# A.C. Generator (Electromagnetic Induction)

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#1
I'm currently revising for my GCSE CCEA Physics exam Unit 2 on Friday.

Whenever a generator is inducing a current with a wire coil, is the maximum current induced whenever the coil is parallel to the magnetic field lines or whenever the coil is perpendicular to the magnetic field lines?
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3 years ago
#2
(Original post by shannonc01)
I'm currently revising for my GCSE CCEA Physics exam Unit 2 on Friday.

Whenever a generator is inducing a current with a wire coil, is the maximum current induced whenever the coil is parallel to the magnetic field lines or whenever the coil is perpendicular to the magnetic field lines?

Induced current and induced emf are in phase, the above picture should answer your query.
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#3
(Original post by Eimmanuel)

Induced current and induced emf are in phase, the above picture should answer your query.
This looks so complicated... I'm only doing GCSE... Could you explain it to me please?
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#4
(Original post by Eimmanuel)

Induced current and induced emf are in phase, the above picture should answer your query.

My book tells me it has the most current when it's parallel but on another page says when it's parallel, it doesn't cut ant field lines and so doesn't experience a force or current when parallel? It confuses me a lot...
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3 years ago
#5
(Original post by shannonc01)

My book tells me it has the most current when it's parallel but on another page says when it's parallel, it doesn't cut ant field lines and so doesn't experience a force or current when parallel? It confuses me a lot...
It needs both conditions to be met for maximum emf and current:

1) Perpendicular to the magnetic field

If the long length of coil (DE and FA) is perpendicular to the magnetic field (as shown in your second diagram) but is stationary, then no emf will be generated and without emf there will be no current.

2) Moving Perpendicular to the magnetic field

If the conditions of 1) above are met (long length of coil DE and FA perpendicular to the filed) whilst the coil is rotating through the field (said to be cutting lines of flux), then an emf will be induced and a current will flow. The emf induced is a function of the length og the conductor (increasing number of turns of the coil effectively increases the conductor length) cutting the field lines, the strength of the magnetic field, the velocity of the rotating coil cutting the field lines and the angle of rotation.
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#6
(Original post by uberteknik)
It needs both conditions to be met for maximum emf and current:

1) Perpendicular to the magnetic field

If the long length of coil (DE and FA) is perpendicular to the magnetic field (as shown in your second diagram) but is stationary, then no emf will be generated and without emf there will be no current.

2) Moving Perpendicular to the magnetic field

If the conditions of 1) above are met (long length of coil DE and FA perpendicular to the filed) whilst the coil is rotating through the field (said to be cutting lines of flux), then an emf will be induced and a current will flow. The emf induced is a function of the length og the conductor (increasing number of turns of the coil effectively increases the conductor length) cutting the field lines, the strength of the magnetic field, the velocity of the rotating coil cutting the field lines and the angle of rotation.
Ohh okay thank you very much!!
1
3 years ago
#7
(Original post by shannonc01)

My book tells me it has the most current when it's parallel but on another page says when it's parallel, it doesn't cut ant field lines and so doesn't experience a force or current when parallel? It confuses me a lot...
I always refer to this video:

It's very old, but still explains the principle better than any text book. Watch until around seven and a half minutes in. You may need to refer to it a few times but it's well worth the perseverance.
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3 years ago
#8
(Original post by shannonc01)

My book tells me it has the most current when it's parallel but on another page says when it's parallel, it doesn't cut ant field lines and so doesn't experience a force or current when parallel? It confuses me a lot...

You are mixing two different things to confuse yourself.

When the coil is rotating between two opposite magnetic poles, the magnetic flux linkage is changing in a sinusoidal way as shown in post #2 diagram – curve labelled as ΦB.

When the plane of the coil is perpendicular to the magnetic field, the magnitude of the magnetic flux linkage is maximum.

It is the negative rate of change of magnetic flux linkage that gives the induced emf NOT the magnitude of the magnetic flux linkage that gives the induced emf.

You can find the rate of change of magnetic flux linkage by finding the gradient of the graph of magnetic flux linkage versus time. You would find that the rate of change of magnetic flux linkage is maximum at when the plane of the coil is parallel to the magnetic field.

This is in line with what your text says.

“My book tells me it has the most current when it's parallel”

The curve (or graph) of induced current has the same shape as that of the induced emf.

Your other diagram is meant for motor effect NOT generator effect.

(Original post by shannonc01)
In a generator, we make use of mechanical work to turn the coil in the presence of the magnetic field to “generate” current in the coil.

In a motor, we use source such as battery to produce a current in coil in the presence of the magnetic field to produce a turning effect in the coil to turn the coil. So there is current in the coil already and this current is constant.

Note: In GCE O level, we tend to treat generator and motor independently. In higher, you would be told that every motor is also a generator and every generator is also a motor. The story would be a bit complicated.

I would explain magnetic flux linkage in another post.
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3 years ago
#9
(Original post by shannonc01)
This looks so complicated... I'm only doing GCSE... Could you explain it to me please?

The concept of magnetic flux linkage is not that difficult.

In the simple case, we can picture magnetic flux as the total number of magnetic field lines passing through an area A which can be an area of a coil.

When the magnetic field is normal (or perpendicular) to the area of the coil A, the magnetic flux Φ must therefore be equal to the product of magnetic flux density B and the area A. (See the diagram above labelled as a.)

Magnetic flux density B can be treated as the magnitude of magnetic field strength in GCSE.

In physics, we define the magnetic flux Φ through area A as:
Φ = BA
where B is the component of the magnetic flux density perpendicular to the area.

As we can see from the above diagram, there are times when the magnetic field B is not perpendicular to A. In such cases, we need to find the component of the magnetic flux density perpendicular to the area to compute the magnetic flux Φ.

Figure b (as shown above) shows a magnetic field at an angle θ to the normal. In this case:

magnetic flux = (B cos θ) × A = BA cos θ

For a coil with N turns, the magnetic flux linkage is defined as the product of the magnetic flux and the number of turns; that is:
or
magnetic flux linkage = BAN cos θ

Returning to your question of the “complicated diagram”.

The magnetic flux linkage ΦB as shown in post #2 has a cosine graph which is telling us the magnetic field through the loop or coil is changing.

And according to Faraday’s law, it is the rate of change of magnetic flux linkage ΦB that is give the induced emf NOT the magnetic flux linkage ΦB alone.

The negative sign is due to Lenz’s law which tells us the “any induced current or induced e.m.f. will be established in a direction so as to produce effects which oppose the change that is producing it”.

In other words, the current caused by the induced emf travels in the direction that creates a magnetic field with flux opposing the change in the original flux through the circuit or coil or loop.
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