7.5 Magnetic fields

Question 1

What is a magnetic field?

Answer 1

A region surrounding a magnet or current carrying wire that will exert a force on any other magnet or current carrying wire or moving charge placed within it.

Question 2

What is the direction magnetic field lines around a magnet?

Answer 2

From north to south.

Question 3

Wha is the right hand rule?

Answer 3

Magnetic field around a current (or vice-versa)

Question 4

What is Fleming’s left hand rule?

(aka motor rule)

Answer 4

Question 5

What is the equation for force on a wire perpendicular to a magnetic field?

Answer 5

Question 6

What do the letters represent in this equation?

Answer 6

Question 7

What is the force on a wire parallel to a magnetic field?

Answer 7

0

Question 8

What is the force on a wire at an angle theta to a magnetic field?

Answer 8

Question 9

What is magnetic flux density a measure of?

Answer 9

How tightly packed magnetic field lines are together or the strength of the field.

Question 10

Define the Tesla.

Answer 10

1 Tesla is equal to 1 Newton per Amp per Metre.

Question 11

What direction does force act on a charged particle as it moves through a magnetic field?

Answer 11

Perpendicular to the field and direction of motion.

Question 12

What is the difference between a postive charge moving and a negative charge moving through a magnetic field?

Answer 12

The magnetic forces act in opposite directions.

Question 13

What is the equation for force on a moving charged particle?

Answer 13

Question 14

What do the letters represent in this equation?

Answer 14

Question 15

If a charged particle is moving with constant speed through a magnetic field, what type of path does it take?

Answer 15

Circular.

(Hence can equate magnetic force with centripetal)

Question 16

What is the equation for the radius of a charged particle moving through a magnetic field?

Answer 16

Question 17

What do the letters represent in this equation?

Answer 17

Question 18

Define magnetic flux.

Answer 18

The measure of magentic field that passes through a given perpendicular area.

Question 19

What is the equation for magnetic flux?

Answer 19

Question 20

What do the letters represent in this equation?

Answer 20

Question 21

Define magnetic flux linkage.

Answer 21

The measure of magentic field that passes through a given perpendicular area enclosed by a coil of wire.

(aka number of turns of wire cutting the flux at one time)

Question 22

What is the equation for magnetic flux linkage when the coil is perpendicular to the magnetic field?

Answer 22

Question 23

What do the letters represent in this equation?

Answer 23

Question 24

What is the equation for magnetic flux linkage when the coil is parallel to the magnetic field?

Answer 24

0

Question 25

What is the equation for magnetic flux linkage if the normal of the coil is at an angle theta to the magnetic field?

Answer 25

Question 26

What is Faraday's law?

Answer 26

The induced emf is directly proportional to the rate of change of flux linkage.

Question 27

What is the equation for Faraday's law?

Answer 27

Question 28

What do the letters represent in this equation?

Answer 28

Question 29

What is the equation for the **magnitude** of induced emf?

Answer 29

Question 30

What is the equation for induced emf in a conductor moving through a magnetic field?

Answer 30

Question 31

What do the letters represent in this equation?

Answer 31

Question 32

What is Lenz's law?

Answer 32

Induced emf/current is always in such a direction as to oppose the change that induced it. | (hence the negative sign in Faraday's law)

Question 33

What is the explaination for Lenz's law?

Answer 33

Energy is never created or destroyed. Induced emf/current could never be in a direction to help the change that causes it; that would mean producing electrical energy from nowhere.

Question 34

What is the equation for induced emf for a rotating coil in a magnetic field?

Answer 34

Question 35

What do the letters represent in this equation?

Answer 35

Question 36

What is the gradient of a flux linkage vs time graph?

Answer 36

Induced emf.

Question 37

What is the area of an induced emf vs time graph?

Answer 37

Flux linkage.

Question 38

What is alternating current?

Answer 38

An electric current that reverses direction with constant frequency | (aka sinusoidal)

Question 39

What is the frequency of an alternating current?

Answer 39

Number of cycles it passes through each second.

Question 40

What is peak voltage?

Answer 40

Distance from the equilibrium to the highest (or lowest) point. | (Same goes for peak current)

Question 41

What is peak-to-peak voltage?

Answer 41

Distance from lowest point to highest point. | (Same goes for peak-to-peak current)

Question 42

What is rms voltage?

Answer 42

The average of all the squares of the possible voltages. | (Same goes for rms current)

Question 43

What are the equations for rms voltage and rms current?

Answer 43

Question 44

What do the letters represent in this equation?

Answer 44

Question 45

What is a transformer's function?

Answer 45

Steps up (or down) an alternating current.

Question 46

What are the components of a transformer?

Answer 46

A primary coil and a secondary coil, both wound around a soft iron core.

Question 47

How does a transformer work?

Answer 47

* AC in primary coil induces alternating magnetic field in core * Alternating magentic field passes through secondary coil * Resulting in changining flux linkage and inducing AC in the secondary coil

Question 48

What is the transformer equation?

Answer 48

Question 49

What do the letters represent in this equation?

Answer 49

Question 50

What is a step-up transformer?

Answer 50

A transformer where the secondary coil has more turns than the primary coil.

Question 51

What is a step-down transformer?

Answer 51

A transformer where the secondary coil has fewer turns than the primary coil.

Question 52

What is the equation for transformer efficiency?

Answer 52

Question 53

What do the letters represent in this equation?

Answer 53

Question 54

What 4 things can reduce a transformer's efficiency?

Answer 54

1. Eddy currents 2. Coil resistance 3. Escaping flux 4. Ease of magnetisation and demagnetisation

Question 55

How would you reduce the inefficiency of coil resistance?

Answer 55

Use thicker wires with low resistance.

Question 56

How would you reduce the inefficiency of escaping flux?

Answer 56

Designing the core such that the two coils are as close as possible.

Question 57

How would you reduce the inefficiency of magnetisation and demagnetisation?

Answer 57

Using soft iron for the core, to reduce the energy needed to magnetise and demagnetise | (here 'soft' mean magnetically soft)

Question 58

What are eddy currents?

Answer 58

Small currents induced in the core by the changing magnetic field.

Question 59

Why do eddy currents cause inefficiency?

Answer 59

Due to Lenz's law, eddy currents create magnetic fields that oppose the changing magnetic field that induced them in the first place. This causes heating in the core.

Question 60

How would you reduce the inefficiency of eddy currents?

Answer 60

Laminating the core with insulating material, reducing the the size of the eddy currents and hence the magnetic field strength they create.