Magnetic Fields

Question 1

State the motor effect and its associated hand rule.

Answer 1

• A current-carrying wire in a magnetic field will experience a force.
• Right-hand rule

Question 2

What type of motion does a charged particle undergo in a magnetic field?

Answer 2

Circular

Question 3

Outline how a cyclotron works.

Answer 3

• Magnetic field causes protons emitted by the source in the centre to undergo circular motion inside the hollow metal dees.
• Oppositely charged dees caused protons to be accelerated across the gap, increasing v and r.
• Every half a cycle, polarity of dees reverses for protons to be accelerated across again.

Question 4

How does a velocity selector ensure all ions have the same speed?

Answer 4

Magnetic force (BQv) = Electric force (EQ)

Question 5

What is the difference between a film and sensor array mass spectrometer?

Answer 5

• Film: ions travel through semicircle, producing spots on film. Brightness depends on number of incident particles.
• Sensor array: positioned to detect ions of specific masses. Can detect relative abundance.

Question 6

State Faraday’s Law (hint: emf)

Answer 6

Induced emf is proportional to the rate of change of flux linkage.

Question 7

State Lenz’s Law

Answer 7

The direction of induced emf/ current opposes the change that caused it.

Question 8

Which hand rule is used for induced current?

Answer 8

Fleming’s right hand rule

Question 9

How can you induce an emf in a static coil?

Answer 9

Generate an alternating current in another static coil, which will create a constantly changing magnetic field the other coil will be in.

Question 10

How does a simple generator work?

Answer 10

A coil is made to spin in a magnetic field, and its sides cut through the flux.

Question 11

What is the purpose of the iron core in a transformer?

Answer 11

Become magnetised by the alternating current in the primary transformer to induce alternating current in the secondary coil.

Question 12

Why is a laminated core useful?

Answer 12

Reduces eddy currents which increases the efficiency. Using low resistance windings also reduces the energy lost.

Question 13

What is back emf?

Answer 13

• When a motor spins, generator effect produces a counter emf which reduced p.d. and current across coil.
• Under heavy load, motor spins slowly: small generator effect = small back emf = large current in coil (could burn out)