# Emf induced in rotating coil

## Homework Statement

A rectangular coil of 200 turns has a length of 200 mm and width
120 mm. The coil rotates with a constant angular speed of 1200
revolutions per minute about an axis through the midpoints of its
longer sides in a uniform magnetic field of 2.4 x 10-2 T.

Starting from a time when the coil’s plane is parallel to the
magnetic field, calculate the average induced electromotive
force whilst the coil is turning 1/(2pi) radians.

## Homework Equations

E(emf) = N x B x w x A x sin (theta)
where N = number of turns
B = magnetic field (T)
A = area of coil
w = angular velocity (rad/s)
theta = angle made by coil wrt perpendicular to the field

## The Attempt at a Solution

Angular velocity = 40pi (1200rpm)

As the coil rotates through the field plane the induced emf is at the maximum (sin (theta) = 1)
where
E = 200 x 2.4 x 10-2 T x 40pi x 0.120m x 0.200m x sin (pi/2) = 14.47V

After rotating towards the vertical through the rather small angle 1/(2pi) radians:
E = 200 x 2.4 x 10-2T x 40pi x .120m x .200m x sin (pi/2 - (1/(2pi)) = 14.29V

The given result is 9.216V so I've obviously missed something. Yet - it seems unusual that after rotating through such a small angle that the induced voltage should fall as low as the given answer.

Help gratefully received!

## Answers and Replies

rock.freak667
Homework Helper
if w=1200rpm why do you have 1200*40pi?

I am not using 40pi * 1200: the rotational speed is 40pi which is used in the expression which requires angular velocity in rad/s. The 40pi comes from converting 1200rpm to rad/s. I then use the value 40pi for w (omega) in the expression..l.

Comments anyone? Help and advice gratefully received... :)