A compass with its needle pointing north sits on a table. If you place a straight wire over the compass parallel to its needle and send a current running north to south through the wire, how does the compass needle respond to the current?
a. The compass needle deflects toward the west
b. The compass needle deflects towards the east
c. The compass needle does not do anything
d. The compass needle turns around to point South
The compass needle responds to the current flowing through the wire by deflecting. To understand the direction of the deflection, you can use the right-hand rule for electromagnetism. Here's how you can determine the answer:
1. Extend your right hand with your thumb, index finger, and middle finger perpendicular to each other, creating three mutually perpendicular axes.
2. Point your thumb in the direction of the current, from North to South (as given in the question).
3. Imagine the magnetic field lines around the wire running from East to West, forming concentric circles.
4. The compass needle is sensitive to the Earth's magnetic field, which is approximately North to South. The needle tries to align itself with the magnetic field, so it will point towards North.
5. Using the right-hand rule, if you point your index finger (representing the magnetic field lines) towards the West on the imaginary circles, the compass needle (represented by the middle finger) would deflect towards the East.
Therefore, the correct answer is b. The compass needle deflects towards the east.