Magnetic Fields

Question 1

What happens when a current-carrying wire goes in a magnetic field ?

Answer 1

the current-carrying wire will experience a force

Question 2

State the condition for F = BIL

Answer 2

• the direction of the force acting in a current-carrying wire is PERPENDICULAR to a magnetic field
• the current flow in wire must be perpendicular to a magnetic field

Question 3

Fleming’s Left hand rule ?

Answer 3

First finger =Field
Second finger = current
Thumb = force (motion)

Question 4

What happens when charged particle moves through a magnetic field ?

Answer 4

the charged particle will experience a force
- current I = Q/t
- distance L = vt
F = BQv

Question 5

What happens when charged particle moves perpendicular into a magnetic field ?

Answer 5

it will flow a circular motion

• the force is always towards the centre of the circle
• the direction of the particle is the Second finger (normally current)
• if negative charge then point finger in the opposite direction to its motion

Question 6

How is the radius and frequency of the circular motion of the charged particle affected ?

Answer 6

RADIUS r = mv/BQ
-mass or velocity increase = radius decrease (less deflect)
- magnetic field/charge increase = radius increase
FREQUENCY f=BQ/2πm
- frequency of rotations is independent of its velocity.

Question 7

What is Magnetic Flux density (B) ?

Answer 7

force acting on one metre of wire carrying a current on one amp

• is a vector
• measured in teslas (T)
• known as magnetic field strength

Question 8

What is one Tesla equal to ?

Answer 8

equal to one newton per amp per metre (1T = 1N/Am)

Question 9

what is the right hand rule used for ?

Answer 9

to work out the direction of a magnetic field around a current -carrying wire
- thumb = current
= fingers = magnetic field

Question 10

Representation of magnetic field on magnet and paper ?

Answer 10

MAGNET
- the magnetic field lines go from NORTH to SOUTH
- NORTH = anticlockwise current (N)
- SOUTH = clockwise current (S)
PAPER
- the magnetic field goes INTO paper when a +
- the magnetic field goes OUT paper when a •

Question 11

How particles accelerators such as CYCLOTRONS make use of the force that acts on charged particles moving perpendicular to a magnetic field ?

Answer 11

HOW

• two electrodes
• charged particle start in centre - circular motion out
• particle accelerates between electrodes so potential difference create and particle increase speed
• spend equal time in each electrode as frequency is independent of velocity

Question 12

Uses of CYCLOTRONS ?

Answer 12

USES

• Medicine
• Provide radioactive traces
• High energy beams for use in radiotherapy

Question 13

What is Magnetic Flux (Φ) ?

Answer 13

The magnetic flux passing through an area is given by the magnetic flux density multiplied by the area
Φ = BA
- Measured in Wb (Weber’s)
- the number of magnetic field lines passing through the area

Question 14

When is an E.M.F induced across a conductor ?

Answer 14

when the conductor moving through the magnetic field or a conductor inside a changing magnetic field
- if the conductor in a circuit then a current will flow

Question 15

What is Magnetic Flux linkage (NΦ) ?

Answer 15

NΦ = BAN
N - number of turns on the coil cutting the wire
- Measured in Wb (Weber’s)

Question 16

What does the Rate of change in Flux linkage tell you ?

Answer 16

It tell you how strong the electromotive force (EMF) will be in volts
- A CHANGE IN FLUX LINKAGE OF ONE WEBER PER SECOND WILL INDUCE AN EMF OF 1 VOLT IN A LOOP OF WIRE

Question 17

How to calculate either the Magnetic Flux or Flux linkage of a rectangle coil when the coil is not normal to B ?

Answer 17

1) Φ = BACOSѲ

2) NΦ = BANCOSѲ

Question 18

What is the Radius and frequency equation in magnetic field circular motion ?

Answer 18

r = mv/BQ
f = BQ/2πm

Question 19

What is Faraday’s Law ?

Answer 19

The induced EMF = the rate of change of Flux linkage in a conductor
E = NΔΦ/Δt
- Measured in Volts (V)
- Need to know GRAPHS for Φ/t and E/t

Question 20

What is Lenz’s Law ?

Answer 20

The DIRECTION of the induced EMF is such that it will try to oppose the change of flux that is producing it
E = -(NΔΦ/Δt)
- agrees with the conservation of energy - they energy used to pull a conductor though a magnetic field, against the resistance caused by magnetic attraction, is what produces the induced current

Question 21

What happens when you drop a magnetic through a solenoid ? - Lenz’s Law

Answer 21

1) north and north end meet so repulsion - A

Question 22

What happen if no current is induced in circuit ?

Answer 22

• no current = no magnetic field
• but still EMF induced
• fall at acceleration = g

Question 23

How is an EMF induced in a rotating coil ?

Answer 23

• rotating caused alternating EMF is induced
  E = BANwSINwt
• graph is E/t - sin graph - EMF is greatest when coil is parallel to Magnetic field (NΦ - Fluxage linkage= 0)

Question 24

What is Alternating Current ?

Answer 24

Changing direction with time

Question 25

How to operate an Oscilloscope ?

Answer 25

It displays voltage 1) Alternative Display - y-gain = 2v per division - time base = 1ms per division - time basee off - straight line vertical 2) Direct Display - time base on - straight line horizontal - time base off - dot

Question 26

Who to calculate Time period, Peak Voltage and Peak-Peak Voltage from graph ?

Answer 26

1) Time period - one complete oscillation 2) Peak Voltage = Vo - amplitude 3) Peak-Peak Voltage = Vpp = whole vertical

Question 27

How to calculate 'root mean square current and potential differences ?

Answer 27

``` Vrsm = Vo/√2 (main electricity - 230v) Irsm = Io/√2 Prms = Po/2 or Po = Vo x Io ```

Question 28

What is a Transformer and the shape of it ?

Answer 28

a device that makes use of electromagnetic induction to change the size of the Voltage for an alternating current - Its a laminated iron core with a magnetic field around it and Primary and Secondary coil around

Question 29

How to calculate the number of turns on coils or voltage across coils ?

Answer 29

Ns/Np = Vs/Vp

Question 30

Who does a step-up and step-down transformer work ?

Answer 30

STEP-UP TRANSFORMER - increase voltage ( reduce power loss P=VI) - having more coils turns (N) on Secondary coil STEP-DOWN TRANSFORMER - decrease voltage - having more coils turns (N) on Primary coil

Question 31

What causes Energy loss in transformers and how to reduce them ?

Answer 31

1) EDDY CURRENT (heat lost) - Laminating core so current can't flow 2) Electrical resistance - Thick copper (low r) 3) magnetism/de-magnetism core - SOFT IRON 4) Air gaps/impurities - high quality iron

Question 32

How to calculate the efficiency of a transformer ?

Answer 32

``` Power = V x I - POWER IN = POWER OUT - IpVp = IsVs EFFICIENCY = IsVs/IpVp - energy lost - E = Pt ```

Question 33

How is electrical power is transmitted in the National Grid ?

Answer 33

Power station -> Step-up -> Pylogens -> Step-down -> House (230V) - Transformers step-up = high voltage = low current = LOW ENERGY LOSSES