EasyPhysicsHigh School (9–10)Electricity & Magnetism › Magnetic Effects of CurrentConceptualBoard Exam
Oersted placed a compass needle under a wire…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
Oersted placed a compass needle under a wire.
- What did the needle do when the current was switched on?
- What did it do when the current was switched off?
- What did it do when the current was reversed?
- What did this show?
- Would the needle swing more or less if the wire were raised to twice the height above it? Why?
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) The needle was deflected, turning away from north.
Step 2. (b) It came back to point north.
Step 3. (c) It was deflected the other way.
Step 4. (d) A current has a magnetic field, whose direction follows the current. Electricity and magnetism are linked.
Step 5. (e) Less. At twice the distance, the wire's field is half as strong, so it turns the needle less against the Earth's field.
Answer: deflected; back to north; deflected the other way; a current makes a magnetic field; less, because the field is weaker farther away.
Check. The only new thing near the needle was the current, so the current must make the field.
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