# True or false? a conductor through a uniform magnetic field produces NO emf

1. Nov 18, 2012

### Xabrewulf

Many sources on the web claim that when moving a conductor through a magnetic field, an emf is produced.

However, some sources claim, that the intensity of the magnetic field also needs to change in order to produce a voltage over the conductor, for example:

What's the deal? Where lies the misconception?

In my understanding, when you move a conductor through a uniform magnetic field, you have no rate of change of flux, hence you have no emf produced.

Is this correct? Or am I wrong?

2. Nov 18, 2012

### Staff: Mentor

It depends on the shape of the conductor. If it is a closed loop, the magnetic flux through the conductor has to change, otherwise the (non-zero) potential differences in the loop will cancel each other. If it is a straight line (and you close the loop somewhere else to measure the voltage), a constant magnetic field is fine. You can calculate the flux through that big loop, and it will change (unless you move the cable parallel to the magnetic field).

3. Nov 18, 2012

### andrien

there is always an induced emf whenever flux change i.e. B.A changes with time.when it moves through magnetic field then in most cases area changes and hence an emf.If B is changing then again an emf is there.

4. Nov 26, 2012

### Xabrewulf

Ok, so if I have coil with 5 loops, and it is not closed (because it is closed with a voltage meter) than no EMF will be generated if I move the coil through a changing magnetic field??

5. Nov 27, 2012

### Staff: Mentor

That is closed, just with a high resistance at one point.

If you move the coil through an inhomogeneous field, you will get a voltage in the general case.

6. Nov 27, 2012

### digvijay kuma

if magnetic field is non uniform then its not a problm u can directly calculate emf induced since flux is changing with time factor but in case of uniform mag. field u need to consider cases wether its a loop or of any other shape if a ring is rotating about its axis parallel to mag field than no net emf is induced but in case of disc it behaves as a rod an emf is induced equl to the rod of length =radious of disc. actually your thinking is correct but emf is viwed here wether feild lines are being cut or not ok

7. Nov 27, 2012

### Enthalpy

The subtlety is where you consider the induction: at the conductor or in the loop.

The flux must change to produce a voltage, which means the induction in the loop must change, but this can happen as the induction at the wire remains constant - at least over soem distance.

One example is an abnormally designed loudspeaker, with long concentric pole pieces producing a uniform radial induction, and a short coil fully immersed in the uniform induction. The induction is constant at the wire, but the flux (per turn...) through the coil changes between two coil positions by the amount that passes between the poles over the distance.

This is for fields varying slowly as compared to light propagation's time over the dimensions. In an antenna, everything is more complicated.

8. Nov 27, 2012

### 2milehi

This reminds me of a lesson back in college - if a wire is cutting lines of magnetic flux, a voltage is produced.