Electromagnetic Induction

Question 1

Induced Electromotive Force

Answer 1

occurs when a conductor moves relative to a magnetic field, the e- in the conductor have a force exerted on them and they gain electrical energy
IMPORTANT - emf is only induced if the magnetic field lines are “cut” by the conductor (i.e. moving parallel to field lines will not induce emf) Max emf is induced when the conductor is moving perpendicular to the magnetic field lines

Question 2

Deriving induced emf formula

Answer 2

if the e in the middle of the conductor is in equilibrium then:
magnetic force due to the motion (UP) = electrical force due to the emf (DOWN)
Bqv = Eq
but E=V/d = V/l so Bqv = (Vq)/l
V = Bvl
as no current is flowing, emf = V = Bvl

Question 3

Magnetic flux definition

Answer 3

ø = BAcosθ
where B = strength of magnetic field
A = area the field covers
and θ = angle between field lines and the normal to the plane of area

units = Tm2 or Wb (Weber)

Question 4

Faraday’s Law

Answer 4

gives SIZE of induced emf

the magnitude of an indced emf is proportional to the rate of change of flux linkage

(flux linkage = product of the number of turns in a coil and the flux through the coil)

emf = -N(deltaø/deltat)

Question 5

Lenz’s Law

Answer 5

gives DIRECTION of induced emf

the direction of the induced emf is such that if an induced current were to flow, it would oppose the change which it caused

emf = -N(deltaø/deltat)

explains - at front of formula

Question 6

How is AC current generated?

Answer 6

if a coil of wire is spun in a magnetic field at a constant speed and is connected to a circuit outside the magnetic field then the induced emf will be sinusodial and a AC current will flow

when the coil is vertical:
- induced emf = 0 as no cutting of flux lines occurs and rate of change of flux is minimum
- max flux occurs

when coil is horizontal:
- induced emf = max as coil cuts flux lines at 90 degrees and rate of change of flux is greatest
- flux = 0

i.e. ø is the -ve gradient of emf

direction of induced voltage changes every half cycle and as the induced current flows in a direction to create a magnetic field that opposes the rotation of the coil, work must be done to keep the coil rotating

Question 7

How can size of induced voltage be increased?

Answer 7

• having more loops of wire
• having a stronger magnetic field
• rotating the coil at a faster rate

Question 8

What affect will increasing the speed of rotation of the coil have?

Answer 8

• reduce the period of emf oscillation
• increase the amplitude of emf (increases rate of change of flux linkage)

e.g. doubling speed of rotation doubles amplitude and hlaves period

Question 9

AC rms def.

Answer 9

the AC rms value of current or voltage is that which would produce the same amount of [pwer as the equivalent DC value

Question 10

What is a transformer?

Answer 10

used to increase or decrease voltages
the primary coil where AC current is input creates alternating flux in the iron core which induces alternating emf in a secondary coil
varying the ratio of turns of wire between the two cold will determine by how much the voltage is stepped up or down

they are very efficient

Question 11

Step - up transformer

Answer 11

secondar voltage > primary voltage
secondary coil has greater number of turns than primary

Question 12

Step - down transformer

Answer 12

primary voltage > secondary voltage
primary coil has a greater number of turns than secondary

Question 13

How are energy losses reduced by a transformer?

Answer 13

• having an iron core to ensure that a strong magnetic field between the primary and secondary windings. This ensure that very little magnetic flux leaks and does not pass through the secondary coil
• iron core is made of soft iron so it can be rapidly magnetised and demagnetised
• the core is laminated (has alternating layers of iron and insulating material) to stop the formation of eddy currents in the core. Eddy currents are currents induced by the core itself by the øand causes energy losses by heating up the core
• using low resistance wire for the coils

Question 14

Power transmission

Answer 14

• transmitted at very high voltages to reduce energy losses
• power lines have some R so heat up
• the power lost is prop. to I^2, so transmitting at high V and low I, power losses are reduced

however high V are dangerous

Question 15

Rectification

Answer 15

process by which AC is converted to DC
involves the use of diodes (electrical devices that only allow current through in one direction)

2 types:
- half -wave
- full wave

Question 16

Half - wave rectification

Answer 16

a diode is connected in series with the AC supply
current through load is not steady but is DC

Question 17

Full - wave recification

Answer 17

uses a diode bridge

Question 18

Capacitors

Answer 18

a device that can store electrical charge
a parallel plate capacitor is one formed from 2 parallel metal plates with an insulating material (dielectric) between to separate charges

Question 19

Capacitance

Answer 19

ability of a capacitor to store charge

defined as: the ratio of charge stored to potential difference between the plates

Question 20

How do dielectrics work in capacitors?

Answer 20

• dielectric materials have polarised molecules (+ve and -ve ends)
• when placed in an electric field, the molecules align themselves with the field
• electric field strength between the plates is reduced
• E = V/d, d is fixed so V between plates is reduced
• as C = q/V, if V is reduced and q stays the same, C is increased
• more charge can be stored for a lower V

Question 21

Charging of capacitors

Answer 21

when being charged the V across the capacitor and I in the circuit will have an exponential relationship with time
usually to charge a capacitor, it is connected in series with a resistor to a DC power supply

during charging:
- the V across the capacitor grows to its max value (= emf of supply)
- the current of the circuit falls to zero

time constant for charging = time taken for the voltage to reach 63% of its maximum value, or the current to fall to 37% of its maximum value

Question 22

Discharging of capacitors

Answer 22

during discharging:
- the V across capacitor falls to zero
- the current in the (discharging) circuit falls to zero

time constant for discharging = time taken for the voltage or current to fall to 37% of its maximum value (decrease by 67%)

Question 23

Capacitors in Series

Answer 23

the +ve plate of one capacitor will be connected to the -ve plate of the next capacitor
capacitors in series will all have the same charge (but different V)

Question 24

Capacitors in Parallel

Answer 24

the +ve plate of one capacitor is connected to the +ve plate of the next capacitor
capacitors in parallel have the same V (but store a different charge)

Question 25

Capacitors and Rectification

Answer 25

adding a capacitor in parallel to the load will smooth the current through the load
the capacitor charges when current flows and discharges when current drops
a large time constant will provide good smoothing but the disadvantage is that the capacitor will draw a large current whilst charging