# Some questions about electromagnetism

• PainterGuy
In summary: B to A on the lower surface.Yes, that's right. I think my concept about the flow of induced current was wrong. Thank you for pointing that out.In summary, the conversation focused on the production of eddy currents in a rotating disk in a uniform magnetic field, the direction of induced currents, and the concept of an accelerating charge radiating electromagnetic waves. It was concluded that while there will be an emf induced in the disk, there will not necessarily be eddy currents produced. Additionally, the direction of induced currents was clarified and it was determined that an accelerating charge does not necessarily lose its charge or mass due to radiating electromagnetic waves. The conversation also touched on the idea of magnetic monopoles

#### PainterGuy

Hi,

Could you please help me with the queries below? Thank you.

Question 1:
Please have a look on this attachment. In the given attachment, the disk is mounted vertically and only a part of the disk passes thru the magnetic field at any instant. If entire surface of the disk was rotating in a uniform magnetic field then I don't think any eddy currents would be produced. Do you agree?

Question 2:
If the disk was instead mounted horizontally but still all of the disk rotating in a uniform magnetic field, would it result into production of any eddy currents?

Question 3:
It is said that an accelerating charge radiates electromagnetic waves, does this mean that the quantity of charge would reduce as a result of emanating electromagnetic waves?

Question 4:
Please have a look on this attachment. If the attachment is not clear please have a look here. I believe that "v" represents the direction of disk rotation. How is the direction of "v" is being determined? The direction of magnetic field is from left to right and disk is rotating clockwise when seen from the left side.

Question 5:

We can see that there is circular magnetic field around wire but where are the north and south poles of this circular magnetic field?

Thanks a lot for your time and help.

#### Attachments

• 1st_generator.jpg
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• wire_magnetic field.gif
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• eddy1.jpg
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PainterGuy said:
If entire surface of the disk was rotating in a uniform magnetic field then I don't think any eddy currents would be produced. Do you agree?
Yes, but there will be a voltage between centre and rim of the disc.
PainterGuy said:
If the disk was instead mounted horizontally but still all of the disk rotating in a uniform magnetic field, would it result into production of any eddy currents?
Yes. Emf will be indued along the thickness of the disc.
PainterGuy said:
It is said that an accelerating charge radiates electromagnetic waves, does this mean that the quantity of charge would reduce as a result of emanating electromagnetic waves?
No.
PainterGuy said:
How is the direction of "v" is being determined?
It is the instantaneous linear velocity of a point on the circular disc (with a direction tangential to the circle).
PainterGuy said:
We can see that there is circular magnetic field around wire but where are the north and south poles of this circular magnetic field?
There cannot be a single isolated current carrying conductor, there's always at least one more conductor that acts as a return path for the current. Together they two form a single-turn coil which acts as a bar magnet and you can find its polarity.

PainterGuy
PainterGuy said:
We can see that there is circular magnetic field around wire but where are the north and south poles of this circular magnetic field?
It is not a dipole field so it doesn't have a well defined pole.

PainterGuy
Thank you.

cnh1995 said:
Yes, but there will be a voltage between centre and rim of the disc.

So, no eddy current but an emf will be induced. I was just trying to find some good picture to make my point clear and found out that it used to be called Faraday paradox back in the days. Please check this out: https://en.wikipedia.org/wiki/Homopolar_generator#Disk-type_generator

In the picture below I believe that the rim would have positive potential and the center negative potential. Please let me know if I'm wrong.

cnh1995 said:
Yes. Emf will be indued along the thickness of the disc.

So, you are saying that the eddy will be produced and an emf will be also induced. I had the construction below in mind.

Using right hand rule, it shows the direction of induced current at points A and B, and it looks like that lower and upper halves of the disk will have different charge distribution.

cnh1995 said:
No.

So, it means that an an accelerating charge won't loose its charge. As radiating electromagnetic waves is energy therefore the charge must loose something, and I think it would be mass then.

cnh1995 said:
There cannot be a single isolated current carrying conductor, there's always at least one more conductor that acts as a return path for the current. Together they two form a single-turn coil which acts as a bar magnet and you can find its polarity.

Dale said:
It is not a dipole field so it doesn't have a well defined pole.

@cnh1995 : Let's think of an infinite wire and in such a case we can ignore the return path.
@Dale: Yes, it does look like that it's not a dipole field but a magnet cannot exist as a uni-pole which also implies no magnetic field without a 'dipole' magnet.

Thank you for your time and help.

#### Attachments

• Faraday_disc.jpg
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• vertical_disc.jpg
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PainterGuy said:
@Dale: Yes, it does look like that it's not a dipole field but a magnet cannot exist as a uni-pole which also implies no magnetic field without a 'dipole' magnet.
Just because there are no magnetic monopoles does not imply that all magnetic fields are dipoles. There are many fields that are neither.

