EM Induction

Question 1

What is magnetic flux?

Answer 1

Magnetic flux is the product of the area and the component of the magnetic flux density perpendicular to that area

Question 2

What is one weber?

Answer 2

One weber is defined as the quantity of magnetic flux passing through an area of one square metre when the flux density normal to the plane of the area is one tesla

Question 3

Is magnetic flux scalar or vector?

Answer 3

Scalar

Question 4

Can magnetic flux be negative?

Answer 4

Yes because B can point in the negative direction

Question 5

Can magnetic flux be zero even though B is non-zero?

Answer 5

Yes because when B // A, magnetic flux = 0

Question 6

What is the formula for magnetic flux?

Answer 6

ϕ = B⊥A

Question 7

What is magnetic flux linkage?

Answer 7

It is the product of the magnetic flux and the number of turns in the coil

Question 8

What is the formula for magnetic flux linkage?

Answer 8

Φ = Nϕ = NBA
N = no. of turns
A⊥B

Question 9

What does Faraday’s law state for a coil?

Answer 9

It states that the magnitude of induced emf of a coil is directly proportional to the rate of change of magnetic flux linkage through the coil

Question 10

What is the rough meaning of induced emf?

Answer 10

Generation of a potential difference in a coil due to the changes in the magnetic flux through it

Question 11

What changes magnetic flux linkage?

Answer 11

• N: number of turns
• B: magnetic flux density
• A: area
• Angle

Question 12

What happens when magnetic flux linkage changes?

Answer 12

Emf is created and current flows

Question 13

What is the equation for Faraday’s law?

Answer 13

EMF = - d/dx Φ (magnetic flux linkage)
= - d/dx (NBAcos(ωt))

Question 14

What does Faraday’s law state for conductor?

Answer 14

Magnitude of induced emf in a straight conductor is direcly proportional to the rate of cut of magnetic flux by the conductor

Question 15

What is Lenz’s law?

Answer 15

The direction of induced emf hence the direction of induced current produces effects to oppose the change causing it

Question 16

Why is Lenz’s law a law of conservation of energy?

Answer 16

Work is done by external force against the repulsive force produced by the induced current. The work done is converted to electrical energy which is manifested as induced current

Question 17

Why is energy dissipated in the resistor when a magnet is pushed into the solenoid?

Answer 17

A moving magnet gives rise to an increase of magnetic flux in the solenoid leading to an induced emf hence induced current in the solenoid. Induced current creates flux in the solenoid with a direction that opposes the increasing flux due to the motion of the magnet. Work done by external force is needed to push the magnet into the solenoid at constant speed. By conservation of energy, this work done is converted to thermal energy in the resistor

Question 18

What is the working principle of a metal detector?

Answer 18

• transmitter coil in a metal detector has a varying electric current flowing through it which generates an magnetic field (outer coil)
• magnetic field generated by the transmitter coil induces a magnetic field in nearby metal targets, the induced magntic field is picked up by the receiver coil

Question 19

Why when the switch is closed, the current doesn’t immediately change from zero to its maximum value?

Answer 19

• As the current increases with time, the magnetic flux through the loop due to this current also increases, the increasing magnetic flux induces an emf in the circuit that opposes the change in magnetic flux
• By Lenz’s law, the induced emf produces a current that is opposite of the current of the circuit
• The net potential difference across the resistor is emf of the battery minus the opposing induced emf
• As the magnitude of current increases, the rate of increase lessens and hence the induced emf decreases, this opposing emf results in a gradual increase in current
• this effect is called self-induction

Question 20

What is the working principle behind an induction cooker?

Answer 20

• When high frequency alternating current passing through the coil, high frequency alermating magntic field will be produced and pierce through the frypan
• emf and hence eddy current will be generated in the pan, heating it up

Question 21

What are eddy currents?

Answer 21

induced current circulating in solid conductors when there is a cutting of magnetic flux

Question 22

What is the effect of eddy current?

Answer 22

eddy currents can generate significant amounts of heat and cause power loss

Question 23

How to reduce the effects of eddy currents?

Answer 23

effective lamination which limits the amount of eddy current generated