1. Chapter 25 Electromagnetic Induction
19-Apr-17
Physics 1 (Garcia) SJSU

2. WARNING Strong magnets will be passed around the room.
Keep these magnets away from iPods and laptops (especially my laptop)!
19-Apr-17
Physics 1 (Garcia) SJSU

3. Electromagnetic Induction
Voltage is induced whether the magnetic field of a magnet moves near a stationary conductor or the conductor moves in a stationary magnetic field.
Can show this with presentation galvanometer, single wire, and large magnet. Very weak signal.
19-Apr-17
Physics 1 (Garcia) SJSU

4. Demo: Magnet Induces Current
Voltage is induced when a magnet moves towards or away from a coil, inducing a current in the coil.
Faster the magnet’s motion, the greater the induced current.
Connect large coil to demo galvanometer. Move large magnet near the coil; move coil near large magnet. Pass-around box with coil, bar magnet, and galvanometer.
19-Apr-17
Physics 1 (Garcia) SJSU
Note: Do this in lab too.

5. Magnetic Recording
Write data by magnetizing recording media (e.g., video tape, hard disk) using electromagnets.
Data is read back using the induced current produced when magnetized media moves past receiver coils (reverse of writing data).
Hard disk
19-Apr-17
Physics 1 (Garcia) SJSU

6. Electric Generator
Electric generator moves a conductor in a magnetic field to produce voltage via electromagnetic induction
Bring small generator, similar to the one pictured (one in stock room, another in 2B lab)
19-Apr-17
Physics 1 (Garcia) SJSU

7. Demo: Electric Generator
Turn the shaft by hand and as the coils pass the magnets a voltage is induced.
DC current is generated.
DC Output
SHAFT
Magnet
Set this up in a pass-around box using St. Louis motor (many in Phys 1 lab) and galvanometer.
Magnet
Coils
Note: Do this in lab too.
19-Apr-17
Physics 1 (Garcia) SJSU

8. Induction: No Free Lunch
Takes work to turn the generator crank to produce electric current.
The faster we turn the crank to produce more current, the more difficult it is to turn.
More difficult to push the magnet into a coil with more loops because the magnetic field of each current loop resists the motion of the magnet.
19-Apr-17
Physics 1 (Garcia) SJSU

9. Faraday’s Law
The induced voltage in a coil is proportional to the product of the number of loops and rate at which the magnetic field changes within the loops.
Large
Voltage
Small
Voltage
Medium
Voltage
19-Apr-17
Physics 1 (Garcia) SJSU

10. Demo: Electromagnetic Oscillations
Put alternating current into an electromagnet and you create an oscillating magnetic field.
This oscillating magnetic field induces electrical currents by inducing electrical field oscillations.
This is, effectively, a very low frequency electromagnetic antenna.
Coil with bulb
Oscillating
Magnetic
Field
Electro-
Magnet
Just like it’s shown. Point out that the ring becomes hot.
Connect to alternating current (AC)

11. Demo: Electric Motor
Can create an electrical motor by passing a current through a set of electro-magnets mounted on a rotating shaft.
Use big St. Louis motor and lawnmower battery
Current out
Current in
Electro-magnets

12. Electric Motor, Analyzed
Electromagnet mounted on a shaft with opposing magnets on each side.
Current direction always such that electromagnet is repelled, causing shaft to turn.
Current N S N S N S
Current
Use big St. Louis motor and lawnmower battery

13. Demo: Generator Becomes Motor
Pass a current into the generator and it becomes an electric motor.
DC Input
Battery
Magnet
St. Louis motor in a pass-around box (with battery and switch).
Magnet
Rotor
Spins
19-Apr-17
Physics 1 (Garcia) SJSU
Note: Do this in lab too.

14. Self-Induction
When a current is induced by a changing magnetic field, that current itself produces its own magnetic field. This effect is called self-induction.
Primary Magnetic Field
Current
Self-Induced
Magnetic Field
19-Apr-17
Physics 1 (Garcia) SJSU

15. Demo: Lenz’s Law
Oscillating
Magnetic
Field
Induced current produces a secondary magnetic field that is always opposed to the primary magnetic field that induced it, an effect called Lenz’s law.
Metal ring is
levitated by
self-induced
secondary
magnetic field
Electro-
Magnet
Just like it’s shown. Point out that the ring becomes hot.
Connect to alternating current (AC)
19-Apr-17
Physics 1 (Garcia) SJSU

16. Check Yourself
Does the ring levitate if we connect the electromagnet to direct current (e.g., to a battery)?
Does the ring levitate if it has a gap?
Electro-
Magnet
19-Apr-17
Physics 1 (Garcia) SJSU

17. Check Yourself
Hold the ring down on the shaft and it starts to get hot. Why?
What if we cool the ring in liquid nitrogen and repeat the demo?
Oscillating
Magnetic
Field
Electro-
Magnet
Just like it’s shown. Point out that the ring becomes hot.
Connect to alternating current (AC)
19-Apr-17
Physics 1 (Garcia) SJSU

18. Demo: Magnetic Brakes
Strong magnet dropped into a copper pipe falls slowly due to secondary magnetic field induced by its motion.
Great America’s Drop Zone has a 22 story freefall, lasting four seconds, decelerated by magnetic braking.
19-Apr-17
Physics 1 (Garcia) SJSU

19. Eddy Currents
Changing magnetic field induces eddy currents within any conductor.
These internal currents produce self-induced magnetic fields, which by Lenz’s law are in opposition of the primary magnetic field.
Primary Magnetic Field
Eddy
Currents
Use large, strong magnet or rare earth magnets passed over a piece of copper. Can drop rare earth down aluminum pipe.
Self-Induced
Magnetic Field
19-Apr-17
Physics 1 (Garcia) SJSU

20. Metal Detectors
Can detect the presence of metals by using a transmitter coil to create an oscillating primary magnetic field.
This creates a secondary magnetic field due to eddy currents in the metal.
Can detect this secondary magnetic field by using a receiver coil.
19-Apr-17
Physics 1 (Garcia) SJSU

21. Demo: Back Electromotive Force
Changing magnetic field in an electromagnet induces a voltage in the electromagnet itself.
When we disconnect a large electromagnet the magnetic field rapidly collapses.
This changing magnetic field
self-induces a large voltage in the coil of the electromagnet.
Large spark occurs at the switch.
Connect lawnmower battery to large electromagnet coil using a switch. Large spark when switch opened.
19-Apr-17
Physics 1 (Garcia) SJSU