PainterGuy
PainterGuy said:
So, you are saying that the eddy will be produced and an emf will be also induced. I had the construction below in mind.
You have joined the center and the rim. I was talking about the same configuration, but not with the center and rim connected together like that, just the rotating disc.
In that case, current will flow from A to B on the upper surface and from B to A on the lower surface.

PainterGuy
PainterGuy said:
So, it means that an an accelerating charge won't loose its charge. As radiating electromagnetic waves is energy therefore the charge must loose something, and I think it would be mass then.
I am not sure if I can comment on this since I haven't studied EM radiation in depth, but just because radiating EM waves is radiating energy doesn't mean the charge has to lose something of its own (charge or mass). The charge is accelerated by means of some energy source and this radiation energy should ultimately come from that source.

PainterGuy
Thank you.

Dale said:
Just because there are no magnetic monopoles does not imply that all magnetic fields are dipoles. There are many fields that are neither.

If you say so, I'd agree with you. But could you please tell me of some fields which are neither monopole or dipole? I'm just curious.

cnh1995 said:
You have joined the center and the rim. I was talking about the same configuration, but not with the center and rim connected together like that, just the rotating disc.

Okay. But in the other configuration where the disc is horizontal, the center and rim were connected too so why is it different for this case when the disc is vertical as shown below?

cnh1995 said:
I am not sure if I can comment on this since I haven't studied EM radiation in depth, but just because radiating EM waves is radiating energy doesn't mean the charge has to lose something of its own (charge or mass). The charge is accelerated by means of some energy source and this radiation energy should ultimately come from that source.

No problem then. But I believe that it should be mass because I did read somewhere in the past that electrically charged body is hard to accelerate compared to an equivalent neutral body, and we can use the relation mass=E/c^2.

Thanks a lot.

PainterGuy said:
If you say so, I'd agree with you. But could you please tell me of some fields which are neither monopole or dipole?
The field around a straight wire for one, and all of the multipole fields greater than dipole, and a plane wave.

PainterGuy
PainterGuy said:
But could you please tell me of some fields which are neither monopole or dipole? I'm just curious
Induced electric field in Faraday's law is not a dipole field.
PainterGuy said:
But in the other configuration where the disc is horizontal, the center and rim were connected too
That connection is necessary for a current to flow. Without that connection, there will only be voltage present between center and rim of the disc.
PainterGuy said:
so why is it different for this case when the disc is vertical as shown below?
For the vertical disc in your diagram, no connection is required between center and rim as the current is flowing along the path I've shown in #6. If you connect the center and the rim, you are providing a parallel path for the current and some current will be diverted through that path.
PainterGuy said:
But I believe that it should be mass because I did read somewhere in the past that electrically charged body is hard to accelerate compared to an equivalent neutral body, and we can use the relation mass=E/c^2.
I don't have enough knowledge about EM radiation and mass-energy equivalence. I can't comment on that.

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PainterGuy

## 1. What is electromagnetism?

Electromagnetism is a branch of physics that deals with the study of the interactions between electrically charged particles and the electromagnetic field. It is a fundamental force that governs the behavior of charged particles and is responsible for many phenomena in our everyday lives, such as electricity, magnetism, and light.

## 2. How does an electromagnet work?

An electromagnet is created by passing an electric current through a coil of wire, which then generates a magnetic field. The strength of the magnetic field can be increased by increasing the number of turns in the coil or by increasing the current flowing through it. This allows electromagnets to be used in a variety of applications, such as lifting heavy objects, generating electricity, and controlling electronic devices.

## 3. What is the difference between an electromagnet and a permanent magnet?

The main difference between an electromagnet and a permanent magnet is that an electromagnet can be turned on and off by controlling the flow of electricity, while a permanent magnet always has a magnetic field. Electromagnets are also typically stronger than permanent magnets and can be adjusted in strength, making them more versatile for practical applications.

## 4. How is electromagnetism used in technology?

Electromagnetism is used in many technological devices, such as electric motors, generators, transformers, speakers, and MRI machines. It is also used in wireless communication technologies, such as radios, televisions, and cell phones. Electromagnetism is essential to the functioning of our modern world and has greatly improved our quality of life.

## 5. What are some common real-life applications of electromagnetism?

Some common real-life applications of electromagnetism include power generation and transmission, electric motors and generators, household appliances, medical imaging, transportation systems (such as trains and subways), and electronic devices (such as computers and smartphones). Electromagnetism also plays a crucial role in many areas of research, such as particle accelerators and fusion reactors.