Electromagnetic Induction

Question 1

How is magnetic flux(Wb) calculated? And what is it?

Answer 1

Magnetic flux density x Perpendicular area. It is the measure of the amount of field lines passing through a given area

Question 2

How do you calculate the effective flux through this coil?

Answer 2

Must take component of field perpendicular to surface

(Or component parallel to normal of surface)

Question 3

How does the number of turns in a coil affect its flux?

Answer 3

Flux doesn’t change

But flux linkage increases

Question 4

How do you find the flux linkage through this coil?

Answer 4

Must take component of field perpendicular to coil

(Or component parallel to normal of coil)

Question 5

How do you induce an emf in this coil?

Answer 5

Moving the magnet in and out of the coil
Causes a change in flux linkage
(Faraday’s Law) which causes an emf which causes a current to flow

Question 6

If the magnet is stationary why is there no emf induced?

Answer 6

Emf only induced if the flux linkage is changing

(Faraday’s Law)

Question 7

Which way does the current act in this coil and why?

Answer 7

Current acts to create a magnetic field opposing the increase in flux linkage
(Tries to keep magnet from entering)

Due to Lenz’s law

Question 8

Which way does the current act in this coil and why?

Answer 8

Current acts to create a magnetic field opposing the decrease in flux linkage
(Tries to keep magnet from leaving)

Due to Lenz’ law

Question 9

Why does moving the magnet faster induce a larger emf?

Answer 9

The rate of change of flux linkage is greater

(Faraday’s law)

Question 10

What is Faraday’s Law of electromagnetic induction?

Answer 10

The magnitude of the emf is equal to the negative of the rate of change of flux linkage

Question 11

What is Lenz’ law?

Answer 11

The direction of an induced emf tends to produce a current which opposes the
change causing it

(The direction of an induced emf, due too faradays law, tends to produce a current which creates a counter magnetic field that opposes the
first agnetic field)

Question 12

Why can’t the current act in this direction?

Answer 12

The magnetic field produced by lenz’s law attracts the magnet (in the same direction as the first magnetic field)
Increasing its kinetic energy
Energy has been created out of nothing!!!

(Against Lenz’ law)

Question 13

What is the right hand grip rule?

Answer 13

A current in a coil produces a magnetic field (this is how lenz’s law works) as shown the same way a magnet moving into a coil induces an emf in the coil

Question 14

What is wrong here?

Answer 14

Flux linkage becomes negative when coil flips!!!

Question 15

How is the induced emf related to the graph of flux linkage?

Answer 15

(From Faraday’s law)

Question 16

What is the corresponding graph of induced emf?

Answer 16

emf = -ve gradient

Question 17

If the coil moves across the field at a constant speed what do the flux linkage and emf graphs looks like?

Answer 17

emf = -ve gradient

Question 18

How do you calculate the emf induced between the ends of this conducting bar moving across a magnetic field?

Answer 18

Question 19

When do you use Fleming’s Right Hand Rule

Answer 19

For generators!!!
F - the electrostatic force
B - magnetic flux density (north to south)
I - the direction of current (+ve terminal to -ve terminal)

Question 20

For the AC generator when is the max emf induced?

Answer 20

When change in flux linkage is max

(When flux linkage = 0)

Question 21

For the AC generator when is 0 emf induced?

Answer 21

When change in flux linkage is 0

(When flux linkage = max)

Question 22

What is the corresponding emf graph for the AC generator?

Answer 22

Question 23

How is the max induced emf calculated for the AC generator?

Answer 23

Question 24

How do you increase the max induced emf of the AC generator?

Answer 24

Increase any of the terms in the equation:

Question 25

How is the AC generator different to the DC motor?

Answer 25

Generator: Kinetic energy -> Electrical energy

(Motor: Electrical energy -> Kinetic energy)

Generator: Slip rings keep each side of coil connected to same side of circuit

(Motor: Commutator ring switch polarity every half cycle)

Question 26

What is the equation for flux linkage?

Answer 26

N Φ = NBA
Make sure you use the perpendicular components of flux and area

Question 27

What is the definition of flux linkage?

Answer 27

The total magnetic field (flux) traveling through a coil with multiple turns

Question 28

How does moving a magnet into a coil induce an emf?

Answer 28

As the magnet moves into the coil the electrons in the coil experience a force from Fleming’s LHR. This causes the electrons to travel around the coil, building up opposite charges on either side of the coil. The difference in charges causes a potential difference across the coil inducing an emf. When the magnet is moved out of the coil the electrons experience an opposite force so the charge build up is reversed, meaning the emf induced is reversed and the current if connected up to a circuit would flow the other way. The magnet must be moving as magnets only effect charged particles if either one of them is moving

Question 29

What are the two applications of Lenz’ law?

Answer 29

1. Magnet in a pipe
2. Interacting coils (magnet in a coil)

Question 30

Describe the ‘magnet in pipe’ scenario

Answer 30

As a magnet falls through a pipe, there is a change in flux linkage above and below the magnet. Therefore, due to Faraday’s law of EM induction, there is an emf induced inside the pipe (coil) on either side of the magnet. Due to Lenz’ law, the currents act to oppose the change in flux linkage by creating a counter magnetic field that repels the magnet from below and attracts the magnet from above.

Question 31

Describe the ‘magnet in half pipe’ scenario

Answer 31

As a magnet falls through a pipe, there is a change in flux linkage above and below the magnet. Therefore, due to Faraday’s law of EM induction, there is an emf induced above and below the magnet. However, as the pipe isn’t closed (doesn’t act like a coil), currents can’t flow so no counter magnetic fields are formed by Lenz’s law. Therefore, there is no force on the magnet falling and travel time is unaffected.

Question 32

Describe the ‘interacting coils’ scenario

Answer 32

When you have two coils, and you connect one to a battery, the current flows through the first coil creating a magnetic field (Faraday’s law) **. There will be a positive change in flux linkage which causes an induced emf to oppose the change in flux linkage in the other coil (Lenz’s law) which causes current to flow in the other coil . This all happens in reverse when the battery is disconnected from the first coil.

Question 33

What is the formula for the emf of a conducting bar moving through a magnetic field?

Answer 33

E = BLv

E - induced emf
B - magnetic flux density
L - length of conductor
v - average velocity of conductor

Question 34

Why is an emf induced when a conducting bar travels through a magnetic field?

Answer 34

As the bar covers an area there is a force acting on that bar and from FLHR the electrons in the bar all move to one side. This difference in charges across the bar causes an induced emf. The area the bar has covered while moving can be imagined as the area of the coil and we can therefore work out Flux Linkage

Question 35

How does Lenz’ law work?

Answer 35

As a magnet goes through a coil, the change in flux linkage in the coil induces an emf which causes a current to travel around the coil, producing a magnetic field which opposes the change in flux linkage (repels the magnet entering and attracts the magnet leaving the coil)