(7) Magnetism and Electronagnetism

Question 1

Where are the magnetic fields strongest on a magnet?

Answer 1

The poles (north and south)

Question 2

What happens when two magnets are brought close to each other?

Answer 2

The exert a force on each other, the two poles repel each other if they are the same however opposite poles attract.

(Attraction and repulsion are examples of non contact forces)

Question 3

What is a permanent magnet?

Answer 3

Permanent magnets produce their own magnetic fields which cannot be turned on or off. Bar or horseshoe magnets are examples of this.

Question 4

What is an induced magnet?

Answer 4

A material that becomes a magnet when it is placed in a magnetic field, when removed from it, it looses all or most of its magnetism very quickly. Induced magnetism only causes a force of attraction.

Question 5

Name three magnetic materials:

Answer 5

Iron, nickel and cobalt

Question 6

How can you test for magnetism?

Answer 6

A permanent magnet can attract or repel another permanent magnet and attract (but not repel) a magnetic material.

Question 7

What is a magnetic field?

Answer 7

The area around a magnet where a force acts on another magnet or magnetic material, the strength of the magnetic field depends on the distance from the magnet (its strongest at the poles).

Question 8

How can you use a magnetic compass to detect a magnetic field?

Answer 8

The compass contains a small bar magnet on a pivot (so it can rotate), the needle points in the direction of the earths magnetic field or the magnetic field of a magnet.

Place the plotting compass near the magnet on a piece of paper, mark the direction the needle points. Move it to many different positions in the magnetic field then join the points to show the field lines.

Question 9

What are 4 point to remember when drawing magnetic field lines?

Answer 9

1) they never cross each other
2) the closer the lines, the stronger the magnetic field
3) draw arrow heads to show direction
4) the arrow heads point from the North Pole of the magnet to the South Pole

Question 10

What happens when a current flows through a conducting wire?

Answer 10

A (circular) magnetic field is produced around the wire

Question 11

What does the strength of the magnetic field depend on (in a wire)?

Answer 11

The strength of the magnetic field depends on the current through the wire and the distance from the wire.

Question 12

What is a solenoid?

Answer 12

A solenoid is a coil of wire (in a spiral shape), this increases the strength of the magnetic field created by the current though the wire

Question 13

Describe the magnetic field inside and outside a solenoid:

Answer 13

Inside a solenoid the magnetic field is strong and uniform.

Around the solenoid is similar to a bar magnet.

Overall it has a stronger magnetic field than a straight piece of wire

Question 14

What is an electromagnet?

Answer 14

Solenoid + iron core = electromagnet

• the iron core increases the solenoid’s magnetic field strength
• typically made by coiling wire around a iron nail

Question 15

How can you increase the strength of an electromagnet?

Answer 15

1) increase the current
2) increase the number of coils
3) the iron core is soft and magnetises easily

Question 16

What devices use electromagnets?

Answer 16

1) scrapyard crane
2) circuit breaker
3) the electric bell
4) the relay

Question 17

How are electromagnets used in the scrapyard crane?

Answer 17

• scrapyard vehicles are lifted using powerful electromagnets, the steel frame of the vehicle sticks to it when a current passes through the coil of the magnet
• when switched off the vehicle frame drops

Question 18

How are electromagnets used in the circuit breaker?

Answer 18

• a circuit breaker is a switch closed by a spring (in series) with an electromagnet
• when the current is too strong the switch is pulled open by the electromagnet
• it’s stays open until manually reset

Question 19

How are electromagnets used in the electric bell?

Answer 19

• when the electric bell is connected to a battery the iron armature is pulled onto the electromagnet
• this opens the make-and-break switch and the magnet is switched off
• the armature springs back and the make-and-break switch closes again
  (Whole cycle repeats itself)

Question 20

How are electromagnets used in the relay?

Answer 20

• the relay is used to switch an electrical machine on/ off
• a small current is passed through the coil of the electromagnet (magnetising the iron core) which pulls the armature onto the magnet closing the switch and turning the machine off

Question 21

What is the motor effect?

Answer 21

When a conductor (wire) carrying a current is placed in a magnetic field the magnet producing the field and the conductor exert a force on each other.

There is no motor effect if the current and magnetic field are parallel to each other.

Question 22

Flemings left hand rule:

Answer 22

Thumb = movement/ direction of force
Forefinger = field (N to S)
Second finger = current (+ to -)

(90 degrees)

Question 23

What is the equation for force?

Answer 23

Force = magnetic flux density x current x length

Question 24

What is magnetic flux density?

Answer 24

The magnetic flux density is the measure of strength of the magnetic field. Unit Tesla (T)

Question 25

What is the basis of an electric motor?

Answer 25

A coil of wire carrying a current in a magnetic field tends to rotate.

Question 26

How can you control the speed of an electric motor?

Answer 26

Changing the current ( reversing the current would reverse the direction)

Question 27

What is a split ring commutator?

Answer 27

Metal contacts on the coil of a DC motor that connects the rotating coil continuously to its electrical power supply