Magnetic Fields

Question 1

Right hand rule

Answer 1

Used to determine the direction of a magnetic field around a current-carrying wire

Thumb represents direction of current

Curled fingers represent direction of field

Increasing current

Also used to determine the direction of a magnetic field in a solenoid - fingers represent current, thumb for field direction

Question 2

Fleming’s left hand rule

Answer 2

Used to determine direction of force when a current-carrying wire has a flow of current parallel to the direction of the external magnetic field

Thumb - force direction

First finger - direction of uniform magnetic field

Second finger - direction of current

Question 3

Magnetic flux density

Answer 3

AKA magnetic field strength - increases with current

The strength of the magnetic field that produces a force of 1 N on 1 metre of wire carrying a current of 1 Amp perpendicular to the magnetic field (Tesla definition)

Measures in teslas, T

1 Tesla = 1 Newton per Amp metre

T=N/Am

Question 4

Moving a charged particle in a circular path

Answer 4

F=mv^2/r and F=BQv

r = mv/BQ

Radius of curvature increases (less deflection) with increases in mass or velocity

Radius of curvature decreases (more deflection) with increases in field strength or charge

Question 5

Line of force - magnetic field line

Answer 5

A line along which a North Pole would move in the field

Question 6

Motor effect

Answer 6

The force on a current-carrying conductor due to a magnetic field

Question 7

Investigating flux density

Answer 7

Use a wire connected to a mass on a balance going through a magnetic field perpendicular to the field

Record initial mass and length

Record mass for each value of current

Covert mass to force = mg

Plot a graph of force against current - gradient = Bl

Question 8

Magnetic force on a charged particle

Answer 8

Results in a circular orbit - force of the magnetic field is at right angles to the motion

Doesn’t change the speed no work is done

F = BQv F = mv^2/r

r = (mv)/BQ

Question 9

Cyclotron

Answer 9

Made of two semicircular electrodes with a uniform magnetic field applied perpendicular to the plane of the electrodes

An alternating potential difference is applied between the electrodes

Used to produce radioactive tracers or high energy radiation beams - radiotherapy

Question 10

How does a cyclotron work?

Answer 10

Charged particles are fired into than electrode the magnetic field makes them follow a semicircular path and leave the electrode

P.d accelerated the particles across the gap into the other electrode

This increases the speed so it’s path has a larger radius

P.d reversed and the process repeated increasing speed and spiralling outwards until it leaves the cyclotron

Question 11

Magnetically hard and soft

Answer 11

Hard -> Permanent magnets

Soft -> temporary magnets

Question 12

Magnetic materials

Answer 12

Iron
Nickel
Cobalt
Steel

Question 13

Inducing magnetism

Answer 13

Magnetism can be induced of a material is placed in a magnetic field

Question 14

Increasing field strength of a solenoid

Answer 14

Increase the current

Increase the number of turns in the coil

Question 15

Increasing force on a wire in a magnetic field

Answer 15

Increase magnetic field strength - flux density

Increase current

Increase length (e.g. More coils)

Question 16

Mass spectrometer

Answer 16

Used to analyse the type of atoms present

The atoms are ionised and directed at a narrow beam into a uniform field

Ions are all at the same velocity - a velocity selector ensures this

Each ion is deflected by a different amount onto the detector - allowing them to be identified

Question 17

Velocity selector (for mass spectrometer)

Answer 17

Only allows ions through travelling at a certain velocity, v

Same velocity is important when identifying ions as it impacts radius

v must be such that the force from the electric field is equal to the force from the magnetic field

BQv = EQ

Question 18

Alternating current

Answer 18

The charge carriers in a circuit repeatedly reverse their direction if an alternating p.d. is applied

Frequency of a.c. = number of cycles per second

Mains electricity has a frequency of 50Hz

Peak value of p.d. or current is maximum value in either direction

Peak to peak value = 2 X peak value

Peak value of mains = 325 V

Root mean square value of mains = 230V

Question 19

Fleming’s left hand rule for a moving electron

Answer 19

Remember current direction is in the OPPOSITE direction to the direction of the electrons motion

Then just use the left hand rule to find the direction of the force

Question 20

When is the magnetic force on a charge when in a magnetic field 0?

Answer 20

Charge is stationary

Charge is moving parallel to the field