Chapter 25 - Electromagnetic induction

Question 1

What happens when a conductor is moved through a magnetic field? What is this called?

Answer 1

Cuts through the field lines, inducing an emf in the conductor

electromagnetic induction

Question 2

What happens when a conductor with an induced emf is part of a complete circuit?

Answer 2

Induced emf forces electrons round the circuit, inducing a current

Question 3

How can an induced emf be increased?

Answer 3

• Moving the wire faster
• Using a stronger magnet
• Making the wire into a coil

Question 4

When does the induced emf become zero?

Answer 4

When the relative motion between the magnet and the wire ceases

Question 5

How do you calculate the rate of energy transfer of energy from the source of emf to the other components of the circuit?

Answer 5

Induced emf x current = energy transfered per unit charge from the source x charge flow per second

= energy transfered per second from the source

Question 6

What can be used to determine the direction of induced current?

Answer 6

Fleming’s right hand rule (Dynamo rule)

Direction of induced current is opposite to the direction of the flow of electrons in the conductor

Question 7

What is magnetic flux?

Answer 7

Magntic flux (Ø) = BA

B = magnetic flux density and A = Area swept out

Question 8

What is magnetic flux linkage?

Answer 8

Magnetic flux linkage through N turns of a coil = NØ = BAN

where B is magnetic flux density perpendicular to area A

= BAN (cos theta)

Question 9

When the coil is turned through 180º what does the flux linkage equal?

Answer 9

-BAN

Question 10

When the magnetic field is parallel to the coil area what does the flux linkage equal?

Answer 10

0 as no field lines pass through the coil area

Question 11

What does faraday’s law of electromagnetic induction state?

Answer 11

States that the induced emf in a circuit is equal to the rate of change of flux linkage through the circuit.

Question 12

What is the equation relating to Faraday’s law?

Answer 12

emf = -N change in flux/change in time

= - rate of change of flux linkage

Question 13

What does Faraday’s law essentially tell us?

Answer 13

The size of the induced emf

Question 14

What is Lenz’s law

Answer 14

The direction of the induced emf is such that it will try to oppose the change in flux that is producing it

Question 15

What determines the direction of the induced current when a bar magnet is pushed into a coil connected to a meter?

Answer 15

Induced current passing round the circuit creates a magnetic field due to the coil. The coil field must act against the incoming north pole, otherwise it would pull the North-pole in faster - making the induced current bigger- pulling the N-pole in even faster. Due to conservation of energy (forbids creation of kinetic and electrical energy from nowhere) the induced current creates a magnetic field in the coil which opposes the incoming north pole. The induced polarity of the end of the coil (X) must therefore be a N-pole, to repel the incoming N-pole. Therefore, current must go around in an anticlockwise direction.

Question 16

What direction does the induced current flow when a bar magnet is pulled out of the coil?

Answer 16

Induced S-pole at end X of the coil due to opposing the magnet moving away, so induced current passes around X in a clockwise direction

Question 17

What is the overall explanation of Lenz’s law?

Answer 17

Energy is never created or destroyed. Induced current could never be in a direction to help the change that causes it; that would mean producing electrical energy from nowhere.

Question 18

For a moving conductor in a magnetic field, what does the emf equal?

Answer 18

emf = BLdistance moved/ change iin time

emf = BLv

Question 19

For a fixed coil in a changing magnetic field, what is the equation for emf?

Answer 19

emf = AN x change in B/change in time

Question 20

For a rectangular coil moving into a uniform magnetic field, what is the equation for emf?

Answer 20

BNLv

Time taken for coil yo enter field completely = coil width/speed = w/v

During this time flux linkage increases from 0 to BNLw. Therefore change in flux linkage per second = BNLw/w/v = BNLv

When coil is completely in the field, the fux linkage through it does not change (A doesn’t change) - so no emf is induced

Question 21

What happens when a coil in a simple ac generator rotates uniformly in a magnetic field?

Answer 21

Alternating emf is induced

Flux linkage giiven by BANcos(theta) and as the coil rotates, theta changes - so the flux linkage varies sinusoidally between +BAN and -BAN

Question 22

What does sinusoidally mean?

Answer 22

Follows same pattern as a sin (or cos) graph

Question 23

What is the equation for flux linkage knowing how fast theta changes depends on angular speed

Answer 23

NØ = BANcos(angular speed x t)

Question 24

Draw the graphs of Flux linkage and emf against time for a rotating coil in a uniform magnetic field

Answer 24

Question 25

What is the gradient of a graph of flux linkage against time?

