Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

Magnetism > Magnetism > Flashcards

Magnetism Flashcards

1
Q

What is a magnetic field?

A

A force field surrounding a magnet/current - carrying wire which acts on any other magnet/current - carrying wire in the field.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

What is a line of force of a magnetic field?

A

A line along which the north pole would move in the field.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

When is magnetic force greatest on a current - carrying wire?

A

When the wire is at right angles to the magnetic field lines.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

Why do beam of electrons move in circular motion when entering a region of constant magnetic field strength?

A

The magnetic force due to the magnetic field acts perpendicular to the velocities of the electrons, which means there is a centripetal force which leads to a centripetal acceleration.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

Why does the force do no work on the charged particles in a magnetic field? What changes? What doesnt change?

A

Force acts at right angles to direction of motion, so no work is done. KE isnt affected, only its momentum, as velocity changes.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

How do you derive r = mv/bq?

A

set bqv = mv^2/r. Equal to centripetal force.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

What is ionisation?

A

The process of forming ions by atoms losing or gaining electrons.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

What happens in mass spectrometry?

A

Used to analyse the type of atoms present in a sample. Atoms are ionised and directed in a narrow beam at the same velocity into a region of uniform magnetic field, each ion is deflected in a semicircle path.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

What occurs in the velocity selector?

A

There are opposing magnetic fields and electric fields which ensures that particles travel at a certain velocity. If particles is too fast, it curves towards magnetic field direction, since f is proportional to v.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

What are cyclotrons used for? What do they consist of?

A

To produce high energy beams for radiation therapy. Two hollow shaped ‘DEES’ inside a vacuum chamber.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

How does a cyclotron work?

A

uniform magnetic field applied normal to the cylclotron. Electrons are injected at centre and move in a circular path. A constant high frequency AC voltage is applied to accelerate the electrons over the gaps of the dees so that electrons move in a path with a greater radius, since they are moving faster.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

When is an EMF induced?

A

When there is relative motion between a magnet and a conductor, and the field lines are cut. It becomes zero when the relative motion stops.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

What is a solenoid?

A

A cylindrical coil of wire which produces a magnetic field when it is connected to a battery and a current is passed through it.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

What direction does current pass around the north pole end of solenoid?

A

Anti clockwise around the north pole end. Clockwise around the southpole end.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

What is Lenz’ law, and how does it work with conservation of energy?

A

The direction of the induced current is always such as to oppose the change of flux that caused the current. Otherwise, magnetic field could pull the magnet in faster than usual and this would mean kinetic energy created out of nothing. Violates law of conservation of energy.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

What is magnetic flux formula? Flux linkage?

A

Flux = BA. Flux linkage = BAN, where n = number of turns.

17
Q

What is Faraday’s law?

A

The induced emf in a circuit is equal to the rate of change of the flux linkage through a circuit.

18
Q

Why is there a minus sign in faraday’s formula?

A

In relation to lenz’ law. Induced field acts in opposite direction to change in flux that caused it.

19
Q

When is induced emf zero and when is it at a maximum?

A

When flux lines are cut at right angles its a max, but when magnetic field is parallel to the coil area , the flux linkage is zero.

20
Q

What is the magnitude of the induced emf proportional to in a solenoid?

A

The rate of change of current, since a change in current leads to a change in magnetic field strength.

21
Q

When is EMF at a maximum in the rotating coil in magnetic field?

A

When flux is zero, and sides of coil are parallel to the magnetic field lines.

22
Q

What is back emf?

A

Emf induced in the conductor which acts against the pd applied, therefore reducing the current

23
Q

When is current largest in rotating coil?

A

At low rotational speeds, since induced emf is proportional to the speed of rotation of the motor, and
V - E = IR, E is small and current will be larger.

24
Q

What is the frequency of electricity from the mains?

A

50 Hz.

25
Q

What is an alternating current?

A

Current that repeatedly reverses it’s direction

26
Q

What is the peak to peak value of an ac current or pd?

A

The difference between the peak one way and the peak value the other way.

27
Q

What is the mean power supplied from an alternating power graph?

A

Half the peak value.

28
Q

What is the root mean square value of an alternating current?

A

The value of direct current that would give the same heating effect as the alternating current would in the same resistor.

29
Q

What is the purpose of a transformer?

A

To change an alternating pd into a different peak value.

30
Q

What does a transformer consist of?

A

A primary coil, a secondary coil and an iron core at the centre.

31
Q

How does a transformer work?

A

Primary coil is connected to an alternating pd, which produces an alternating magnetic field in the primary coil. This alternating magnetic field passes into the secondary coil by passing through the iron core and an alternating emf is induced because of the changing magnetic field.

32
Q

How are transformers designed?

A

So that all the magnetic flux in the primary coil passes through to the secondary coil.

33
Q

What is a step us transformer? What is a step down transformer.

A

Has more turns on the secondary coil than the primary coil. Voltage therefore increases, and current decreases in secondary coil. Opposite for a step down transformer.

34
Q

What is the efficiency of transformers?

A

Nearly 1, therefore IpVp = IsVs.

35
Q

Give 3 ways in which the efficiency of transformers is improved?

A

1) Low resistance windings to reduce power wasted due to heating effect of the current.
2) Laminated core which reduces eddy currents, so magnetic flux is as high as possible in the core.

A soft iron core - easily magnetised / demagnetised. This results in a reduced power wasted through repeated magnetisation/demagnetisation of the core.

36
Q

Why is transmission of electrical power over long distances much more efficient at a high voltage than at a low voltage?

A

A high current leads to more power wasted by heating the cables.

37
Q

Define the tesla?

A

The strength of the magnetic field that produces a force of 1 newton in a wire of length 1m with 1A current flowing.

Magnetism (1 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions