Understanding Magnetic Forces: Drawing Force F Diagrams

[UrbanXrisis]

forces exerted by a magnet

After learning about the right hand rules for a magnet on a wire, my packet asks for me to draw the vector diagram for the force F from a magnetic field B on a moving positive charge with velocity v. What is it looking for? I don't know if B is into or out of the page. How do I know what to draw?

 

[gnome]

If you want to find the force exerted by an external magnetic field on a current-carrying wire (or any moving POSITIVE charge), use the first rule that you described. Start with your fingers straight, pointing in the direction that the positive charge is moving, and with your hand in a position so that you can bend your fingers to point in the direction of the external magnetic field. Now your extended thumb points in the direction of the force. (If the moving charge is negative, the force is in the opposite direction.)

If you want to find the direction of the magnetic field surrounding a conductor carrying an electrical current (the magnetic field that is PRODUCED by the current), point your thumb in the direction that the current is flowing, and your curled fingers show the direction (circular, around the wire) of that magnetic field.

I don't understand the bit that you mentioned about the palm.

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Regarding the direction of the field in your "packet":

Usually, if the field in a diagram is represented by dots: . . . . . the field direction is out of the page (the dots represent arrow-tips). If the field is shown by x's: x x x x the field direction is into the page (the x's are the tails of the arrows).

 

[UrbanXrisis]

If you want to find the direction of the magnetic field surrounding a conductor carrying an electrical current (the magnetic field that is PRODUCED by the current), point your thumb in the direction that the current is flowing, and your curled fingers show the direction (circular, around the wire) of that magnetic field.

what about the force?

 

[gnome]

> what about the force?


Which force?

 

[UrbanXrisis]

the direction of the magnetic force on a charge

 

[gnome]

You have to be more specific if you want to understand what's going on here. Which magnetic force on which charge?

What I described as the second version of the rule

If you want to find the direction of the magnetic field surrounding a conductor carrying an electrical current (the magnetic field that is PRODUCED by the current), point your thumb in the direction that the current is flowing, and your curled fingers show the direction (circular, around the wire) of that magnetic field.

is a magnetic field that a moving charge PRODUCES. That field does not exert any force on the charge that produces it.

If you're asking about a situation where there is a magnetic field from some EXTERNAL source, and the current (or charge) is flowing through that field, use the first version of the rule.

 

[UrbanXrisis]

I think I get it now, I was getting confused with "find the direction of the magnetic field surrounding a conductor carrying an electrical current" but I understand it now.

As for the packet, I think I know what to do. Here's what it says:
"5. But, a current is just charges in motion. From the definition of current (i.e. what charge in what direction), draw the vector diagram for the force F from a magnetic field B on a moving positive charge with velocity v. (This is rather simple; just replace the current arrow with an arrow signifying the direction of the flow or positive charges in the wire.)"

 

[gnome]

OK, so you know you're dealing with positive charges, and you know the direction the charges are moving. It's easy as long as you know the direction of B.

 

[UrbanXrisis]

What direction is the charges moving? All it says is "a moving positive charge with velocity v." So is the vector for current the same as the vector for velocity?

 

[UrbanXrisis]

The actual magnitude is F=qvB*Sin(theta)

Why isn't it cosine?

 

[gnome]

> What direction is the charges moving? All it says is "a moving positive charge with
> velocity v." So is the vector for current the same as the vector for velocity?

Current direction is defined as the direction of flow of positive charge.

> The actual magnitude is F=qvB*Sin(theta) Why isn't it cosine?

Why do you think it should be cosine?

The magnetic force is the charge (a scalar quantity) times the vector cross-product of the velocity and the field. A cross-product involves the sine of the angle between the vectors.

You use cosine when you want a dot-product.