# Which is the correct equation to use

1. Aug 25, 2015

### David J

1. The problem statement, all variables and given/known data
If the magnetic flux linking all of the turns of a 50 turn coil changes from 10 mWb to 20 mWb and induces an e.m.f. of 62.5 V in the coil, calculate the time over which the flux changes

2. Relevant equations
Equation 1: E = - N x the change in flux / the change in time
Equation 2: E = N x the change in flux / the change in time

3. The attempt at a solution

I have seen this question on a number of occasions and the answer appear to be 0.8ms but my question is which is the correct equation to use? Equation 1 or Equation 2 and why are there 2 equations, one using -N and the other using N?

2. Aug 25, 2015

### BvU

You are talking about Faraday's law here. In your exercise they don't worry about the signs. If you have to worry about the sign, then it's good to remember that nature likes to counteract change. If an external magnetic field drops to zero, the induced emf will be such that a magnetic field due to that emf in the loop points in the same direction.

(And vice versa. See hyperphysics on the lower left: if you push a loop into a field, the induced field opposes the external field)

Apparently this counteracting has a name too: Lenz's law.