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Physics > Magnetic Fields > Flashcards

Magnetic Fields Flashcards

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1
Q

How are magnetic fields represented?

A

They are represented by field lines, the closer together the lines, the stronger the field. The lines go from north to South

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2
Q

What happens when a current is passed through a wire?

A

A magnetic field is induced

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3
Q

What is the right - hand rule?

A

Thumb = current

Curled fingers = direction of field

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4
Q

What happens when you loop a current-carrying wire into a coil?

A

The field produced is doughnut shaped and when lots of coils are present a solenoid is formed - which produces a field like a bar magnet

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5
Q

What happens when a current-carrying wire is placed in a external magnetic field?

A

The fields around the magnet and bar interact. The field lines from the magnet contract to form a ‘stretched catapult’ effect where the field lines are closer together and a force acts on the wire.

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6
Q

When does no force act on a wire?

A

If the current is parallel to the field lines.

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7
Q

How does a wire vibrate when an alternating current is passed through it?

A

The direction of the force acting on the wire is perpendicular to the direction of the current and when the current is reversed, so is the force. This constant reversal of the force causes a vibration of the wire.

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8
Q

What is proportional to the strength of the magnetic field?

A

The force on a current-carrying wire at a right angle to the magnetic field

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9
Q

What is magnetic flux density?

A

The force on one meter of wire carrying a current of one amp at right angles to the magnetic field

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10
Q

What is one tesla?

A

The magnetic flux density of a field in which 1 Newton acts on one meter length of conductor - carrying a current of one Ampere and is perpendicular to the field.

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11
Q

What is proportional to the current, length of wire and magnetic flux density?

A

The size of the force, F, on a current-carrying wire.

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12
Q

F=BIL is the what? And why?

A

This is the maximum force the wire could feel, it would be smaller if not perpendicular

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13
Q

What happens when a charged particle moves through a magnetic field?

A

A force is produced, and is deflected in a circular path.

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14
Q

How do you change Flemings Left Hand Rule for a charged particle?

A

Use right hand and the second finger, instead of meaning current, means the direction of motion for a positive charge.

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15
Q

What is independent of a charged particles mass when moving in a magnetic field? What does depend on mass?

A

The force due to magnetic field experienced by a particle travelling through a magnetic field.
Centipetal acceleration

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16
Q

How does the radius of curvature for a charged particle in a magnetic change?

A

The radius is increased if mass or velocity increases but decreases if strength of magnetic field or charge on particle increases.

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17
Q

What is independent of a charged particles velocity in a magnetic field?

A

The frequency of rotation of a charged particle in a magnetic field?

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18
Q

What happens if you increase a charged particles velocity in a magnetic field?

A

It will follow a larger radius circular oath but will take the same amount of time to complete.

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19
Q

How does a particle sped up in a particle accelerator?

A

The particle follows a circular path through a uniform magnetic field, perpendicular to the plane of the electrodes, in hollow semi-circular electrodes with a small gap between them. An alternating p.d is applied between the electrodes. Charged particles are produced and fired into one of the electrodes, when the pass through the applied potential difference, it accelerates the particles until they reach the next electrode, where as they are travelling at a higher velocity have a larger radius.

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20
Q

What is a magnetic field?

A

A region where a magnetic force acts

21
Q

What happens when a conducting rid moves through a magnetic field?

A

It’s electrons experience a force, which means they accumulate at one end, which induces an emf across the ends of the rod

22
Q

What is electromagnetic induction?

A

When a conducting rod is part of a complete circuit, causing an induced current to flow

23
Q

What is needed for an emf to be induced?

A

Relative motion between a conductor and a magnet

24
Q

What will always induce an emf but only induce a current if part of a complete circuit?

A

Flux cutting - moving a conductor through a magnetic field

25
Q

What is magnetic flux density?

A

It is a measure of the strength of the magnetic field per unit area

26
Q

What is flux linking?

A

When an emf is induced by changing the magnitude or direction of the magnetic flux, by using alternating currents. A change in flux of one weber per second will induce an emf of 1 volt in a loop of wire

27
Q

What is flux linkage?

