# Homework Help: Parallel wires

1. Jan 30, 2005

### decamij

If you have two parallel wires, each producing a magnetic field, what would the direction of the magentic field be if:
1) Current flowed in the same direction in each wire.
2) Current flowed in opposite direction in each wire.

2. Jan 30, 2005

### da_willem

Do you know how a magnetic field around a current carrying wire looks like? What direction are the field lines? Do you know what the force on a currnet carrying wire in a magnetic field is? What is the direction of this force?

3. Jan 30, 2005

### HallsofIvy

Right hand rule! Point the thumb of your right hand in the direction of current in the wire and your fingers curl around the wire in the direction of the field lines. What happens between the wires when the current is in the same direction? In opposite directions?

4. Jan 30, 2005

### decamij

When the current is in the same directions, the magentic field between the wires are in different directions. I know that the magnitude of magnetic field will decrease relative to if the current was in different directions, but what do I say for direction? I know about the right hand rule, but how do word the direction? Into/out of the page? Away from the wires? I'm not sure.

5. Jan 30, 2005

### da_willem

Now you know what the magnetic fieldswill look like all you have to know is what is the direction of the force on a wire in a given magnetic field.

6. Jan 30, 2005

### decamij

If you know the direction and magnitude of the force on each wire, you use vectors to determine the net direction, right?

7. Jan 30, 2005

### da_willem

The force on a charged particle in a magnetic field is given by the Lorentz force law: $$\vec{F}=q(\vec{v} \times \vec{B})$$

Now current is flowing charge right? And qv is in the direction of the current! With this information, and some knowledge of the cross product (right hand rule, or corkscrew rule), you should be able to figure out the direction of the force.

8. Jan 30, 2005

### christinono

To know the DIRECTION of the force, all U have to do is:
1) use the right hand rule (quoted below). No fancy formulae are required.
2) use this hand rule: right hand open, fingers straight out, thumb to the right. Fingers (except the thumb) point in the direction of the magnetic field, thumb points in the direction of the moving POSITIVE charge (convertional current) and palm of hand points in the direction of the force.

9. Jan 30, 2005

### decamij

But if you don't know the direction of the current, just that they are in the same or different directions on each wire, how do u know the general direction? i.e. in what way will the magnetic fields "interfere" with each other, and what would the net electric field be?

10. Jan 31, 2005

### da_willem

Just draw a picture, in your head or on paper of a current carrying wire with trhe direction of its current Now find out what the magentic field will look like. Draw another wire parallel carrying current in the same or opposite direction. Now what is the direction of the force? You know the direction of the magentic field at the place of the second wire, and you know the driection of the current its carrying. I'm sure you can work it out!

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