Chapter 18: Electromagnetism

Question 1

What is the relationship between the changing the magntitude and/or direction of the current on the magnetic field

Answer 1

• When the direction of current changes in a solenoid, the polarities of the magneitsed solenoid change.
• When the current flowing through the solenoid increase, the magnetic field lines become closer together.

Question 2

How does electric bell work?

Answer 2

• When we press an electric bell, we close a switch causing a current to flow through the solenoid.
• The solenoid becomes an electromagnet hwich attracts a soft iron armature. The hammer which is attachedd to the armature strikes the gong producing a sound
• The contact with the circuit breaks open, turning off the elctromagnet. The armature then springs back to its original position to close the contact and allow the curren to flow through the electromagnet again.
• This repetition causes the hammer to strike the gong repeatedly

Question 3

How does a circuit breaker work?

Answer 3

• A circuit breaker is a safety device that breaks off the contact from the live wire when there. is an excessive current due to a fault.
• In a circit breaker, when strong current flows through a solenoid, the strenght of the magnetic field increases.
• This creates a magentic force strong enough to attract the soft-iron barm, breaking the contacts to stop the current from flowing through the household appliances.
• The reset button is used to bring the soft iron bar back to its original position in which the contact is closed.

Question 4

How does a split ring commutator work?

Answer 4

• When the plane of the coil is horizontal, the force acting on the arms of the coil is perpendicular to the plane of the coil. These forces causes the largest turning effect about the middle axis of the coil.
• When the coil then rotates 90°, the gap between the split-commutator stops the current from entering the coil. The momnetum of the coil allows it to continue to turn.
• The purpose of the split-ring commutator is to reverse the direction of current flowing ino the pocil after every half a turn (180°). This reverses the direction of forces acting on the arms of coul, ensuring that the coil rotats in the same direction.

Question 5

A student makes a do it yourself speaker as shown.
Explain how a varying current produces a sound?

Answer 5

A varying current flowing in the coil of wire produces a varying magnetic field which interacts with the magnetic field of the permanent magnet. The coil of wire constantly attracts and repels the permanent magnet and vibrates. The forward and backward vibrations of the plastic cup is passed on to the air particles. This causes alternate regions of the air particles to be compressed and extended.

Question 6

How does an electromagnet work?

Answer 6

• The electromagnet has many long and tight coils of wire to produce a strong magnetic field.
• It has an iron core to concentrate the magnetic field lines
• The electric current can be swtiched on to pick up a car and switched off to drop the car.

Question 7

Explain the phenomenon in Fig 2(a)

Answer 7

• When two parellel conductos with currents flowing in same direction are placed near to each other, the magnetic fields in the centre cancel otut each other to obtain a weaker resultant magnetic field.
• The conductors with currents flowing in the same direction will be attracted to each other.

Question 8

Explain the phenomenon in Fig 2(b)

Answer 8

• When two parallel conductors with current flowing in opposite directions are placed near to each other, the magentic field in the centre add up to obtain a stronger resultant magnetic field.
• The condcutors with currents flowing in opposite directions will repel each other

Question 9

How can you increase the turning effect on the coil `

Answer 9

• The strength of the magnetic field increases when there is a larger currrent flowing through the conductor. This causes a larger force acting on the coil. Hence, the coil experiences a larger turning effect.
• A larger turning effect can be produced by increasing the number of turns on the coil
• A larger turning effect can be produced by increasing the number of turns on the coil.
• A larger turning effect can be produced by winding the coil around a soft iron core to concentrate the magnetic field lines.