# Homework Help: Rotating Coil in Magnetic Field

1. Mar 15, 2016

### dukesolice

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

Magnetic field values are often determined by using a device known as a search coil. This technique depends on the measurement of the total charge passing through a coil in a time interval during which the magnetic flux linking the windings changes either because of the motion of the coil or because of a change in the value of B. As a specific example, calculate B when a 61-turn coil of resistance 194Ω and cross-sectional area 44.5m^2 produces the following results: A total charge of 4.76E-4C passes through the coil when it is rotated in a uniform field from a position where the plane of the coil is perpendicular to the field to a position where the coil's plane is parallel to the field.

2. Relevant equations
emf = -change in flux

3. The attempt at a solution

emf = -d/dt (integral of B dot dA)

The change in flux is from when the coil is perpendicular to the field to when the coil is parallel to the field. So 90 degrees. But I don't know how to get the change the in area, and how to use the charge and resistance.

2. Mar 15, 2016

### TSny

The shape of the coil doesn't change as the coil rotates. So, the area of the coil remains constant.

You'll need to bring together several elementary concepts in this problem. How is charge related to current and time? How is current related to resistance and emf?

3. Mar 15, 2016

### dukesolice

Current is charge/time. Emf = current * resistance.

4. Mar 15, 2016

### TSny

OK. These relations along with Faraday's law give you everything you need.

Try combining them to get a relation between the total charge and the change of flux.

5. Mar 15, 2016

### TSny

This is not correct. Faraday's law also involves time.

6. Mar 15, 2016

### dukesolice

So now I have dQ/dt * R = -d(flux)/dt, so then I took integral of both sides and got NRQ = -BA, where N is number of coils, this gave me the wrong answer

7. Mar 15, 2016

### TSny

OK, but should the number of turns, N, appear somewhere here?

Show the steps in getting to this result. Why does N appear on the left?