1. Oct 13, 2014

### rbn251

1. The problem statement, all variables and given/known data

A rectangular loop of wire, of sides 5 and 3 cm moves away from a a long straight wire at 4ms-1 carrying a current of 2 amps. What is the EMF induced in the moving loop, when the near side of the loop is 3cm away from the wire?

2. Relevant equations

B = μ0I/2πr
EMF = -dΦ/dt

3. The attempt at a solution

EMF = -dΦ/dt = -dAB/dt where A is the area, and B is the field.

I am not sure what to do now as the answers I have seen treat A (the area) as changing?

As I see it the area of the loop (A) is constant, and B changes as the loop moves out along the radius (the field gets weaker). Can anyone clarify which is right?

2. Oct 14, 2014

### BvU

A stays the same.
What's unclear to me is which of the two (3 cm or 5 cm) sides is parallel to the current wire ...

3. Oct 14, 2014

### rude man

You can also use the Blv law. That is actually preferred since for moving media emf = -dΦ/dt doesn't always hold.

The Blv law is emf = (B x l).v
or more generally
d(emf) = (B x dl).v
l = length of wire
v = velocity of wire
Apply the formula to both loop lengths (parallel to the wire) and subtract.