A long vertical wire carries a current downward, toward the floor.
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A long vertical wire carries a current downward, toward the floor. You look at it from above.
- Does the magnetic field go clockwise or anticlockwise round the wire?
- A small compass is placed on the floor just north of the wire. Ignore the Earth's field. Which way does its north end point?
Need a reminder? The key ideaShow
A current has a magnetic field (Oersted). Round a straight wire the lines are circles; the right-hand thumb rule gives their direction. The field of a long wire is , in tesla, with in metres: it grows with the current and falls as .
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. (a) Point the thumb down, away from you. The fingers curl clockwise as seen from above.
Step 2. (b) North of the wire is the top of the circle as you look down. On a clockwise circle, the top point moves to the right, which is east.
Answer: (a) clockwise. (b) east.
Check. A current into the page always gives a clockwise field, and this is the same case.
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