11. Magnetic Fields

Question 1

What is a ‘force’ field?

Answer 1

a region where an object experiences a non-contact force

Question 2

What is a magnetic field?

Answer 2

an area where a magnet or magnetically susceptible material experiences a force of attraction or repulsion

Question 3

What are magnetic field lines referred to as?

Answer 3

flux lines

Question 4

What do magnetic flux lines show?

Answer 4

the direction of the force on the north pole of a magnet

Question 5

Where do the arrows of magnetic flux lines always point when drawn?

Answer 5

from North to South

Question 6

What is the magnetic field strength called?

Answer 6

magnetic flux density

Question 7

What is magnetic flux density?

Answer 7

• a measure of the density of the flux lines
• the force on one metre of wire carrying a current of one amp at right angles to the magnetic field

Question 8

What is the symbol for magnetic flux density?

Answer 8

B

Question 9

What are the units of magnetic flux density?

Answer 9

Teslas (T)

Question 10

How are flux lines going into the page represented?

Answer 10

with an X

Question 11

How are flux lines coming out of the page represented?

Answer 11

with in O

Question 12

What happens when a current flows through a wire?

Answer 12

a circular magnetic field is induced around it

Question 13

What does the thumb show in Fleming’s right hand rule?

Answer 13

the thumb points in the direction of the conventional current

Question 14

What do the fingers show in Fleming’s right hand rule?

Answer 14

fingers curl in the direction of the magnetic field

Question 15

What happens when a current carrying wire is placed in a magnetic field?

Answer 15

it will experience a force

Question 16

What does the thumb show in Fleming’s left hand rule?

Answer 16

the direction of the force

Question 17

What does the index finger show in Fleming’s left hand rule?

Answer 17

the direction of the magnetic field

Question 18

What does the middle finger show in Fleming’s left hand rule?

Answer 18

the direction of the conventional current

Question 19

What four factors could affect the force on a current carrying wire?

Answer 19

• magnetic flux density
• size of current
• length of wire in magnetic field
• angle between the wire and field

Question 20

When is the magnetic flux density equal to 1 Tesla?

Answer 20

when a 1 metre wire, carrying 1 amp perpendicular to the magnetic field experiences 1 Newton of force

Question 21

What is the full equation used to calculate the force on a current carrying wire?

Answer 21

F = BILsin(θ)

Question 22

What are the two conditions where a charged particle will not experience a force in a magnetic field?

Answer 22

• if it is moving parallel to the field lines (so Fleming’s left hand rule doesn’t apply)
• if it is stationary (v = 0 therefore F = BQv = 0)

Question 23

What path do charged particles follow in a magnetic field?

Answer 23

A circular path

Question 24

Why does a charged particle follow a circular path in a magnetic field?

Answer 24

• Fleming’s left hand rule states that the force experienced by a particle is always at right angles to the direction it is travelling in
• so this force will change the direction the particle is travelling in
• which changes the direction of the force

Question 25

How can the radius of the circular motion of a charged particle in a magnetic field be derived?

Answer 25

Centripetal force (using linear velocity) = magnetic force

Question 26

How can the time period and frequency of one rotation of a charged particle in a magnetic field be derived?

Answer 26

Centripetal force (using angular velocity) = magnetic force

Question 27

What is a cyclotron?

Answer 27

A compact particle accelerator used often in hospitals to produce radioactive tracers and high energy radiation beams

Question 28

What do cyclotrons consist of?

Answer 28

Two “Dees” (the area where there is a magnetic field) with a gap in the middle (where an alternating electric field is present)

Question 29

How does a cyclotron work?

Answer 29

• A charged particle is inserted in the middle of the cyclotron and it experiences a force in the electric field, accelerating it towards the negative plate
• It follows a circular path once it enters the magnetic field
• The particle with re-enter the electric field
• The potential difference of the plates must be flipped to accelerate the particle to a higher linear speed
• The particle changes direction as it enters the other Dee
• The potential difference must be flipped again as the particle re-enters the electric field
• This process repeats until the particle reaches the desired speed
• It will then exit the cyclotron

Question 30

Why can the magnetic field not increase the linear speed of the particle?

Answer 30

Because there is no force in the direction of travel, it only changes the direction

Question 31

Why must the potential difference of the plates be flipped?

Answer 31

To accelerate the particle to a higher speed, otherwise the particle would get decelerated

Question 32

Why does the radius of the particle’s circular path increase?

Answer 32

The radius of the circular path will be larger as the particle will have a greater linear speed