# Magnetic fields Flashcards Preview

## Physics 1 - Particles, quantum phenomena and electricity > Magnetic fields > Flashcards

Flashcards in Magnetic fields Deck (80)
1
Q

A magnetic field is formed around a _____ carrying ____

A

A magnetic field is formed around a current carrying wire

2
Q

How to find direction of field in current carrying wire

A

use right hand grip rule (thumb pointing along the direction of the current, other fingers give the direction of the field)

3
Q

A current carrying wire experiences a ______ in a magnetic field

A

A current carrying wire experiences a force in a magnetic field

4
Q

How to find direction of force, current or motion in current carrying wire

A

Use Fleming’s Left Hand rule (thumb force/motion, first finger field, second finger current) magnitude of force

5
Q

Magnitude of force of in current carrying wire

A

F = BIL

6
Q

A charge moving in a magnetic field experiences a ____

A

A charge moving in a magnetic field experiences a force

7
Q

A moving charge is equivalent to

A

current

8
Q

To find the direction of a moving charge in a magnetic field

A

Use Fleming’s Left Hand rule

9
Q

The magnitude of the force experienced by a moving charge in a magnetic field

A

F = BQV

10
Q

Cyclotrons use … in order to …

A

Cyclotrons use magnetic fields to keep charged particles in circular orbits

11
Q

If a wire moves in a magnetic field then

A

A current is induced in a wire if it moves in a magnetic field

12
Q

To find the direction of current, field, motion of wire in magnetic field

A

Use Fleming’s Right Hand rule (same)

13
Q

Magnetic flux density =
Magnetic flux =

A

magnetic flux density = B,
magnetic flux, phi = BA,
magnetic flux linkage = N*phi = BAN

14
Q

A field is

A

the region around a mass, charge or magnetic in which a force can be experienced

15
Q

The flux density =

A

the strength of the field = the force on a unit North pole or current-carrying conductor

16
Q

The lines of force of a long, straight wire carrying current are

A

concentric circles, in the plane at right angles to the wire, with the separation of the lines 1/r. (circles around a line going up)

17
Q

If you look at the end of the coil and the current is going clockwise

A

then you are looking at a south pole, conversely, if the current is anti-clockwise then you are looking at a north pole

18
Q

Solenoid with diagram of field lines

A

arrows going in from south (left) to north (right) and one circle going anticlockwise at top and clockwise at bottom
Current going down at left and up and right

19
Q

Strength of magnetic field in solenoid is increased by

A

increasing the current in the coil
increasing the number of coils in the solenoid
using a soft iron core within the solenoid

20
Q

When looking at the end of the solenoid, if the current is clockwise then

A

you are looking at south pole

21
Q

The field in a solenoid is identical to that of a bar magnet except that it continues inside the solenoid. It has the advantage over the bar magnet in that

A

the strength of the field can be varied, whereas a bar magnet has a fixed strength

22
Q

A current carrying conductor in a magnetic field experiences a force which is

A

at right-angles to the field and to the wire, due to the combination of two magnetic fields

23
Q

One tesla (the unit of magnetic field strength/flux density)

A

is the field in which a wire carrying a current of 1 amp, produces a force of 1 N for each 1 metre of the length of the wire in the field, at right angles to both the wire and the field.

24
Q

A hall probe

A

uses the effect of the magnetic field on the electrons in a semi-conductor to give a potential difference across a slice of semi-conductor which is proportional to the field. This gives a measure of the field strength

25
Q

r =

A

mv/BQ

26
Q

What causes the path of a charged particle to curve/decrease

A

if B is increased or v is decreased, if particles with a larger specific charge (Q/m) are used

27
Q

w = v/r =

A

BQ/m

28
Q

Cyclotron use

A

The cyclotron is used in hospitals to produce high energy beams for radiation therapy

29
Q

Conventional current goes in

A

opposite direction to electrons, same direction as positrons, +ve to -ve

30
Q

Conductor, force =

A

BIL

31
Q

Beam, for charged particles, force =

A

BQV

field is perpendicular to velocity

32
Q

(X) means

A

magnetic field lines into the board

33
Q

(.) means

A

magnetic field lines out of the board

34
Q

A magnetic force only acts on particles which are ____ and ____

A

which are charged and moving and when the field is perpendicular to the path of the moving charge

35
Q

A uniform field has

A

constant magnitude of force and acceleration

36
Q

A magnetic force perpendicular to the trajectory of moving charge -

A

speed of particle is unaffected

37
Q

The cyclotron frequency, f =

A

w/2pi = BQ/2mpi

38
Q

Cyclotron cosists of

A

It consist fo two hollow D-shaped electrodes (dees) in a vacuum chamber. With a uniform magnetic field applied perpendicular to the plane of the dees, a high-frequency alternating voltage is applied between the dees. Each time a charged particle crosses the gap it gains kinetic energy and hence speed and momentum so the radius of the path increases

Charged particles are directed into one of the dees near the centre of the cyclotron. The charged particles are forced on a circular path by the magnetic field causing them to emerge form the dee they were directed into. As they cross into the other dee, the alternating voltage reversed so they are accelerated into the other dee where they are once again forced on a circular path by the magnetic field. On emerging from this dee, the voltage reverses again and accelerates the particles into the first dee where the process is repeated. This occurs because the time taken by a particle to move round its semi-circular path in each dee does not depend on the particle speed

39
Q

A magnetic field/force line is

A

a line along which a free north pole would move in a field

40
Q

Motor effect =

A

current-carrying wire placed at a non-zero angle to the lines of force of an external magnetic field experiences a force due to the field. The force is perpendicular to the wire and to the lines of force

41
Q

The magnitude of the force depends on

A

the current
the strength of magnetic field
the length of the wire
the angle between lines of force of the field and the current direction

42
Q

The force is greatest when

A

the wire is at right angles to the magnetic field

43
Q

The force is zero when

A

the wire is parallel to the magnetic field

44
Q

B = magnetic flux density =

A

= force per unit length per unit current on a current-carrying conductor at right angles to the magnetic field lines, unit is Tesla(T)=N/m/A, B and I are perpendicular

= episolon0 * I/2pi * r

45
Q

Force = BQV if direction of motion of charged particle is perpendicular to the direction of the magnetic field, if not

A

F = BQVsin(X)

46
Q

Stationary chargers in a magnetic field experience

A

no magnetic force

47
Q

When applying Fleming’s left-hand rule to charged particles, the current direction for negative particles

A

is in the opposite direction to the direction of motion of the particles.

