Electromagnetic Induction

Subject Area

Electricity and magnetism

Age or Grade

9^th grade, Active Physics Curriculum

Estimated Length

30-40 minutes

Prerequisite knowledge/skills

Students should be familiar with current, ammeters, and magnets.

Description of New Content

Electromagnetic induction

Goals

Students should understand that electric current produces a magnetic field, and that a changing magnetic field through a coil of wire produces current through the wire.

Materials Needed

Part I:

-copper tube (1 ft or so long, and ¾” in diameter)
-identical looking metal and magnetic ball small enough to fall through the copper tube.

Part II:

- solenoid (55o turns)
- ammeter
- a battery (3V)
- a compass

Part III:

-A solenoid
-ammeter
-A very strong magnet that can fall, or be pushed through the solenoid.

Part IV:

-a miniature simple, home made motor analogous to THIS ONE.
- battery (3V)

Procedure

This is a very interactive lesson framed as a mystery that is to be solved by the students with the guidance of the instructor.

Opener:

Start with a “magic trick.” Ask two volunteers to come up to the front of the class. Give to one student the metal ball, and the magnetic ball to the other. Then ask each to drop the ball through the pipe. The magnetic ball should take significantly longer time to get through – changing magnetic field through the copper pipe induces a current in the pipe, and the magnetic field due to the induced current exerts a vertical force on the falling magnet.

Then ask students for an explanation. They by default yell “its a magnet,” and will be quite baffled when they see that the magnetic ball doesn't get stuck to the copper pipe. So there is a mystery to be solved!

Development:

Part II

Put together a circuit with a battery, ammeter, and a solenoid. Show each element of the circuit carefully to the students. Then, pull out a compass and ask them if they know what the compass does. There should be at least one student who knows that the compass tries to align with the magnetic field of the Earth. Then put the compass next to the solenoid, but not too close, and ask one of the students to close the circuit, and ask the rest to watch the compass. As the circuit is closed, the compass needle should deflect. Conclusion: current creates a magnetic field, and the loop of wire acts like a magnet.

Part III

Take out the battery out of the circuit, but keep the ammeter. Ask a student to quickly put in and pull out the magnet out of the solenoid. The ammeter should read some current. So magnets make the currents?

Ask a student to slowly place the magnet inside of the solenoid. No current will be detected. Then ask students how are the two cases different. The conclusion of this part should be that the moving magnet-changing magnetic field through a loop of wire causes a current to flow through the loop.

Part IV

Assemble the motor and describe to students the parts of it. Ask a volunteer to come up and connect the motor to the battery. The motor should spin. Take a way the magnet, and the motor will stop spinning. Put the magnet back in, and the motor should start spinning again. Compare the motor to two magnets that are repulsing – they should know by now that a current produces a magnetic field.

Closure: Go back to the copper pipe and the magnetic ball. Draw similarities between the solenoid and the pipe.

Evaluation

Students were ask to write down three things they learned during the investigation. At least one should be about the current producing magnetic field, and the changing magnetic field through a coil producing current.

Extensions

One can as students what would happen if a slit had been cut down the side of the pipe – in the best case scenario, one would have such a cut pipe with them. With such a pipe, metal and magnetic balls fall at the same speed, as the “circuit” has been cut and the current cannot be induced.

References