Magnets

Question 1

What direction do magnetic flux lines go in?

Answer 1

North to South

Question 2

Define the Tesla

Answer 2

The magnetic flux density which produces a force of 1N in a wire of length 1m with a current of 1A

When the wire is perpendicular to the magnetic field

Question 3

Flemings right hand rule

Answer 3

Thumb=direction of conventional current
Fingers curl in direction of magnetic field

Question 4

Flemings right hand rule

Answer 4

Thumb=direction of conventional current
Fingers curl in direction of magnetic field

Question 5

Flemings left hand rule

Answer 5

Thumb= Force
Index finger= Magnetic field
Middle finger= current

Question 6

Magnetic force equation (Main)

Answer 6

F=BIL
F=BILsinθ

Question 7

Equation for magnetic force on a single particle

Answer 7

F=BQV

Question 8

How to derive radius equation

Answer 8

Centripetal force = magnetic force
Mv^2/r = BQv
r=mv^2/BQv
r=mv/BQ

Question 9

Derive frequency/time period equation

Answer 9

Centripetal force=magnetic force
mω^2r = BQv
mω^2r = BQωr
mω=BQ
mx2πf=BQ
f=BQ/2πm

T=2πm/BQ

Question 10

How do you find the time period / frequency of a half rotation?

Answer 10

T=πm/BQ

f=BQ/πm

Question 11

How does a cyclotron work?

Answer 11

1) Charged particle is inserted in middle
2) Particle experiences a force in electric field -> it’s accelerated to negative plate
3) Particle enters M field and follows circular path
4)Particle re-enters magnetic field. PD must be flipped to accelerate it to higher speed
5) particle enters other Dee. radius is larger because speed is greater
6)Proton re-enters electric field. PD must be flipped again to accelerate particle
7)Process repeats until desired speed is reached

Question 12

State 2 situations where a charged particle will experience no magnetic force when placed in a magnetic field.

Answer 12

1) It’s moving parallel to field lines
2) It’s stationary

Question 13

How to induce an EMF and a current

Answer 13

In the wire the positive charges will move up and the negative charges down.

Therefore an EMF has been induced.

If the wire was a complete circuit then a current would be induced too.

The direction of the conventional current would be in the direction the positive charges move.

Question 14

When does flux cutting/ changing the magnetic flux occur?

Answer 14

When a conductor moves through flux lines

Question 15

When does flux cutting/ changing the magnetic flux occur?

Answer 15

When a conductor moves through flux lines

Question 16

What factors affect the size of the EMF induced?

Answer 16

-The speed of movement: more flux lines cut per second.
-The number of coils: like having batteries in series.
-Strength of magnet: greater density of field lines.
-Area cut through: amount of field lines.

Question 17

Define magnetic flux

Answer 17

The total amount of field lines cut through with the conductor

Question 18

Magnetic flux equation

Answer 18

Φ=BA
Φ=BAcosθ

Question 19

Why does including more coils increase the EMF?

Answer 19

Each wire acts like a battery

Looping same wire around multiple times means it gets “cut” multiple times

Like batteries in series

Question 20

Flux linkage equation

Answer 20

ΦN. (BAN)

BANcosθ

Question 21

Flux linkage against time graph (when rotating)

Answer 21

Cos graph

Question 22

Lenz’s law

Answer 22

The direction of the induced EMF/current is in the opposite direction to the change that caused it

Question 23

Faraday’s law

Answer 23

The induced EMF/current is directly proportional to the rate of change of flux linkage.

Question 24

Faraday’s law EMF equation

Answer 24

EMF=. - △(ΦN) /△T

Question 25

Area of EMF against time graph

Answer 25

Change in flux linkage

Question 26

How to derive EMF equation

Answer 26

EMF= BAN/t (N=1) EMF= BA/t A=ld EMF=Bld/ t d/t = v EMF = BLV

Question 27

Equation for EMF produced by a rotating coil. Equation for maximum EMF

Answer 27

EMF= BANωsin(ωt) Max EMF= BANω

Question 28

How to calculate Vrms (root mean square)

Answer 28

V0/√2 V0=peak voltage

Question 29

How to calculate Irms (root mean square)

Answer 29

Irms=I0/√2 I0=peak current

Question 30

What does the Y gain dial do?

Answer 30

Changes the value of each vertical division

Question 31

What does the time base dial do?

Answer 31

Changes the value of each horizontal division

Question 32

What do the y shift and x shift dials do?

Answer 32

Move the graph up / left and right

Question 33

How to get peak voltage from oscilloscope

Answer 33

Find peak to peak voltage Divide by 2

Question 34

How to reduce percentage uncertainty when using oscilloscope to measure voltage?

Answer 34

Increase y gain sensitivity to make the curve take up whole screen vertically. Turn time base off so it’s just a vertical line and easy to measure.

Question 35

How to reduce percentage uncertainty when using oscilloscope to measure time period?

Answer 35

Decrease the time base sensitivity to make one oscillation take up whole screen vertically Use freeze function

Question 36

State the four inefficiencies in a transformer and how they can be reduced.

Answer 36

Flux leakage: place iron core in transformer. Eddy currents: Laminate iron core. Resistance of wires: Use a thicker wire or a material with lower resistivity. Heat: Use cooling vents to reduce heat

Question 37

Transformer equation

Answer 37

Np/Ns = Vp/Vs

Question 38

How does flux leakage happen?

Answer 38

As the magnetic field grows and shrinks around primary coil the field lines move left/right Only needed on right to flux cut secondary coil So field lines to the left are wasted

Question 39

How to reduce flux leakage?

Answer 39

Place an iron core in the transformer Iron is magnetically soft Helps guide field lines towards secondary coil

Question 40

How do transformers work?

Answer 40

1) An alternating current is passed through the primary coil 2)Primary coil creates an alternating magnetic field which grows and shrinks 3) Therefore the secondary coil gets flux cut 4)Inducing an alternating current in the secondary coil

Question 41

Why can’t a direct current be used in a transformer?

Answer 41

The magnetic field would be stationary in this case. It must be changing so flux cutting occurs

Question 42

What are Eddy currents?

Answer 42

A current is induced in the iron core. This can’t go anywhere.

Question 43

How to calculate efficiency?

Answer 43

Pout/Pin x100

Question 44

How to calculate efficiency?

Answer 44

Pout/Pin x100

Question 45

Define root mean square voltage

Answer 45

The square root of the mean of the squares of all the values of voltage in one cycle

Question 46

What happens when time base it switched off?

Answer 46

It will just be a vertical like Because dot doesn’t move horizontally

Question 47

Why does a magnetic material slow down when it goes through a magnetic tube?

Answer 47

-Moving magnet flux cuts walls -This induced a current in the walls creating a 2nd field -This opposes the field of the moving magnet slowing it down

Question 48

When does F=BQv apply?

Answer 48

Magnetic field must be at right angles to the velocity

Question 49

Why does the balance reading increase when there’s a magnetic field around the wire?

Answer 49

Current through wire creates magnetic field creating a force An equal and opposite force is present on the magnet pushing the balance down.

Question 50

Why use an alternating current?

Answer 50

Resistance opposes current reducing power With a lower current, less power wasted Alternating current can be reduced using a transformer

Question 51

Explain why proton follows semicircular path in a Dee

Answer 51

Force due to uniform magnetic field is always at 90 degrees to direction of travel. This force is the centripetal force for circular motion

Question 52

Why isn’t is possible for the magnetic field to alter the speed of a proton when it’s in a Dee?

Answer 52

The force is perpendicular to the motion