer 32

• Current stops
• Iron plate no longer magnetized

Question 33

Explain why the metal rods move apart

Answer 33

• Rods are magnetized so they repel

Question 34

When the current is switched off, the metal rods return to their starting places.

Suggest what material the metal rods are made from

Explain your answer.

Answer 34

• Steel
• Because the material is capable of being magnetized