Rotating magnet inside coil = flux change?

In summary, a lot of car speedometers work by using a rotating magnet inside a coil to induce a current. This is possible because pushing a magnet in and out through a coil can induce current, as the magnetic field acting upon the coil becomes stronger. However, the concept of spinning a magnet inside a coil to create eddy currents that oppose the spinning and create a torque may be difficult to visualize. To better understand this, one can visualize placing half of the coil on one side of the magnet and the other half on the other side. As the magnet turns, its flux goes one way through the coil, then the opposite way as it turns to 180 degrees. This can be seen in a diagram or video, such as the one
  • #1
lluke9
27
0
Apparently, that's how a lot of car speedometers work... but I don't understand how a rotating magnet inside a coil (or any conductive material, really) can induce a current in the coil.

I can understand how pushing a magnet in and out through a coil can induce current, because
Flux = Area * Magnetic field strength * sin θ, and the magnetic field acting upon the coil is becoming stronger.

But I don't see how spinning a magnet inside a coil is going to do anything, much less create eddy currents to oppose the spinning and somehow create a torque.

Could someone refer me to a video or an image of some kind? I've tried to figure this out but nothing really comes to mind.
 
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  • #2
lluke9 said:
Apparently, that's how a lot of car speedometers work... but I don't understand how a rotating magnet inside a coil (or any conductive material, really) can induce a current in the coil.

I can understand how pushing a magnet in and out through a coil can induce current, because
Flux = Area * Magnetic field strength * sin θ, and the magnetic field acting upon the coil is becoming stronger.

But I don't see how spinning a magnet inside a coil is going to do anything, much less create eddy currents to oppose the spinning and somehow create a torque.

Could someone refer me to a video or an image of some kind? I've tried to figure this out but nothing really comes to mind.

Do it like this -- put half the coil on one side of the magnet and the other half on the other side. You can space the two half-coils arbitrarily close together, if you use a short magnet. As the magnet turns, its flux goes one way through the coil, and then the opposite way as it turns to 180 degrees... Make sense?

http://sub.allaboutcircuits.com/images/02002.png

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02002.png
 
  • #3
If you mean a car speedo of the 'old fashioned' type where a needle moves across a scale then this works by causing eddy currents in an aluminium disc by means of a spinning magnet. The disc has a spring to counteract the turning effect and the pointer is connected to the disc. The faster the magnet spins the greater the eddy currents and therefore the greater the drag on the disc caused by the magnet... the greater the pointer deflection.
 

Related to Rotating magnet inside coil = flux change?

1. How does a rotating magnet inside a coil create flux change?

When a magnet rotates inside a coil, it creates a changing magnetic field. This changing magnetic field induces an electric current in the coil, which in turn creates a flux change.

2. What is flux change and why is it important?

Flux change refers to the change in the amount of magnetic flux passing through a surface. It is important because it is directly related to the strength of the magnetic field, and can be used to generate electricity in devices such as generators.

3. How is the direction of the induced current determined in a rotating magnet and coil system?

The direction of the induced current is determined by the right-hand rule. If the fingers of the right hand curl in the direction of the rotating magnet, the thumb will point in the direction of the induced current.

4. What factors affect the amount of flux change in a rotating magnet and coil system?

The amount of flux change depends on the strength of the magnetic field, the speed of the rotation, and the number of turns in the coil. Additionally, the angle between the magnet and the coil, as well as the size and shape of the coil, can also affect the amount of flux change.

5. How is the principle of rotating magnet and coil used in real-life applications?

The principle of rotating magnet and coil is used in various devices such as electric motors, generators, and transformers. It is also used in some types of sensors, such as tachometers and magnetic flow meters.

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