Chapter 15

Question 1

Define Faraday’s Law

Answer 1

Faraday’s law of electromagnetic induction states that the induced emf in a circuit is proportional to the rate of change of total magnetic flux through the circuit

Question 2

What is the equation from Faraday’s law?

Answer 2

V=-d(phi)/dtwhere phi is the flux linkagealso emf, E=-Nd(phi)/dtwhere N is number of turns and phi is the flux density

Question 3

Define Lenz’s law

Answer 3

The direction of the induced current in a circuit is always such as to oppose the change that causes it

Question 4

Define magnetic flux

Answer 4

A term that refers to the lines of force. The lines of force are known as lines of flux, so we get the term flux density.

Question 5

How do you calculate magnetic flux?

Answer 5

phi=BA where B is magnetic flux density and A is area swept out

Question 6

How do you calculate flux linkage?

Answer 6

Phi=BAN=phiN where phi is magnetic flux and N is number of turns

Question 7

How do you calculate the induced voltage when a rod of length L moves at 90deg. to magnetic field lines at speed v?

Answer 7

V=BLv

Question 8

How are magnetic fields produced?

Answer 8

By current-carrying conductors or permanent magnets

Question 9

What is a magnetic domain?

Answer 9

A region in a magnet with uniform magnetisation

Question 10

How do you create a magnet?

Answer 10

Melt a ferrous metal slightly and tap it so all magnetic moments of the atoms align

Question 11

How do you demagnatise a magnet?

Answer 11

Hit it really hard or heat and cool it quickly

Question 12

How do you draw a wire, current going into the page?

Answer 12

Circle with a cross in it

Question 13

What are the rules of drawing field lines?

Answer 13

Arrows go from north to southCan’t cross (vector quantity so just get resultant)Stronger fields when field lines are closer together

Question 14

How do you tell the poles on a soleroid?

Answer 14

Clockwise current is north; anticlockwise current is south

Question 15

What are the units of flux and flux density?

Answer 15

Flux: weberFlux density: Testler

Question 16

How are current and flux related?

Answer 16

As current changes, so does flux as current-turns drive flux

Question 17

How are flux and induced emf related?

Answer 17

emf is the gradient of the flux graph

Question 18

What is the transformer equation?

Answer 18

Vs/Vp = Ns/Np = Ip/Is

Question 19

How are the voltage in the primary coil of a transformer and voltage in the secondary coil related?

Answer 19

Antiphase relationship

Question 20

How are current and voltage related?

Answer 20

Voltage is differential of current

Question 21

How does a transformer work?

Answer 21

Alternating current in the primary coilProduces a changing magnetic field in the secondary coilMutual induction:Alternating pd across primary coilProduces an alternating fluxIron core increases flux and guides it to the secondary coilSecondary coil experiences changing flux which induces an emf

Question 22

How are the currents related in the coils of a transformer?

Answer 22

Antiphase relationship

Question 23

What are the 2 equations for permeance?

Answer 23

Capital lamda, ^, permeance = flux/current turns = phi/NI^ = permeability*area/length =muA/L

Question 24

What is the left hand motor rule?

Answer 24

First finger field; second finger conventional current; thumb force

Question 25

What is a catapult force?

Answer 25

The force experienced by a wire when placed in a magnetic field. Initially, field lines are distorted, but wire moves to make them straighter and shorter

Question 26

How do you calculate a catapult force?

Answer 26

F=BIL where B is magnetic field strength, I is current and L is length of conductor

Question 27

What is the right hand generator rule?

Answer 27

First finger field; second finger conventional current, thumb motion

Question 28

What is back emf?

Answer 28

The emf due to the generator effect

Question 29

How is the power drawn split in a motor (equation)?

Answer 29

VI=E(back)I + I^2R where VI is power drawn, E(back)I is power to move current against back emf and I^2R is power lost to thermal energy

Question 30

What is the equation for calculating the emf at a specific rotation (angle from horizontal to perpendicular to coil)?

Answer 30

E=NBA*w*sin(wt)where N is no. turns, B is flux density, A is area, w is actually omega is is angular frequency and t is time.

Question 31

At what rotation is the maximum emf achieved?

Answer 31

When coil is parallel to field lines

Question 32

How is angular frequency calculated?

Answer 32

w=d(theta)/dt or w=2(pi)/T=2(pi)f where T is time period and f is frequency

Question 33

How do you calculate the maximum emf in a coil?

Answer 33

V=2(pi)f*peak flux linkage