Magnetic Fields

Question 1

Define magnetic flux

Answer 1

product of the component of the magnetic flux density perpendicular to the area, and the cross sectional area

Question 2

Define magnetic flux density

Answer 2

magnetic flux density = F / IL

where F is the force experienced by a wire placed at right angles to the magnetic field,
L is its length
I is the current in the wiredefine

Question 3

Why does centripetal force not affect speed?

Answer 3

Force is perpendicular to motion so no work is done

Question 4

how can you experimentally determine magnetic flux density

Answer 4

• place magnets on a top-pan balance
• magnetic field between them is almost uniform
• a stiff copper wire is held perpendicular to the magnetic field between the two poles
• length, L, of the wire in the field is measured with a ruler
• using crocodile clips, a section of the wire is connected in series with an ammeter and a variable power supply
• balance is zeroed
• with a current, I, the wire experiences a vertical upward force
• due to newton’s third law, magnets experience an equal downward force, F, measured using F = mg
• magnetic flux density determined using B = F/IL

Question 5

what device measures magnetic flux density directly

Answer 5

hall probe

Question 6

how is a current produced through electromagnetic induction

Answer 6

• work is done to move the magnet using lenz’s law
• motion of coil relative to the magnetic field makes the electrons move because they experience a magnetic force
• the moving electrons constitute an electrical current in the coil so the process has produced electrical energy
• principle of conservation of energy

Question 7

define magnetic flux linkage

Answer 7

the product of the number of turns in the coil and the magnetic flux

Question 8

when is an e.m.f induced

Answer 8

when there is a change in the magnetic flux linking the circuit

Question 9

what is faraday’s law

Answer 9

the magnitude of the induced e.m.f is directly proportional to the rate of change of magnetic flux linkage

Question 10

what is lenz’s law

Answer 10

the direction of the induced e.m.f is always such as to oppose the change producing it

Question 11

explain lenz’s law

Answer 11

• when a magnet is pushed towards the coil, the end of the coil closest to the magnet has the same polarity as that of the magnet
• this means work is done in pushing the magnet towards the coil, meaning electrical energy is produced in the coil
• when the magnet is pulled away from the coil, the end of the coil closest to the magnet will switch polarity such as to attract the magnet , meaning work is done in moving the magnet away
• this is why the coil still has electrical energy when the magnet is pulled away, nut the e.m.f is in the opposite direction
• uses the principle of conservation of energy

Question 12

what is the gradient of a flux-linkage - time graph

Answer 12

minus e.m.f

Question 13

when is flux linkage maximum

Answer 13

when the coil is perpendicular to the magnetic field

Question 14

when is flux linkage 0

Answer 14

when the coil is parallel to the magnetic field

Question 15

explain how a transformer works

Answer 15

• an alternating current is supplied to the primary coil
• this produces a varying magnetic flux in the soft, laminated iron core
• the secondary coil is linked by the changing flux
• the iron core ensures that all the magnetic flux created by the primary coil links the secondary coil and none is lost
• according to faraday’s law, a varying e.m.f is produced across the ends of the secondary coil