Magnetic Fields

Question 1

What does the magnitude of the force on a wire depend on

Answer 1

• Current
• Strength of the magnetic field
• Length of the wire
• Angle between the lines of the field and the direction of the current

Question 2

How can a wire induce a magnetic field

Answer 2

when a current flows through it

Question 3

When will a current carrying wire experience a force

Answer 3

If it is placed at a non-zero angle to the filed lines of another magnetic field. The field lines from the wire and the external magnet interact, this causes the field lines from the external magnet field to contract, so the field lines are closer together, causing a force on the wire.

Question 4

Define Magnetic Flux Density

Answer 4

Magnetic Flux Density, B, is the force per unit length, per unit current, on a current carrying conductor at right angles to the magnetic field lines. It is a vector quantity and measured in teslas, T. Effectively, it is the strength of the magnetic field.

Question 5

Define the Tesla

Answer 5

The tesla is the flux density that causes a force of 1N on a 1M wire carry a current of 1A at right angles to the flux

Question 6

Why do charged particles follow a circular path in a magnetic field

Answer 6

Charged particles in a magnetic field follow a circular path with the force always acting towards the centre of curvature; this is because the force on the charge from the magnetic filed is always at right angles to the direction of the motion of the particle.

This also means that no work is done on the particle by the field; the speed and kinetic energy of the particle are unchanged

Question 7

What is a cyclotron

Answer 7

A cyclotron is a particle accelerator that accelerates charged particles through a spiral path using a fixed magnetic field and an alternating potential difference

They are used with heavier particles like alpha and protons

Question 8

Explain how a cyclotron works

Answer 8

• The particles experience a potential difference when they travel across the gap, and gain energy equal to QV
• Since the particles have more kinetic energy, they move faster and accelerate to the next dee
• The ac voltage is timed to change direction every time the particle reach the gap with the dees – it must alternate to accelerate the particles each time the reach the gap
• Particle spend the same time inside each dee, but the radius of their path increases after each gap and they travel further in the same time

Question 9

How does special relativity effect a particle’s speed in a cyclotron

Answer 9

• The effect of special relativity limits a particle’s speed in a cyclotron.
• Particles get more massive as they travel close to the speed of light.
• As particles move faster and their mass increases, the time spent in each dee increases and the more massive particles get out of step with the alternating potential difference
• Magnets are used
• Magnetic field acts perpendicular to the path of the particles - meaning the magnetic force acts perpendicular to their path
• The force required depends on the speed of the particles - larger speed - stronger force - stronger magnets
• F=BQv

Question 10

Define Magnetic Flux

Answer 10

Magnetic flux, Φ, is defined as the magnetic flux density multiplied by the surface areas of the surface, where this area is perpendicular to the lines of flux. Magnetic flux is measured in Weber’s (Wb)

Question 11

What factors will increase the number of flux lines a wire cuts through each second

Answer 11

• length L is longer
• The wire moves faster
• The magnetic flux is stronger

Question 12

Define Magnetic Flux linkage

Answer 12

Magnetic flux linkage is defined as NΦ, where Φ is the number of flux lines that pass through with each turns of the coil of N turns.

Measured in Weber Turns

Question 13

What factors effect the size of Flux Linkage

Answer 13

o The flux density
o The orientation of the coil and flux lines
o The coils cross-sectional area
o The number of turns on the coil

Question 14

Explain how electromagnetic induction occurs

Answer 14

• When a length of wire moves between two magnets,
  electromagnetic induction occurs
• An e.m.f is induced in the wire because of an electric
  charge moving perpendicular to the magnet
  experiences a force, BQv
• The electrons in a wire move towards one end of the
  wire when the wire moves perpendicular to the
  magnetic field
• This leaves one end of the wire negatively charged
  overall and the other end positively charged, creating a
  potential difference across the wire
• A current can then flow if the wire is part of a complete
  circuit

Question 15

What is Faradays law

Answer 15

Faradays law states that the e.m.f is equal to the rate of change of magnetic flux linkage

Question 16

Explain Faraday’s law - when and when won’t an e.m.f be induced in a coil

Answer 16

o Relative movement between a magnet and a coil changes the flux linkage in the coil. This generates an e.m.f
o Rotating a coil in the plane perpendicular to the field changes the cross-sectional area – through which the flux passes – this changes the flux linkage and generates an e.m.f
o Increasing the relative motion, or the speed at which the coil rotates, increases the rate of change of the flux linkage, which increases the induced e.m.f
o If there is no relative movement or rotation, the flux linkage does not change, so no e.m.f is generate

Question 17

What is Lenz’s law

Answer 17

Lenz’s law indicates the direction of the induced e.m.f causes effects that oppose the change producing it

Question 18

Explain Lenz’s Law

Answer 18

 Pushing a magnets south pole into the coil produces a south pole – this repels the magnet
 Pulling a south pole out of the coil induces a north pole – this attracts the magnet

In a pratical situation:

• Where the south pole of a magnet is pushed into the
  coil, a current is induced in the wire, which becomes an
  electromagnet
• If the south pole of the electromagnet faces the moving
  magnet, the poles repel, and work must be done to keep
  pushing the magnet into the coil of wire

Question 19

What are Eddy currents

Answer 19

Eddy currents are circulating electric currents flowing in a plane of metal

They are caused by the change of flux linkage when the metal moves perpendicular to the field, and the currents flow in a direction to oppose the motion creating them

Question 20

How can you calculate the area swept out through a conductor moving in a magnetic field and use this to find the e.m.f induced

Answer 20

When a conductor moves at a velocity, v, perpendicular to the flux lines, Faraday’s law applies and an e.m.f is generated

For a conductor of length, travelling in a flux density B, the area swept out per second is length x velocity

The induced e.m.f equals rate of change of flux linkage so:

ε=Blv

Question 21

What equation is used to calculate the induced a.c. voltage in an coil rotates in a magnetic field

Answer 21

ε=BANωsinωt

e = induced e.m.f (volts)
B = magnetic flux density (Teslas) 
A = cross sectional area of the coil 
N = number of turns on the coil 
ω= angular speed of the rotating coil 
t = time (seconds) 

• the equation is as long as the axis of rotation is at right angles to the field.

Question 22

How do you convert degrees into radians

Answer 22

angle x π/180