Night 2 · Core lab
Compare plastic and a steel keeper
Parts · Small classroom horseshoe magnet, compass, mild-steel keeper long enough to bridge both poles, plastic or card strip, and tape.
1Predict
Predict whether the compass deflection at the same point will get smaller, larger, or stay about the same when you swap plastic for steel.
Help with the prediction
A mild-steel keeper provides a path for flux between the poles. The field outside the magnet can change. The compass compares the field at its own location, so placement matters.
2Set up the station
Use a nonmagnetic surface and small classroom magnets. Move steel tools and other magnets away. Keep the compass flat.
Reveal the setup diagram
- With the magnet away from the compass, mark the resting needle direction as zero. Clockwise is positive. Counterclockwise is negative.
- Tape the horseshoe magnet to the work surface. Put the compass just beyond the pole opening, centered between the two poles. Rotate the whole arrangement so the line between the poles is roughly perpendicular to the baseline.
- Check that the magnet gives a clear deflection. If needed, adjust the spacing, then mark the final positions before recording either trial.
- Bridge both pole faces with the plastic or card strip for the control. Replace it with the steel keeper at the same location. Keep fingers clear as steel pulls toward the poles.
3Measure and check
Record signed angles relative to your baseline. Repeat an unexpected reading before changing the setup.
Show the flux-density calculation
For a uniform field perpendicular to the area, B = Φ ÷ A. Convert 8 cm² to 0.0008 m². Divide 0.00024 Wb by 0.0008 m². This is a separate paper example. The compass supplies an angle.
4Explain
What changed in the setup? Explain how a steel path can redirect the field and why one reading cannot describe the field everywhere.
Optional extension
Remove the keeper and repeat the plastic reading to check whether the setup drifted. With your instructor, sample a second point and compare both materials there.
Show the instructor your setup, readings, and explanation.