Topic 12 - Magnetism

Question 1

At which part of a magnet are the magnetic forces strongest?

Answer 1

The poles of the magnet.

Question 2

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

Answer 2

They exert a force on each other

Question 3

What type of force is exerted if two like poles or magnet are brought near each other?

Answer 3

A repulsive, non-contact force

Question 4

What type of force is exerted if two unlike poles of a magnet are brought near each other?

Answer 4

An attractive, non-contact force.

Question 5

What is a magnetic field?

Answer 5

The region surrounding a magnet where another magnet or magnetic material experiences a non-contact force.

Question 6

What is the difference between a permanent magnet and an induced magnet?

Answer 6

● A permanent magnet produces its own magnetic field

● An induced magnet becomes magnetic when placed in a magnetic field

Question 7

Induced magnetism always causes what type of force?

Answer 7

A force of attraction.

Question 8

What is the effect when an induced magnet is removed from a magnetic field?

Answer 8

The induced magnet loses most/all of its magnetism.

Question 9

Give four examples of magnetic materials.

Answer 9

1. Iron 2. Steel 3. Cobalt 4. Nickel

Question 10

What can always be said about the force between a magnet and a magnetic material?

Answer 10

It is always attractive.

Question 11

How does the strength of a magnetic field alter as you move further away from the magnet producing it?

Answer 11

The magnetic field strength decreases the further you move away.

Question 12

In what direction does a magnetic field point?

Answer 12

In the direction of the force that a north pole would experience if placed in the field
● From the north seeking pole to the south seeking pole of a magnet

Question 13

What does a magnetic compass contain?

Answer 13

A small bar magnet that points in the direction of the Earth’s magnetic field.

Question 14

What is produced when current flows through a conducting wire?

Answer 14

A magnetic field is produced around the wire.

Question 15

What determines the strength of the magnetic field around a current-carrying wire?

Answer 15

The magnitude of the current flowing through the wire.

Question 16

Does a high concentration of field lines mean the field is strong or weak?

Answer 16

Strong

Question 17

True or false: magnetic field lines never cross each other?

Answer 17

True; they never cross, touch or overlap.

Question 18

How is a plotting compass used to map out a magnetic field?

Answer 18

● Place a compass (containing a needle magnet) on a piece of paper near the field
● Draw an arrow in the direction the compass points
● Repeat at different points on the paper
● Join the arrows to make a complete field pattern

Question 19

How does a compass prove earth core is magnetic

Answer 19

• The core is magnetic, and creates a large magnetic field around the Earth
• We know this because a freely suspended magnetic compass will align itself with the earth’s
  field lines and point North.
• A compass is effectively a suspended Bar Magnet, with its own north pole lining up with Earth’s North Pole
• This cannot be right - like poles repel
• So in fact, Earth’s magnetic pole in the north is a magnetic South Pole and the geographic south pole is close to the magnetic North Pole

Question 20

Right hand rule

Answer 20

Current produces a magnetic field around a wire
- The direction is dictated by the “right hand rule”
- Plotting compasses on a piece of paper through which a wire is
pierced shows this
- Current direction is perpendicular to the magnetic field
direction

Question 21

Strength of magnetic field

Answer 21

Magnetic field strength depends on current size; Greater current, stronger magnetic field
- Strength also varies with distance from the conductor; Greater distance from wire, weaker
field

Question 22

What is a solenoid

Answer 22

A coil of wire which when current passes through creates a strong magnetic field.

Question 23

Describe the magnetic field found inside a solenoid

Answer 23

Strong and uniform.

Question 24

Is the field on the outside of a solenoid strong or weak? Why?

Answer 24

Weak.

The fields from each coil cancel out, making the outside field weak.

Question 25

Solenoids:summary

Answer 25

• Magnetic Field Shape is similar to a bar magnet
• Coiling the wire causes the field to align and form a giant single, almost uniform field along
  the centre of the Solenoid.
• Having an iron core in the centre increases its strength as it is easier for magnetic field lines
  to pass through than air
• The fields from individual coils cancel inside to produce a weaker field outside the solenoid

.

Question 26

Factors that affect solenoid field strength

Answer 26

o Length
o Cross sectional area
o Number of turns (coils) o Using a soft iron core
Size of current

Question 27

What is an electromagnet?

Answer 27

A solenoid with an added iron core

● Adding the iron core increases the
strength of the magnetic field

Question 28

What is the motor effect

Answer 28

When a force is exerted between a magnetic field and a current-carrying conductor placed in that field.

Question 29

What rule is used to determine the force produced by the motor effect?

Answer 29

Fleming’s Left-Hand Rule

Question 30

Flemings left hand rule: summary

Answer 30

Each component is at 90° to the others

• Use this to work out the unknown factor out of the
  three (usually the direction of the force felt)
• Remember current is conventional current (motion of positive charge), which moves in opposite direction to electron flow.

Question 31

When using Fleming’s left hand rule what does the forefinger represent?

Answer 31

The forefinger points in the direction of the magnetic field.

Question 32

When using Fleming’s left hand rule what does the second finger represent?

Answer 32

The second finger points in the direction of current flow in the conduct

Question 33

When using Fleming’s left hand rule what does the thumb represent?

Answer 33

The thumb points in the direction of the force produced by the motor effect

Question 34

What factors affect the size of the force on a current-carrying wire in a magnetic field?

Answer 34

The magnitude of the current flowing through the conductor

● The strength of the magnetic field that the conductor is placed in

Question 35

If the direction of current in a current-carrying wire placed in a uniform magnetic field is reversed, what happens to the force?

Answer 35

The direction of the force is reversed.

Question 36

If the strength of the current in a current-carrying wire placed in a uniform magnetic field is increased, what happens to the force

Answer 36

The strength of the force is increased.

Question 37

Give the equation linking force, magnetic flux density, current and length

Answer 37

force (N) = magnetic flux density (T) x current (A) x length (m)
F = BIL

Where Magnetic Flux Density is measured in Tesla [T] o The number of flux lines per metre squared

Question 38

What criteria must be met for the equation linking force, magnetic flux density, current and length to hold?

Answer 38

The conductor must be at right-angles to the magnetic field it is placed in

Question 39

What is the unit used for magnetic flux density?

Answer 39

Tesla, T

Question 40

If the current and the magnetic field are parallel to each other, which direction would the force act in?

Answer 40

No forcé will act

Question 41

How does an electric motor work?

Answer 41

A coil of wire, carrying a current, is placed in a magnetic field ( between two permanent magnets)
● The forces on the two sides perpendicular to the field experience forces in opposite directions
● This causes a rotational effect

Question 42

How are electric motors kept rotating?

Answer 42

A split ring commutator is used. This switches the current direction every half turn, which ensures that the coil keeps spinning.