5.5 Magnetic Fields

Question 1

Define a magnetic field

Answer 1

the space surrounding a magnet where a magnetic force is experienced

Question 2

What is the direction of a magnetic field?

Answer 2

North to South

Question 3

In the right hand grip rule, what direction does the thumb point in

Answer 3

the direction of conventional current

Question 4

How do you increase the strength of a magnetic field in a solenoid? (3 things)

Answer 4

1. increasing current in coil
   2.increasing no. turns in coil
2. placing a rod of soft iron into the coil

Question 5

Define magnetic flux density

Answer 5

the force per unit current carrying length

Question 6

What are the units of magnetic flux density?

Answer 6

T, teslas

Question 7

Define a tesla

Answer 7

the strength of the magnetic field when a current of 1A is flowing in a conductor perpendicular to a magnetic field causes a force of 1N to be produced on each metre of conductor in the field

Question 8

In Fleming’s left hand rule, where does the thumb point?

Answer 8

the direction of the force

Question 9

In Fleming’s left hand rule, where does the pointer finger point?

Answer 9

The magnetic field direction

Question 10

In Fleming’s left hand rule, where does the middle finger point?

Answer 10

the direction of conventional current (because fuck conventional current)

Question 11

The force acting on a current carrying conductor depends on what 2 things?

Answer 11

1. The length of the conductor, l, in the magnetic field
2. The current, I, flowing in the conductor

Question 12

What is the purpose of the split-ring commutator in an electic motor?

Answer 12

To reverse the direction of current every time the coil passes through the vertical, so that the coils keep rotating in the same direction

Question 13

What is magnetic flux?

Answer 13

It represents the total number of magnetic flux lines that pass at 90degrees thru’ a given area

Question 14

What is magnetic flux measured in?

Answer 14

Wb, Webers

Question 15

Define magnetic flux linkage

Answer 15

the total magnetic flux linking a coil of N turns of current carrying conductor and an area of cross-section A

Question 16

What is ‘Faraday’s Law of Electromagnetic Induction’?

Answer 16

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

Question 17

What 3 things is the magnitude of increased emf proportional to?

Answer 17

1. strength of magnet
2. number turns on coil
3. speed of moving magnet

Question 18

For an emf to be induced, what MUST there be between the magnet and conductor?

Answer 18

relative motion

Question 19

Define Lenz’s Law

Answer 19

The direction of the induced current is such that it opposes the changes in the magnetic flux that is producing it

Question 20

In step up transformers, are there more turns in the primary or secondary coils?

Answer 20

There are more turns in the secondary coil

Question 21

In step up transformers, does voltage increase or decrease?

Answer 21

voltage increases

Question 22

In step up transformers, does current increase or decrease?

Answer 22

current decreases

Question 23

In step down transformers, are there more turns in the primary or secondary coils?

Answer 23

There are more turns in the primary coils

Question 24

In step down transformers, does voltage increase or decrease?

Answer 24

voltage decreases

Question 25

In step down transformers, does current increase or decrease?

Answer 25

current increases

Question 26

How do resistive heat losses in transformers cause power losses?

Answer 26

As coils heat up, resistance increases and so the amount of energy lost increases

Question 27

How is the iron core in a transformer heated, causing power loss?

Answer 27

Repeatedly magnetising the iron core in one direction then the other results in its heating

Question 28

How do eddy currents in a transformer cause power losses?

Answer 28

The iron core is a conductor in a changing magnetic field, so large currents, known as eddy currents, are induced to flow in it. They result in heating the iron core

Question 29

How can eddy currents in a transformer be reduced?

Answer 29

laminating the iron core

Question 30

In a transformer, the fact that not all of the magnetic flux of the primary coil passes thru’ the secondary coil causes what?

Answer 30

power losses

Question 31

To reduce power loss in electricity transmission, Steve suggests using cables with a very large cross-sectional area because, he says, this will reduce resistance. Why do electricity companies not use Steve’s suggestion?

Answer 31

This would be very expensive.

Question 32

What is a good way to reduce power loss in electricity transmission?

Answer 32

Utilise step-up transformers to increase voltage and reduce current in transmission cables. Then utilise step-down transformers to reduce voltage for uses in industry and homes