4.4 Magnetic Fields

Question 1

Why do charged particles move in a circular orbit in a magnetic field?

Answer 1

The magnetic force is always perpendicular to the velocity of the particle, so they end up being forced in a circular orbit. The particles undergo centripetal acceleration, with the centripetal force being the magnetic force.

Question 2

When a magnetic field is perpendicular to a current-carrying wire, does the wire feel a force?

Answer 2

Question 3

Fleming’s left hand rule for motors represents what properties on what fingers?

Answer 3

• Thumb - Thrust/Force
• First finger - Field (Magnetic)
• Second finger - Current

Question 4

What is magnetic flux density (B)?

Answer 4

Flux density measured in Tesla (T) or Webers/meters2 (Wb/m2), is the flux per metre2.

Question 5

Does a charged particle moving through a field feel a force when it is traveling along the field lines or perpendicular to them?

Answer 5

Perpendicular to the field.

Question 6

What is the equation for the force felt by a moving charge in a magnetic field?

Answer 6

F = BQv sinθ

Question 7

Is the force applied to the particles applied perpendicular to the particle’s motion or in one direction?

Answer 7

Perpendicular to its motion, causing it to move in a circular motion.

Question 8

Which fields do cyclotrons use?
A. Electric field
B. Magnetic field
C. Gravitational field
D. Both Electric and Magnetic

Answer 8

D. An electric field and a magnetic field.

Question 9

Define magnetic field.

Answer 9

• A region of space in which moving charged particles are subject to a magnetic force.
• This force is caused by the interaction of two magnetic fields (there is a field around the moving charged particles which interacts with the existing magnetic field they are passing through).

Question 10

What is a magnetic field line?

Answer 10

The path which a north pole would take when placed in a magnetic field.
Field lines go from north to south.

Question 11

How can you map field lines around a magnet?

Answer 11

• You can place iron filings on a piece of paper and then put the magnet on the paper and the filings will align to the field.
• You can also use a plotting compass and place it in various positions around the magnet, before marking the direction of the needle at each point and connecting
  them.

Question 12

How do you represent the strength of a magnetic field on a diagram?

Answer 12

It is represented by how close together the field lines are – the closer they are, the stronger the field. (It is the density of the field lines, which is why magnetic flux density and Magnetic Field strength are interchangable)

Question 13

Define magnetic flux density.

Answer 13

The force per unit current per unit length on a current-carrying conductor placed in a magnetic field perpendicular to the field lines. (Magnetic flux per unit area)

Question 14

What is the unit of magnetic flux density?

Answer 14

Tesla (T)
1 T = 1 N m-1 A-1

Question 15

When a magnetic field is perpendicular to a current-carrying wire, does the wire feel a force?

Answer 15

Yes, the magnitude of the force is =BIl
l = length of the wire
B = Magnetic flux density
I = Current in the wire

Question 16

Give the formula relating magnetic force, flux density, current, length and angle between the field and the conductor.

Answer 16

• F = BILsinθ
• F = Magnetic force (N)
• B = Magnetic flux density (T)
• I = Current in the conductor (A)
• L = Length of conductor in the field (m)
• θ = Angle between the field lines and the conductor(o or rad)

Question 17

What is the Hall voltage?

Answer 17

When a magnetic field, B, is applied to a conductor carrying a current I, perpendicular to the field direction, a Hall voltage appears perpendicular to the B and I directions.

Question 18

What is the the field strength of a long straight wire and a long solenoid?

Answer 18

Question 19

How do you work out the shape of the field around a solenoid?

Answer 19

Question 20

What increases the field strength in a solenoid?

Answer 20

Adding an iron core.