Teaching Physics with the Physics Suite Edward F. Redish

Comparing electric and magnetic forces: 2

In the picture below are shown a long wire carrying a current I to the right and a long amber rod with a charge density (charge/unit length) of l. Assume I and l are both positive.

1. The two are separated by a distance d. The point marked x is half-way between them. On the diagram, draw arrows to represent the following. (Be sure to label your arrows clearly so we can tell which one is which.)
1. the direction of the magnetic field at the point marked x.
2. the direction of the electric field at the point marked x
3. the direction of the electric force that a charge q placed at x would feel
4. the direction of the magnetic force that a charge q placed at x would feel if it were moving to the right.
2. The current, I, is +10 A, the charge density, l, is -1 nC/m (=10-9 C/m) (note that it is negative), and the distance between the wires is 40 cm. At the instant shown, a proton with charge q = 1.6 x 10-19 C is moving into the page with a speed v = 106 m/s. Ignoring gravity, what is the magnitude and direction of the net force it feels at that time?

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