# Homework Help: Force on a current carrying wire

1. Aug 12, 2009

### vorcil

a)
A current‐carrying wire bent into a semicircle of radius r forms a close circuit as shown in the figure. A uniform magnetic field is directed normal to the linear section and in the plane of the loop. Show that the magnetic force acting on the closed wire loop is zero.

b)
List three items which operate on the principle “force acting on a current‐carrying conductor in a magnetic field”.

- n.b these aren't homework questions, i'm doing an experiment tomorrow and this was in a book i'm reading (related to the experiment)

I'm not sure how to solve the first one, can I do this mathematically??? or is it more of a geometrical problem?

b - not sure if I know any practical applications of force on a current carrying wire,
All i can think of is the magneto in microwaves
anyone know any?

Last edited by a moderator: Apr 24, 2017
2. Aug 12, 2009

### vorcil

3. Aug 12, 2009

### ideasrule

I can think of two applications that are in your computer. You can probably hear one of them, and you listen to the other every day.

4. Aug 12, 2009

### Staff: Mentor

How do you find the magnetic force on a current-carrying wire?

5. Aug 12, 2009

### vorcil

Use the lorentz force law,

Fmag = Q*(v*b)

then use the right hand rule, it says the magnetic field is into the semi circle, perpendicular to it's normal

not really sure how to solve it from here

6. Aug 12, 2009

### Staff: Mentor

When dealing with a current, use:
$$\vec{F} = \vec{I}\ell \times \vec{B}$$

7. Aug 12, 2009

### vorcil

8. Aug 12, 2009

### Staff: Mentor

What makes you think that the force on the semicircle is zero?

What's the force on the straight segment?

Hint: Find the force on an element of the semicircle, then integrate over the entire segment.

9. Aug 12, 2009

### vorcil

http://img529.imageshack.us/img529/6398/questionk.jpg [Broken]

that's why i think it's 0, the question basically says it

-

would i get the circumference

(2*pi*r)/2 + (2r) = the length
= pi*r + 2r = total length of wire,

it's a closed loop with no apparent battery source

Last edited by a moderator: May 4, 2017
10. Aug 12, 2009

### Staff: Mentor

There are two segments: the semicircular segment and the straight segment. The net force on the entire closed loop is zero. To show that, compute the force on each segment.