# Find an expression for magnetic flux and calculate

1. Jan 31, 2016

### Matt3175

1. The problem statement, all variables and given/known data
Loop of wire with the following properties in a magnetic field B. Find an expression for the magnetic flux through the loop and evaluate the magnetic flux at t=25s.
The magnetic field is uniform but changes strength at time (t) given by
B(t) = B0 exp (kt)

Resistance = 20ohms
Angle of loop in magnetic field = 45 degrees
B0 (constant) = 7.00x10^-2 T
k (constant) = 0.250 s^-1

2. Relevant equations
Flux = AB cos theta
Area of circle = Pi r^2

3. The attempt at a solution
So I'm working out B first from the function as:
B(t) = 7.00x10^-2 T x exp x 0.250 s^-1 x 25s. = 1.189 T
Area of loop = Pi x 0.10m^2 = 0.0314m^2
Then using the flux equation with these values I get:
0.0314 m^2 x 1.189 T x cos45 = 0.0264 Wb

Is this correct? Thanks in advance

Last edited by a moderator: Jan 31, 2016
2. Jan 31, 2016

### rude man

3. Jan 31, 2016

### Matt3175

Sorry are you asking for clarification?

4. Jan 31, 2016

### J Hann

e^(25 * .25) = e^6.25 = 518
.07 * 518 = 36.3
That differs considerably from the 1.19 that is shown for B.

5. Jan 31, 2016

### rude man

Sorry, didn't see the cos45 term.