You are Here: Home >< Physics

# Edexcel PHY5 Question Magnatism watch

1. Hi,
thanks.

Your given a wire (Y) Carrying a current I (see attached picture)
Your then told that a parrallel wire (Z) is placed 12cm away from wire carrying a current of 2I . We are also given the direction of the force (Fz) on wire Z. From this we have to work out the direction of the current in Wire Z, the direction of the force acting on wire Y and the ratio of the two forces.

Any help would be appreciated.
Attached Images

2. between two long wires,when their direction is opposite the force is repultion and the ratio is 1:1
3. (Original post by habosh)
between two long wires,when their direction is opposite the force is repultion and the ratio is 1:1
Thanks but I don't understand why the ratio is 1:1, if force is calculated by F= BIL and the current in one is twice the current in the other shouldn't the ratio be 2:1
4. ITs from newtons 3rd law i think the force on one thing is equal and opposite on the other thing therefore the ratio must be 1:1
5. Fz = BIL
Z is in a field of - mu0I/2pir created by Y
Fy = BIL
Y is in a field of - mu0I/2pir created by Z
Twice the current means that Y is in twice the magnetic field but since it only carries half the current the force on it is a half giving 1-1.
6. Fz = BIL where I = 2I
Fz = 2BIL
B = muoI/2pir
Therefore Fz = 2mu0I^2/2pir
Fy = BIL where I = I
B = mu0(2I)/2pir
Therefore Fy = 2mu0I^2/2pir
Fy = Fz
7. (Original post by Economist)
Fz = BIL where I = 2I
Fz = 2BIL
B = muoI/2pir
Therefore Fz = 2mu0I^2/2pir
Fy = BIL where I = I
B = mu0(2I)/2pir
Therefore Fy = 2mu0I^2/2pir
Fy = Fz
Oh thanks alot that makes sense, it was a one mark question, apparently its blindingly obvious but never mind. Thanks

### Related university courses

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:
Updated: June 15, 2005
The home of Results and Clearing

### 3,454

people online now

### 1,567,000

students helped last year
Today on TSR

### IT'S TODAY!

A-level results chat here

### University open days

1. Bournemouth University
Fri, 17 Aug '18
2. University of Bolton
Fri, 17 Aug '18
3. Bishop Grosseteste University