# Relationship between Magnet Rotation Speed and Induced Voltage in a Solenoid?

• ajax
In summary, the conversation revolved around the relationship between the speed of rotation of a magnet and the induced voltage in a current-carrying solenoid. The discussion included the effect of rotation on the direction of current in the solenoid, the importance of placing the magnet near the solenoid rather than inside, and the experimental setup involving a solenoid connected to a DC source voltage and a bar magnet attached to a motor. The conversation also touched on the orientation of the rotation axis and the magnet's north-south axis, and how a faster rotation speed results in a greater changing magnetic field and thus a greater induced current, according to Faraday's law. The conversation was taking place between high school students on a physics forum
ajax
Hello all,

We've been studying electromagnetism and motors in physics. I was just wondering, if you placed a rotating magnet near the end of a current-carrying solenoid, what would the relationship be between the magnet's speed of rotation and the induced voltage in the solenoid?

Thanks

Hi ajax,
I don't understand why you say 'current-carrying'. What you are interested in is voltage, so what for do you need a current?

Welcome to Physicsforums Ajax!

I can't remember the name for this effect, but I think rotation of the magnet would change the direction of the current in the solenoid in about every half a turn. This effect stated when the magnet is approaching a current carrying solenoid, the current carrying solenoid would create a magnetic pole repell the magnet.

e.g. If the north pole of a magnet is approaching a current carrying solenoid, the solenoid would adapt a current in which it produces a similar pole to repell the magnet.

Thanks for your replies. As I understand it (and I could well be wrong), the effect is indeed something to do with half-cycle current reversals (ie. AC), but other than that I'm not sure.

As for a current-carrying solenoid, I don't think there would be any effect of a plain magnet on the coil, but when the solenoid has a current, it then has an electromagnetic field that can be "fiddled" with. I suppose rather than voltage, I'm interested in any effects on the solenoid.

That's also why placing the magnet near the solenoid (rather than inside) is important. As I'm sure everyone knows, the field within a solenoid is linear.

Perhaps the answer has something to do with how the magnet interferes with flux?

ajax,
I still don't quite understand your experimental setup. Please tell me what causes the magnet's rotation? And what causes the current in the coil?

Sorry for not having been quite clear with a description.

As I picture it, you would have a solenoid connected to a DC source voltage (perhaps also in series with a rheostat so as not to overheat the solenoid).

As a separated piece of equipment would be a bar magnet attached to a small motor, so that the motor would cause the magnet to rotate (I was thinking the magnet would be attached so as to rotate about its longitudinal axis).

Finally, an oscilloscope or digital multimeter could be connected across the solenoid to measure any effects of altering the rate of the magnet's rotation.

I hope this helps. If I'm still not making sense, I can draw up a circuit diagram for you if you like.

Yes that helps.

- What is the relative orientation of the rotation axis relative to the solenoid axis?
- What is the orientation of the magnet's north-south axis relative to the rotation axis?

Maybe a drawing of the whole setup would indeed do good.

think of it this way,

if the magnet is spinning/rotating faster, then there would also be a greater changing magnetic field [ greater as in the difference between the change of the magnetic field for a particular spot, if u know what i mean].

thus by referring to faradays law, the induced current is proportional to the changing magnetic field, u can deduce that induced current would be greater.

let me know what u think, i am in a similar situation[high school].

## 1. What is induced voltage in a solenoid?

Induced voltage in a solenoid is the voltage generated in a coil of wire, known as a solenoid, when there is a change in the magnetic field passing through it. This change in magnetic field can be caused by a variety of factors, such as the movement of a magnet or the change in current flowing through a nearby wire.

## 2. How is induced voltage in a solenoid calculated?

The induced voltage in a solenoid can be calculated using Faraday's Law, which states that the magnitude of the induced voltage is equal to the rate of change of the magnetic flux through the solenoid. This can be expressed as V = -N(dΦ/dt), where V is the induced voltage, N is the number of turns in the solenoid, and dΦ/dt is the change in magnetic flux over time.

## 3. What factors affect the magnitude of induced voltage in a solenoid?

The magnitude of induced voltage in a solenoid is affected by several factors, including the number of turns in the solenoid, the strength of the magnetic field passing through it, and the rate at which the magnetic field changes. Additionally, the material of the solenoid and its dimensions can also impact the induced voltage.

## 4. How can induced voltage in a solenoid be used?

Induced voltage in a solenoid can be used in a variety of applications, including generators, transformers, and electromagnets. It is also the principle behind many electronic devices, such as motors, speakers, and microphones.

## 5. Is induced voltage in a solenoid always beneficial?

While induced voltage in a solenoid can be useful in many applications, it can also cause problems in certain situations. For example, in electronic circuits, it can create unwanted noise and interference. In power transmission systems, it can cause voltage drops and power losses. Therefore, it is important to carefully consider and control induced voltage in solenoids in different applications.

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