Electromagnetism

Question 1

What is a magnetic field?

Answer 1

A region of space in which a current-carrying conductor experiences a magnetic force.

Question 2

In which direction do magnetic field lines point?

Answer 2

From north to south.

Question 3

What does the closeness of magnetic field lines represent?

Answer 3

Magnetic flux density.

Question 4

Describe the shape of a magnetic filed around a current carrying conductor.

Answer 4

Concentric circle, described by right-hand-grip rule

Question 5

Describe the shape of the magnetic field within a solenoid.

Answer 5

Uniformal, described by right-hand-grip rule

Question 6

Why is an iron core used in solenoids?

Answer 6

To intensify the magnetic field that is produced due to the arrangement of coiled wires.

Question 7

Describe how we use the RHG rule to determine…
- direction of field
- polarity

Answer 7

• For a current-carrying wire, point your thumb in the direction of current
• For a solenoid, coil your fingers in the direction of current

Question 8

Why do current-carrying wires in a magnetic field experience a magnetic force?

Answer 8

• A magnetic field is produced about moving charge carriers
• This magnetic field interacts with the existing magnetic field
• To produce a force perpendicular to the direction of both current and the existing field

Question 9

What rule can be used to determine the direction of a magnetic force?

Answer 9

Fleming’s Left Hand Rule…
- Thumb: force
- Forefinger: B field
- Second minger: E field (current)

Question 10

When is the motor effect considered to be at its’ strongest?

Answer 10

Where the wire (direction of current) is perpendicular to the direction of the magnetic field
This is where sin theta = sin 90 = 1

Question 11

What equation describes the magnitude of a magnetic force?

Answer 11

F=BIL sin theta

Question 12

Give another derivation of the F=BIL equation.

Answer 12

F=Bqv also described as F=Bev for an electron

Question 13

How might we practically determine flux density?

Answer 13

• Place 2 magnetic onto a balance with a wire between them
• Measure the length of the wire
• Record initial ‘mass’
• Pass a known current through the wire
• Record new ‘mass’
• Find difference and multiply by g
• This equals F in F=BIL sin theta

Question 14

How does a Hall probe allow for the determination of magnetic flux density?

Answer 14

• Current passed through Hall probe
• Electrons are deflected due to the interaction between B field of moving electrons and existing unknown B field
• Producing charge separation and so an electric field whose strength is related to potential difference and flux density

Question 15

What shape of path does a moving electron subject to only a B field describe?

Answer 15

Circular path

Question 16

Why do moving electrons describe a circular path in a magnetic field?

Answer 16

• Magnetic force acts perpendicular to the direction of current i.e. motion
• No other force applies
• No work is done so constant speed; vfelacoity changes due to changing direction as the objects accelerates at 90 degrees to velocity

Question 17

Why can we equate Bev to mv2/r?

Answer 17

Electrons describe a circular path when moving in a magnetic field

Question 18

How does a velocity selector work?

Answer 18

• Both B and E fields are applied between 2 plates
• Charged particles are passed between the plates
• For a particle to move undeflected zero resultant force applies so F=Bqv=Eq
• This V=E/B so particles are isolated only by their velocity
• Else they are deflected in some way as forces are unequal

Question 19

What is electromagnetic induction?

Answer 19

The induction of an electric current when a conductor moves relative to a magnetic field

Question 20

What is the underlying cause of induction in all cases?

Answer 20

A change in magnetic flux linkage

Question 21

What is magnetic flux?

Answer 21

Product of flux density and the area through which magnet flux passes

Question 22

What is magnetic flux linkage?

Answer 22

Sum of magnetic flux through all of the turns in a given coil

Question 23

What is the unit for magnetic flux density?

Answer 23

Tesla

Question 24

What is the unit for magnetic flux?

Answer 24

Weber

Question 25

Why does moving a wire relative to a magnetic field produce a changing flux linkage?

Answer 25

- As electrons move through a B field they are subject to a B force - This causes them to move along the conductor - This constitutes a charge separation so. a p.d. occurs - The movement of electrons wrt. time constitutes a current

Question 26

What factors determine the magnitude of the induced emf in a coil?

Answer 26

- Relative speed - Magnetic flux density - Number of turns in the coil

Question 27

How can we use EM induction to produce alternating current?

Answer 27

- Moving a conductor back and forth causes changes in the direction of the B force so current changes direction periodically - This constitutes an AC

Question 28

Describe Faraday's law.

Answer 28

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

Question 29

How did Lenz's law extend upon ideas about EM induction proposed by Faraday?

Answer 29

He discovered that induced emf was in a direction such that it opposes the force that caused it

Question 30

Give an equation for EM induction.

Answer 30

emf = -dBAN cos theta / dt

Question 31

How can Lenz's law be analysed as a product of the principles of conservation of energy?

Answer 31

- As a North Pole moves towards a coil mechanical work must be done to increase electric energy which produces a current - This means that the coil we move the magnet into must also have a North Pole facing - Since opposite poles repel which is the only way in which we could do work against a force

Question 32

How does magnetic flux linkage vary with the rotation of an AC generator?

Answer 32

It is a maximum when vertical (we cut the field lines to the greatest extent) and a minimum when horizontal

Question 33

Why is emf a minimum. when flux linkage is a maximum?

Answer 33

- Flux linkage is described by a cosine graph - Emf is the rate of change i.e. the gradient of the graph - Thus emf is a minimum when the flux linkage graph is a maximum

Question 34

Why do AC generators need slip rings?

Answer 34

- This enables current to be reversed at every half turn - Producing constant rotation in a given direction - Else the generator simply oscillates as B force always acts in the same direction

Question 35

Describe a basic transformer setup.

Answer 35

- Alternating current in primary coil produces an alternating magnetic field in soft iron core - Which links primary and secondary coils whilst intensifying the field - The alternating magnetic field in the secondary coil induces an alternating current due to changing flux linkage

Question 36

Why do we need a soft iron core in a generator?

Answer 36

- Links the two coils - Intensifies the field - 'Soft' property allows easy realignment of 'domains' reducing resistive losses due to AC

Question 37

Why must a soft iron core be laminated in a transformer setup?

Answer 37

This prevents the formation of Eddy currents which in turn minimises thermal losses

Question 38

Why must an alternating current be used in a transformer?

Answer 38

- Constant change in current produces changing field - Producing changing flux linkage - Electric field is constant when DC is applied so no induction occurs; flux linkage is constant

Question 39

Why must transformers be connected to load resistors?

Answer 39

- Minimal resistance would otherwise allow for excess current to flow - This would in turn produce a short circuit

Question 40

Give an equation linking induced emf and number of turns in a transformer coil.

Answer 40

Vp/Vs = Np/Ns

Question 41

Give an equation linking induced current and number of turns in a transformer coil.

Answer 41

Is/Ip = Np/Ns