Chapter 17: EMI

Question 1

Magnetic flux

Answer 1

Product of the component of the magnetic flux density normal to the surface & area of the surface

Bcos@ x A

Question 2

Magnetic flux linkage

Answer 2

Product of the magnetic flux through the loop and the number of turns in the loop
Bcos@ x A x N

Question 3

Faraday’s law

Answer 3

Magnitude of the induced emf is directly proportional to the rate of change of magnetic flux linkage

E=-gradient of magnetic flux linkage/time graph

Question 4

Lenz’s law

Answer 4

Direction of the induced emf is such that it produces magnetic effects to oppose the change causing it.

Question 5

Explain how Lenz’s law demonstrate COE

Answer 5

1. The emf induced is a source of electrical energy that must come from other forms of energy
2. The emf produces an effect to oppose the change given rise to it
3. Consequently, work must be done to overcome this opposition
4. Mechanical energy is transformed to electrical energy. Energy is conserved.

Question 6

What is AC generator type qns? (not memorywork)

Answer 6

Amplitude= NBAw
E=NBAwsinwt
loop rotates (like barbeque), component of the magnetic flux density normal to the area of the loop changes, resulting in change in flux linkage in the loop

Question 7

Why are eddy currents induced in the metal disc in magnetic braking?

Answer 7

1. The flux density is not uniform/ rate of cutting of magnetic flux linkage is not the same throughout/ speed of the disc is not the same over the whole disc
2. Different emf induced at different parts in the disc, leading to eddy currents.

Question 8

Magnetic Damping

Answer 8

1. At each repeated freq. of oscillator, oscillating sheet cuts magnetic flux, inducing emf on metal sheet
2. rate of flux cut is not uniform, so diff emf is induced in diff parts of the sheet
3. Sets up potential differences that causes induced currents to flow in the metal sheet
4. Current dissipates energy/ Lenz’s law, force opposing motion of metal sheet is induced, work is done against force
5. Loss of mechanical energy

Question 9

Ideal transformer

Answer 9

An ideal transformer is where output power of transformer = input power

Question 10

electrical insulators

Answer 10

Electrical insulators do not have mobile electrons, so no induced current in the ring, no repulsive force on ring

Question 11

Use Faraday’s law to explain why an AC in primary coil give rise to alternating emf in secondary coil

Answer 11

1. The AC produces an alternating magnetic flux in the soft iron core
2. When the flux is increasing at the FASTEST rate, induced emf in the secondary coil is maximum
3. When the flux is maximum, induced emf is zero
4. When the flux is decreasing at the fastest rate, induced emf in the secondary coil is maximum but in the reverse direction
   Cycle repeats and so emf in the secondary coil is alternating

Question 12

Soft iron core purposes

Answer 12

1. increase magnetic flux within the core
2. to link the flux in primary coil to secondary coil

Question 13

Phases of p.d. in primary & secondary coil

Answer 13

1. Input p.d. drives a current in the primary coil that is in-phase with it
2. The current in the primary coil then produces a magnetic flux in the iron core that is also in-phase with it
3. So, by Faraday’s law the output p.d. is not in-phase with input p.d.

Question 14

Sources of power loss in a real transformer

Answer 14

1. Heat dissipation due to resistance of coils
2. Heat dissipation due to eddy currents in the soft iron core
3. Heat dissipation due to rapid magnetizing and demagnetizing of iron core

Question 15

Explain how lamination of the core reduces eddy currents

Answer 15

1. Lamination is an electrical insulator, increases electrical resistance to eddy currents in the iron core
2. Less heating effect takes place inn the core

Question 16

How an emf is obtained (in transformer)

Answer 16

1. AC produces an alternating magnetic flux in soft iron core
2. An alternating magnetic flux linkage induces an emf

Question 17

Why emf of secondary coil is less than pd across primary coil (Ns<Np)

Answer 17

Magnitude of induced emf across secondary coil is proportional to the rate of change of magnetic flux linkage, which is proportional to the number of turns of the coil, which is fewer than primary coil.

Question 18

Explain variation of emf induced in coil

Answer 18

Specific for AC generator:
The component of the magnetic flux density normal to the area varies, so magnetic flux linkage varies.
Generic explanation why alternating emf:
1. When the flux is increasing at the FASTEST rate, induced emf in the secondary coil is maximum
2. When the flux is maximum, induced emf is zero
3. When the flux is decreasing at the fastest rate, induced emf in the secondary coil is maximum but in the reverse direction
Cycle repeats and so emf in the secondary coil is alternating