Unit 22 Electromagnetism Physics

Front 1

How did Hans Christian Oersted discover electromagnetism?

Back 1

Hans Christian Oersted proved that an electric current produces a magnetic field as it flows through a wire, thus discovering electromagnetism.

Front 2

What is the unit of magnetic field strength?

Back 2

The unit of magnetic field strength is oersted.

Front 3

Describe an experiment to demonstrate the interrelationship between electric current and magnetism.

Back 3

Objective: To make an electromagnet.

Apparatus : long iron nail, insulated copper wire, battery, paper clips and a switch.

Procedure;

1. Wind the copper wire tightly around the nail.

2. Connect the ends of the wire of the battery and switch.

3. When the circuit is switched on, the nail is observed to pick up some paper clips.

4. Switch off the circuit.

Precautions; The copper wire can be very hot when the circuit is closed.

Front 4

Describe an experiment to demonstrate the magnetic effect of a current (Oersted's Experiment)

Back 4

The wire XY is placed in the North-South direction. When the circuit is closed, current flows through XY.

A compass A placed above the wire would point to the East.

If a compass B is placed below the wire, it would point to the West.

When the switch is open, no current flows through the wire.

The compass needle is now observed to point to the North.

This experiment shows that a magnetic field is present when a current flows through a wire.

Front 5

What does a current-carrying conductor produces around it?

Back 5

A current-carrying conductor produces a magnetic field around it.

Front 6

Describe how to draw the magnetic field pattern of a straight wire carrying a current.

Back 6

We can draw the magnetic field pattern of a straight wire carrying a current with a plotting compass.

Place a wire vertically through a small hole in a horizontal cardboard. The compass is then placed on the card to trace the magnetic field. The resulting magnetic field lines form concentric circles around the wire.

Front 7

State the rule used to predict the direction of the magnetic field around the wire.

Back 7

Right-hand grip rule used to predict the direction of the magnetic field around the wire.

Front 8

Describe the right-hand grip rule in a current-carrying straight wire.

Back 8

Grip the wire with the right hand so that the thumb points in the direction of the current flow. The fingers will then point in the direction of the magnetic field around the wire.

Front 9

What happens when the direction of the current is reversed?

Back 9

When the direction of the current is reversed, the direction of the magnetic field will reverse as well.

Front 10

State when the magnetic field of a long,straight current-carrying conductor is stronger. (2)

Back 10

The magnetic field of a long,straight current-carrying conductor is stronger:

1) when it is closer to the wire,or

2) when a larger current flows through the wire.

Front 11

Where is the magnetic field of a flat coil stronger?

Back 11

The magnetic field of a flat coil is stronger at the centre (i.e. magnetic field lines are closer).

Front 12

State the 2 ways to increase the magnetic field strength at the centre of a flat coil.

Back 12

Ways to increase the magnetic field strength at the centre of a flat coil;

1. Increase the current

2. Increase the number of turns of the coil.

Front 13

What is a solenoid?

Back 13

If we increase the number of turns on a flat coil, we will get a solenoid.

Front 14

State why a solenoid acts like a bar magnet.

Back 14

The resulting magnetic field pattern of a solenoid resembles that of a bar magnet. Hence, we say that the solenoid has two poles and it acts like a bar magnet.

Front 15

Describe how to determine which end is the N pole in a solenoid.

Back 15

To determine which end is the N pole, grip the solenoid with the right hand with the fingers pointing in the direction of the current flow. The end of the solenoid where the thumb points to is the N pole.

Front 16

Draw the magnetic field lines of a solenoid.

Back 16

Front 17

What does the parallel field lines in a solenoid show?

Back 17

The parallel field lines show that the magnetic field strength is almost uniform in the solenoid.

Front 18

Where is the magnetic field stronger in a solenoid?

Back 18

The magnetic field is stronger inside a solenoid.

Front 19

How can the magnetic field strength in a solenoid increase?(3)

Back 19

The magnetic field strength in a solenoid can be increased by;

1. Increasing the current

2. Increasing the number of turns per unit lenght of the solenoid

3. Placing a soft iron core within the solenoid. The soft iron core concentrates the magnetic field lines, thereby increasing the magnetic effect strength.

Front 20

Give 4 devices where electromagnets are used.

Back 20

Electromagnets are used in the following devices :

1) Circuit breaker

2) Magnetic relay

3) Electric bell

4) Magnetic Resonance Imaging (MRI).

Front 21

What is a circuit breaker?

Back 21

The circuit breaker is a safety device that switches off the electric supply when excessive current flows through the circuit.

Front 22

Describe how the circuit breaker works.

Back 22

Terminals T1 and T2 are where the live and neutral wires are connected. When the current is within the limit (e.g. less than 20 A), the solenoid magnetic field is not strong enough to attract the soft iron latch. The interrupt point remains closed and current flows normally through the circuit.

When there is a sudden surge in current (e.g. more than 20 A) such as a short circuit or overloading, the solenoid becomes a very strong electromagnet due to the larger current. It is then able to attract the soft iron latch , releasing the spring, which pushes the safety bar outward. The interrupt point is now open and the circuit is switched off.

After the fault is repaired, the circuit breaker can be reset by pushing the button of the safety bar. This would once again close the interrupt point.