# Induced Voltage

1. Jan 7, 2010

### bigmack

Induced Voltage = -NBA/t
I know the equation, but could someone please explain it to me?

2. Jan 7, 2010

### Staff: Mentor

Try this: http://hyperphysics.phy-astr.gsu.edu/HBASE/electric/farlaw.html" [Broken]

Last edited by a moderator: May 4, 2017
3. Jan 7, 2010

### bigmack

hmm that looks good, but how do you get "A" ?
oh and why is there a negative sign?

4. Jan 7, 2010

### Staff: Mentor

"A" is the area of the coil.
Read about Lenz's law at the bottom of that same page.

5. Jan 7, 2010

### bigmack

ok. but how do you find the area of the coil?

6. Jan 7, 2010

### Staff: Mentor

Using a bit of geometry. For a circular coil, the area is $\pi r^2$.

7. Jan 7, 2010

### bigmack

ok so wait, does the radius of the coil include the diameter of the wiring?
or is it just the area within the coil, and not including the wire

8. Jan 7, 2010

### Staff: Mentor

Measure the radius to the center of the wire. (For thin enough wires, it won't matter.)

9. Jan 7, 2010

### bigmack

oh ok.

so lets just see if i get it.
to calculate the induced voltage, you check the area of the field, you multiply it with the strength of the magnet, divide by the speed of the wire and multiply by the number of turns of the wire.

wait, what are the units?
B is in Tesla, right? but what about A and t ?

10. Jan 7, 2010

### Staff: Mentor

What you really want is: Induced Voltage = -N Δ(BA)/Δt, where Δ(BA)/Δt is the rate of change of the magnetic flux (BA). (Read the explanation on the website I linked.)
Yes, the magnetic field has units of Tesla. Area has units of meters²; time has units of seconds.

11. Jan 7, 2010

### arunma

I'm not sure I understand the question. The induced emf on a coil of wire depends on the wire's shape, the rotation speed, the time dependence of the magnetic fields, and other specifics of the problem. The usual example is a coil of circular wire spinning in a constant magnetic field, in which case the induced voltage is,

$$emf = \omega NBA cos\left(\omega t\right)$$

The equation you gave says that the induced emf will decay over time, tending to zero. What's the physical situation here?

12. Jan 8, 2010

### bigmack

Ok thanks, you've been a lot of help.

Its the upper right picture in Doc Al's link, the one in which you move a magnet in a coil

13. Jan 8, 2010

### bigmack

wait no thats not what i meant.

im confused. what you're saying has a magnet cut the field lines. what i was thinking was the field lines could be cut by the coil, is that possible?

i mean can i place two magnets side by side, and drop a coil of wire in between them to induce voltage?

14. Jan 8, 2010

### bigmack

Help with Gauss

To calculate the voltage induced when cutting magnetic field lines you need to know the strength of the magnets in Gauss.

I was looking at some magnets I would like to buy for a project.
The magnets had 2 different values for the gauss.

"Brmax (Residual Induction) or Residual Flux Density "

and

"Surface Field (Surface Gauss)"

which reading is used to calculate the voltage induced?

15. Jan 8, 2010

### Staff: Mentor

Is there a specific problem you are trying to understand?

That formula you started the thread with is a simplified form of Faraday's law, as I tried to explain. A more useful form is what I gave in post #10 (and is described in the link). But that version is also limited to certain situations--a more complicated situation (where the coil rotates, for example) requires a different version of Faraday's law.

What are you trying to do?

16. Jan 8, 2010

### bigmack

ok.
im trying to generate current by cutting magnetic fields with wiring coiled around an iron core.

all i want to know is how much current im going to get and the voltage too.

17. Jan 9, 2010

### bigmack

if you can help some more id really appreciate it

18. Jan 9, 2010

### Staff: Mentor

That's not a simple problem. You need to know the rate at which the magnetic flux through your coil is changing at any given time. That will allow you to find the induced voltage at that moment using Faraday's law.

19. Jan 9, 2010

### bigmack

so how would i do that?
the speed at which the coil cuts fields?
can you like just give me a worked out example, using whatever numbers you want.

20. Jan 13, 2010

### bigmack

hello??

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