48
Q

The force of the magnetic field on moving charged particles is

A

at right angles to the direction of motion of the particle

49
Q

No work is done by the magnet on the particle as

A

the force always acts at right angles to the velocity of the particle

50
Q

The direction of motion, kinetic energy and speed of a charged particle…

A

direction of motion and kinetic energy is changed by the force but not its speed

51
Q

In accordance with Fleming’s left-hand rule, the magnetic force is always…. to the velocity at any point along the path meaning…

A

perpendicular
This means that the particle therefore moves on a circular path with the force always acting towards the centre of curvature of the circular path.

52
Q

The force causes a centripetal acceleration because

A

its perpendicular to the velocity

53
Q

The deflection of a beam of electrons in a uniform magnetic field follows a complete circle because

A

the magnetic field is uniform and the particle remains in the field.

54
Q

Mass spectrometer

A

for equally charged particles travelling at the same velocity in the same magnetic file the radius of their paths are proportional to their masses, r =mv/BQ Nuclei of different isotopes of a given element could thereby be separated if they could be made to travel at the same velocity.

The mass spectrometer is used to analyse the type of atoms present in a sample. The atoms of the sample are ionised and directed in a narrow beam at the same velocity into a uniform magnetic field. Each ion is deflected in a semi-circle by the magnetic field onto a detector. The radius of curvature of the path of each ion depends on the specific charge of the ion. Each type of iron is deflected by a different amount onto the detector. The detector is linked to a computer which is programmed to show the relative abundance of each type of ion in the sample.

55
Q

Electric motor

A

The simple electric motor consists of a coil of insulated wire which spins between the poles of a U-shaped magnet. When a direct current passes round the coil, the wires at opposite edges of the coil are acted on by forces in opposite directions; the force on each edge makes the coil spin about its axis. Current is supplied to the coil via a split-ring commutator. The direction of the current round the coil is reversed by the split-ring commutator each time the coil rotates through half a turn ensuring the current along an edge changes direction when moving poles

56
Q

The trajectory of charged particles which enter at right angles to a uniform electric field is

A

parabolic

57
Q

The trajectory of charged particles which enter at right angles to a uniform magnetic field is

A

circular

58
Q

A square coil with its plane parallel to a uniform magnetic field induces an emf in the coil when

A

it is rotated about an axis through the perpendicular

59
Q

A coil mounted on an axle has its plane parallel to the flux lines of a uniform magnetic field. When a current I is switched on, and before the coil is allowed to move there are no forces due to the magnetic field on

A

the sides parallel to this field.

60
Q

Condition for F = BIL

A

Current is perpendicular to magnetic flux density

61
Q

Charged ion is projected into a vertical magnetic field that is directed upwards, thedirection of the magnetic force that acts on the ion is

A

initially out of plane of diagram and subsequently it is circular in the horizontal plane

62
Q

If the strength of the magnetic field is doubled then

A

63
Q

if the charge is halved then

A

64
Q

An electron moves due North in a horizontal plane with uniform speed. It enters a uniform magnetic field directed due South in the same plane. What happens to its motion?

A

It is accelerated due North

65
Q

Why do ions travel in a semicircular path whilst in a magnetic field

A

From Fleming’s left hand rule force acts perpendicular to both magnetic field and velocity, force changes direction of velocity but not its magnitude, centripetal force remains perpendicular to velocity as direction changes

66
Q

diameter is directly proportional to

A

mass

1/charge

67
Q

If the wire is parallel to the magnetic field then

A

there is no force

68
Q

Force =

Condition

A

Force = magnetic flux density x current x length = BIL when field is perpendicular to current

69
Q

Two numbers for B, magnetic flux density

A

T = N/Am

70
Q

What is magnetic flux density

A

The strength of the magnetic field

71
Q

Couple =

A

BINAcosX

72
Q

DC electric motor simplified

A

Every half turn the split ring commuter reverses current so it does not get stuck at top

73
Q

Maxwell’s right hand screw rule

A

thumb pointing in direction of current flow (opposite to direction flow), Fingers in direction of magnetic field

74
Q

Increasing strength of solenoid

A

Solenoid = coil of wire, increase force by adding iron core or number of coils or current to increase magnetic force strength, increase length of wire

75
Q

How a bell works

A

switch closes causing a current to electromagnet attracting the iron causing the strike to hit thebell disconnecting the circuit causing it to stop attracting reconnecting the circuit as it rises back up starting cycle again

76
Q

To determine poles of a magnetic field/direction of turn of coil/solenoid

A

North with arrows pointing out from tips (anticlockwise) – field going out
South with arrows pointing out from tips (clockwise) – field going in

77
Q
```Flux linkage (BNLV) y axis
w/v x axis```
A

y=x

78
Q

induced emf y axis

w/v x axis

A

y=c

79
Q

Use left hand rule for

A

Motor effect

80
Q

Use right hand rule for

A

induction/ generators

riGht hand for Generators