You are Here: Home >< Physics

Help with a question on simple motors

Announcements Posted on
Four hours left to win £100 of Amazon vouchers!! Don't miss out! Take our short survey to enter 24-10-2016
1. Hi Everyone,

I am a bit stuck on this question from a past AQA paper, unfortunately I can't find a mark scheme to help!The question is about the coil and magnets of a simple motor. The coil has 24 turns and carries a current of 4.3 A. The coil is made from copper wire of diameter 0.36 mm. The uniform magnetic field produced by the magnets has a flux density of 0.17 T.The work done to rotate a coil 90 degrees is 0.084 J.

5 (b) (ii) The conversion of work done to the kinetic energy of the coil is 83% efficient. Assume that only the long sides of the coil share the kinetic energy.The coil starts from rest. Show that the speed of a side of the coil after it has rotated through 90 degrees is approximately 5.2 m s-1.

density of copper = 8900 kg m-3

My attempt so far:

0.084 * 0.83=0.69 J

KE=1/2 mv2
=0.69 J

v=sqroot(2KE/m)

m=p x A x L

A=(pi) x 0.000182 =1.02 x 10-7 m2

I have tried various values for m because I'm not sure if I need to consider m as the mass of 24 lots of wire on both sides of the coil or just one side? but nothing gives the answer as 5....Anybody able to help? I have attached the whole question paper as I couldn't do the question on its own, but it is question 5 if anybody needs to look at it. Thanks!
Attached Images
2. AQA-PHB5-W-QP-JUN09.PDF (302.5 KB, 30 views)

Register

Thanks for posting! You just need to create an account in order to submit the post
1. this can't be left blank
2. this can't be left blank
3. this can't be left blank

6 characters or longer with both numbers and letters is safer

4. this can't be left empty
1. Oops, you need to agree to our Ts&Cs to register

Updated: June 6, 2016
TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Today on TSR

Who is getting a uni offer this half term?

Find out which unis are hot off the mark here

How to get a 1st class degree

Poll
Study resources

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.