Electromagnetic Effects: Question 10
Syllabus 4.5.1
A straight horizontal metal rod lies at right angles to a magnetic field that points vertically upward. The rod is moved directly out of the page, towards you, while remaining at right angles to the field throughout.
Using Fleming's right-hand rule (the "dynamo rule") for electromagnetic induction, what is the direction of the induced current in the rod?
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Worked solution
Step 1: Recall Fleming’s right-hand rule
For a conductor being moved through a magnetic field (a generator/dynamo situation), Fleming’s right-hand rule gives the direction of the induced current:
- First finger Field
- thUmb direction of motion
- seCond finger Current (induced)
This uses the right hand, unlike the motor-effect rule (Fleming’s left-hand rule), because here the motion is given and the current is the unknown, instead of the current being given and the force being the unknown.
Step 2: Set up the right hand for this situation
Point the First finger vertically upward, in the direction of the field. Point the thUmb directly out of the page towards you, in the direction the rod is being moved. These two, held at right angles, fix the orientation of the right hand.
Step 3: Read off the direction of the seCond finger
With the First finger up and the thumb out of the page towards you, the seCond finger of the right hand points out to the left. This is the direction of the induced current along the rod.
Why the other options are wrong
| Option | Direction | Error |
|---|---|---|
| B | To the right | Right-hand rule applied back-to-front (equivalent to using the left hand) |
| C | Along the field, upward | Current is at right angles to the field, not parallel to it |
| D | Along the motion, out of the page | Current is at right angles to the direction of motion, not parallel to it |
Final answers
- The induced current in the rod flows to the left, option A.