A

When the size of the end induced depends on the magnetic flux passing through the coil and the number of turns on the coil cutting the flux

28
Q

What is often the case with flux linkage?

A

The wire coil is not normal to the direction of the magnetic flux density so the normal to the coil makes an angle x to the direction of the magnetic flux ( add cosx to magnetic flux and flux linkage equations)

29
Q

What is Faraday’s Law?

A

Induced emf is directly proportional to the rate of change of the flux linkage

30
Q

What is the magnitude of the emf and flux linkage equal to on graphs? ( flux linkage and emf over time)

A

On a graph of flux linkage against time, emf is the gradient of the line.
On the graph of emf over time, the area under the graph is equal to the change in the flux linkage

31
Q

What is Lenz’s Law?

A

The induced emf is always in such a direction as to oppose the change change that caused it

32
Q

What does Lenz’s law agree with?

A

It agrees with the principle of the conservation of energy - the energy used to pull a conductor through a magnetic field against the resistance caused by the magnetic attraction is what produces the induced current

33
Q

What happens when a coil rotates uniformly through a magnetic field?

A

The coil cuts the flux and an alternating emf is induced, the amount of flux cut depends of the angle the coil moves through.

34
Q

What does the speed of a uniform, rotating coil in a magnetic field depend on?

A

The angular speed of the coil.

35
Q

What does the emf of a coil uniformly rotating in a magnetic field depend on?

A

The rate of change of flux linkage, also varying sinusoidally.

36
Q

The sinusoidal graphs produced by plotting flux density bs induced emf over time for a rotating coil in a magnetic field are?

A

Flux linkage = cosine graph

Induced emf = sine graph

37
Q

How can the shape of the induced emf over time of a rotating coil in a magnetic field be changed?

A

Increasing the speed of rotation will increase the frequency and increase the maximum emf
Increasing the magnetic flux density will increase the maximum emf but will have no effect on the frequency

38
Q

What does generators do?

A

They convert kinetic energy into electric eagerly by inducing an electric current by rotating a coil in a magnetic field.

39
Q

How does a generator work?

A

It has slip rings a brushes to connect the coil to an external circuit. The output voltage and current change direction with every half rotation of the coil, producing an alternating current.

40
Q

What is a transformer?

A

It is a device that makes use of electromagnetic induction to change the size of the voltage for an alternating current.

41
Q

How does a transformer work?

A

An alternating current flowing through the primary coil causes the core to magnetise, do you magnetise and re-magnetise continuously in opposite directions. This produces a rapidly changing magnetic flux across the core. This rapidly changing magnetic flux in the iron core passes through the second airy coil, where which induces an alternating voltage of the same frequency, but different voltage.

42
Q

What a step up transformers and what are stepdown transformers?

A

Step up transformers increase the voltage by having more turns on the secondary coil than primary.
Stepdown transformers reduced voltage by having fewer turns on the secondary coil

43
Q

How do eddy currents occur?

A

By the magnetic core continuously being cut and changing flux, inducing emf causes eddy current to be produced. This can cause the core to heat up and energy be lost l.

44
Q

What are the inefficiency in a transformer? (4)

A

Eddy currents - by laminating the core, layering layers of core between thin layers of insulator, eddy current are reduced.
Heating loss in coils - thick copper wires.
Energy lost as core heats up - magnetically soft material that magnetises and demagnetises easily.
Not all magnetic field created by primary links up to secondary - design transformer to make field well linked so most energy is concentrated in core or have the could winding on top of each other around the same part of the core.

45
Q

Why does the national grid send energy at the lowest current possible?

A

Energy loss is proportional to I^2 x R

46
Q

How can having a very low current bin the national grid still produce a high power?

A

Power is equal to current x voltage, so a high voltage and low current will give the same power output and high current and lower voltage, but with a significantly lower energy loss.

47
Q

What is the voltage stepped up to in order to flow through pylons without too much energy loss?

A

400,000 V

48
Q

What has to happen to the voltage on the national grid before being used?

A

It has to be stepped down to 230V due to safety and insulation issues.

Physics (9 decks)

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