Magnetic Fields

Question 1

What is a magnetic field?

Answer 1

A region of space where a permanent magnet/current-carrying conductor/moving charge experiences a force.

Question 2

The separation of magnetic field lines indicates _____________ .

Answer 2

the strength of the magnetic field.

Question 3

A current-carrying conductor has _________ magnetic field lines.

Answer 3

concentric

Question 4

A current-carrying conductor has concentric magnetic field lines. The direction of the field is given by the ________ rule.

Answer 4

right-hand

Question 5

The direction of the magnetic field in a solenoid is given by the _______ rule.

Answer 5

right-hand grip

Question 6

The strength of the magnetic field due to a flat coil or a solenoid may be increased by winding the coil on a __________.

Answer 6

bar of soft iron

This is a coil with a ferrous core.

Question 7

There is a force on a current-carrying conductor in a magnetic field.
The direction of the force is given by ___________ rule.

Answer 7

Fleming’s left-hand

Question 8

The magnitude of the force F on a conductor of length L carrying a current I at an angle θ to a magnetic field of flux density B is _________ .

Answer 8

F = BIL sin θ

Question 9

When using Fleming’s left-hand rule, the thumb points in the direction of the ________ .

Answer 9

motion or force

Question 10

When using Fleming’s left-hand rule, the first finger points in the direction of the ________ .

Answer 10

magnetic field

Question 11

When using Fleming’s left-hand rule, the second finger points in the direction of the ________ .

Answer 11

current

Question 12

Define magnetic flux density.

Answer 12

Force per unit length acting on a long straight conductor carrying unit current perpendicular to the magnetic field.

Question 13

Unit of magnetic flux density

Answer 13

tesla (T)

Question 14

Define the tesla.

Answer 14

The magnetic flux density is 1 T when a long straight wire carrying a current of 1 A placed at right angles to the magnetic field experiences a force per unit length of
1 N m-1.

Question 15

The force on a current-carrying conductor can be used to measure the flux density of a magnetic field using a ________ balance.

Answer 15

current

Question 16

Magnetic flux density can also be measured with a ____________ .

Answer 16

Hall probe

Question 17

Explain the forces between 2 current-carrying conductors.

Answer 17

A current-carrying conductor has a magnetic field around it. If a second current-carrying conductor is placed parallel to the first, this second conductor will be in the magnetic field of the first and, by the motor effect, will experience a force. The first conductor will also experience a force. By Newton’s third law, these two forces will be equal in magnitude and opposite in direction.

Question 18

The force F on a particle with charge q moving at speed v at an angle θ to a magnetic field of flux density B is F = _____

Answer 18

F = Bqv sin θ

Question 19

A charged particle entering a uniform magnetic field at right angles will move in a _______ path.

Answer 19

circular

arc of a circle

Question 20

A charged particle entering a uniform magnetic field at right angles will move in a circular path because ____________ .

Answer 20

the magnetic force is perpendicular to the velocity.

Question 21

For a charged particle moving in a uniform magnetic field, _________ provides the centripetal force.

Answer 21

magnetic force

Question 22

What is specific charge?

Answer 22

Ratio of charge to mass.

Question 23

A charged particle is in a region where an electric field and a magnetic field are right angles to each other. The charged particle is undeflected when ___________ is equal to ____________ .

Answer 23

electric force is equal to magnetic force.

Question 24

Derive an expression for the Hall voltage.

Answer 24

Check notes.

Question 25

Explain the principles of the use of magnetic resonance to obtain diagnostic information about internal body structures:

Answer 25

Many atomic nuclei eg. hydrogen, possess a ‘spin’ and behave as tiny magnets. The patient is placed in a large uniform magnetic field. In this magnetic field, the nuclei align and spin about the direction of the magnetic field (precession). The frequency of precession (Lamour frequency) depends on the magnetic field strength.
Radio-frequency pulses are transmitted to patient using RF coils. Resonance occurs because the frequency of the radio waves is equal to the frequency of precession. The nuclei absorb the energy of the radio waves. When the pulse ends, the nuclei return to equilibrium state (de- excitation) and emit radio-frequency radiation.
A non-uniform magnetic field is also applied. This field causes different regions of the patient to have different frequency of precession. This enables the location of the resonating nuclei to be determined. By changing the non-uniform magnetic field, atoms in different parts of the patient can be detected (enables the location of the slice to be changed).
The RF coils detect RF emissions from patient. The received emissions are processed to construct an mage of the number density of hydrogen atoms in the patient.