Answer 25

rate of change of flux linkage = induced emf

Question 26

What is the equation for alternating emf

Answer 26

emf = BAN(angular speed) x sin(angular speed x t)

= emf(o) sin2πft

where emf(o) is the peak emf and 2πft = angular speed

Question 27

What is back emf?

Answer 27

emf induced in the spinning coil of an electric motor becasue the flux linkage through the coil changes.

Acts against the Pd V applied to the motor (in accordance with Lenz’s law)

V - emf = IR where I is the cirucit through the motor coil and R is the circuit resistance

Question 28

Whats the equation for electrical power supplied by the source IV?

Answer 28

Multiply V - emf = IR by I gives IV - Iemf = I²R

electrical power supplied by the source (IV) = electrical power transferred to mechanical power (Iemf) + electrical power wasted due to circuit resistance (I²R)

Question 29

What is an alternating current?

Answer 29

Current that repeatedly reverses it’s direction

Question 30

What is the frequency of an alternating current?

What frequency does mains electricity have?

Answer 30

Number of cycles (moves one way, reverses and then re-reverses) its passes through each second.

50 Hz - one cycle takes 0.02 seconds in mains electricity.

Question 31

What is the peak value of an alternating current?

Answer 31

Maximum current/Pd in either direction

Peak current in a circuit depends on the peak Pd of an alternating current source and on the components in the cicuit.

e.g. peak pd in a mains circuit is 325 V. If this pd is applied to a 100 ohm heating element, the peak current through the heating element through the heating element would be 3.25 A (325/100)

Question 32

What can an oscilloscope be used for?

Answer 32

Display a waveform of the alternating pd from a signal generator

used to observe altenrating current

Question 33

What does increasing the ouput pd from a signal generator connected to an oscilloscope do?

Answer 33

Makes oscilloscope trace taller. Shows peak value of the alternating pd has been made larger

Question 34

What does increasing the frequency of the signal generator do when connected to an oscilloscope?

Answer 34

Increases the number of cycles on the screen. This is because the number of cycles per second of the alternating pd has increased.

Question 35

What is the time period of an ac current?

Answer 35

The time taken for one complete cycle/wave

Question 36

For a heating element with resistance R, what would the equation for maximum. power be?

Answer 36

Io²R

Where Io is peak current

Question 37

Draw graph for variation of power with time for an alternating current

Answer 37

Question 38

For a sinusoidal current, what is the equation for mean power?

Answer 38

1/2 Io²R

Mean power over a full cycle is half the peak power.

Question 39

What is the root mean square value (of an alternating current)?

Answer 39

Value of direct current that would give the same heating effect/power as the alternating current in the same resistor (mean power)?

Question 40

Give the equation for root mean square value of an alternating current and an alternating Pd

Answer 40

I_rms = I_o/(2)^1/2

V_rms = V_o/(2)^1/2

1/(2)^1/2 x the peak value

Question 41

Give equation for mean power supplied to a resistor in an alternating circuit

Answer 41

P = (I_rms)²R = (V_rms)²/R = I_rmsV_rms

Question 42

What does a transformer do?

Answer 42

Chnages an alternating pd to a different peak value

Question 43

What does a transformer consist of?

Answer 43

2 coils (primary coil and secondary coil) wound around a soft iron core

Question 44

How does a transformer work?

Answer 44

An alternating current flows into the primary coil - causing an alternatingmagnetic field to be produced in the core. Field passes through secondary coil so flux linkage of the secondary coil is constantly changing - and so ac volatge is induced across it.

Question 45

Give the tranformer rule

Answer 45

Vs/Vp = Ns/Np

Question 46

What does the time base and the voltage sensitivity/volts per division of an oscilloscope mean?

Answer 46

time base tells you the time represented per division/ width of square

volts per division tells you what the height of one square represents in volts

Question 47

What 2 dials are on a oscilloscope?

Answer 47

Time base dial

Voltage sensitivity

Question 48

Describe how you would use an oscilloscope to check the accuracy of the rms output voltage and the frequency of the supply. Given values are 12V rms output voltage and 50Hz frequency

Answer 48

power supply is connected to the input of the oscilloscope. The time base is switched off and the y gain adjusted until a complete vertical line is seen on the screen. The length of the line is measured and this is converted to peak to peak voltage using the calibration. The peak voltage is divided by root two to get the rms voltage and this is compared with the stated value. The time base is now switched on and adjusted until a minimum of one cycle is seen on the screen. The length of one cycle is measured and this is converted to time using the time base setting. Frequency is the reciprocal of this time.