SP12 Magnetism and the Motor Effect

Question 1

Describe the law of magnetism

Answer 1

• Two like poles (e.g.: S,S or N,N) will repel each other
• Two unlike poles will attract each other

Question 2

What type of force is the attraction and repulsion of different magnetic poles?

Answer 2

Non contact force

Question 3

What elements are magnetic?

Answer 3

• Iron
• Nickel
• Cobalt

Question 4

How do we test if an unknown material is a magnetic metal or a magnet?

Answer 4

• If it can be repelled by the magnet, the material itself is a magnetic
• If it can only be attracted by the magnet, the material is a magnetic metal

Question 5

What are the two types of magnets?

Answer 5

• Permanent
• Induced

Question 6

Define permanent magnet

Answer 6

A material that will produce its own magnetic field. It will not lose its magnetism

Question 7

Define induced magnet

Answer 7

When a magnetic material is placed into a magnetic field, it can temporarily turn into a magnet

Question 8

How can we induce a magnet?

Answer 8

By stroking it with a permanent magnet.

Question 9

Why does stroking the magnetic material with a magnet induce its magnetism?

Answer 9

It aligns all the domains of the material in the same direction, creating a temporary magnet

Question 10

Define magnetic field?

Answer 10

The region around a magnet where a force acts on another magnet or on a magnetic material

Question 11

What are magnetic field lines used for?

Answer 11

Used to represent the strength and direction of a magnetic field

Question 12

How do magnetic field lines travel?

Answer 12

Point from North to South

Question 13

How is the strength of the magnetic field shown?

Answer 13

Spacing of the magnetic field lines:
- close together = strong magnetic field
- far apart = weaker magnetic field

Question 14

What is a uniform magnetic field?

Answer 14

One that has the same strength and direction at all points

Question 15

How is a uniform magnetic field shown?

Answer 15

• equal spacing between all the magnetic field lines
• arrow going from north to south
• produced between the gaps of two opposite poles
• the field will not be uniform outside that point

Question 16

Where is the magnetic field of a bar magnet the strongest?

Answer 16

At the poles

Question 17

Why does the magnetic field become weaker as we move away from the bar magnet?

Answer 17

The magnetic field lines are getting further apart, so it cannot attract or repel as much

Question 18

How do we know that the Earth’s core is magnetic?

Answer 18

A free suspended magnetic compass will align itself with Earth’s field lines and always point North

Question 19

Why is Earth’s magnetic north actually its geographic south pole?

Answer 19

• a compass is basically a suspended bar magnet
• its north pole lines up with that of the Earth’s geographic north pole
• which by the law of magnetic attraction cannot be right
• therefore, the north of the compass must be lining up with the Earth’s magnetic south

Question 20

What happens when a current flows through a wire?

Answer 20

A magnetic field is produced

Question 21

Describe how to find the direction of the magnetic field using the right hand rule

Answer 21

• Thumb: points along the direction of the current
• Magnetic field has a direction similar to the way your fingers are pointing

Question 22

How do we change the direction of the magnetic field using current?

Answer 22

reversing current = reversing direction of the magnetic field

Question 23

How do we increase the strength of a magnetic field produced due to a current?

Answer 23

• Increase the size of the current
• wrapping the wire in coils also increases the size of the current

Question 24

What does the strength of the magnetic field depend on?

Answer 24

• size of the current
• distance from the long straight conductor

Question 25

What is a **solenoid**?

Answer 25

A series of coils that has a similar magnetic field as a bar magnet

Question 26

Describe the magnetic field of a solenoid

Answer 26

- uniform - similar to that of a bar magnet

Question 27

Why is the magnetic field of a solenoid uniform?

Answer 27

Coiling the wire causes the field to align

Question 28

How can we work out the poles of a solenoid?

Answer 28

- needs to be viewed from the end - if the current is travelling in a clockwise direction, then it is the south pole - if the current is travelling in an anticlockwise direction, then it is the north pole

Question 29

How do we increase the magnetic field of a solenoid?

Answer 29

- increase the current - increase the number of coils - adding an iron core

Question 30

When does the motor effect occur?

Answer 30

When a wire with current flowing through it is placed in a magnetic field and experiences a force

Question 31

What is the effect of a motor effect?

Answer 31

The interaction of two magnetic fields: - one is produced around the wire due to current flowing through it - second is the magnetic field where the wire is placed between This produces a force

Question 32

How must the wire be placed in relation to the magnetic field for a motor effect to be produced?

Answer 32

Perpendicular to each other

Question 33

What does the size of the force of a motor effect depend on?

Answer 33

- strength of magnetic field - strength of current in wire - length of wire

Question 34

How can we increase the size of the force in a motor effect?

Answer 34

- increasing the amount of current that flows through a wire - using stronger magnets

Question 35

Describe the magnetic field around a wire

Answer 35

Circular

Question 36

Describe Flemmings Left hand rule

Answer 36

- thumb: represents force - first finger: represents magnetic field - middle finger: represents current

Question 37

What is Flemming's left hand rule used to find out?

Answer 37

The direction of the force/current/magnetic field in a motor effect

Question 38

What is the equation used to calculate force in a motor effect?

Answer 38

Force = Magnetic flux density (field strength) x current x length of wire

Question 39

What can the motor effect be used for?

Answer 39

Force produced by the magnetic field and current is used to cause rotation, therefore producing a D.C. motor

Question 40

How does motor effect cause movement?

Answer 40

- The current will flow in opposite directions on each side of the coil - force produced from the magnetic field will push one side of the coil **up** and the other side **down** - causing the coil to rotate

Question 41

What is the **split ring commutator** used for?

Answer 41

Reverses the direction of the current flow