Magnetism-Magnetic Induction

By David Kimball, Emma Kaplan, Nate Rudolph

Big Idea

• Magnetism can produce electric current, and an electric current can produce magnetism.

Electromagnetic induction

• In 1831, two scientists Michael Faraday, and Joseph Henry individually discovered that magnetism could produce an electrical current

• Electric current can be produced in a wire by moving the magnet through the coiled wire.

Electromagnetic Induction cont.

• No power source is required to produce the current

• The production of voltage depends only on the relative motion of the conductor with respect to the magnetic field

• The amount of voltage depends on how fast the magnetic field lines are traversed by the wire

Induction

• The number of coils also increases the induced voltage and current in the wire.– Twice as many coils will induce twice as much

voltage

Faraday’s Law

• The induced voltage in a coil is proportional to the product of the number of loops, the cross sectional area of each loop, and the rate at which the magnetic field changes within these loops.

• This law describes the relationship between induced voltage and the rate of change of a magnetic field

Generators/Alternating Current

• A motor converts electrical energy into mechanical energy

• A generator converts mechanical energy into electrical energy

• When the magnet is pushed into and out of the coiled wire the voltage switches between a positive and negative value

Simple generators

• Starting perpendicular to the field, the loop crosses the maximum amount of magnetic field lines.

• As it rotates it switches between the magnetic field lines, voltage is produced

• Generators change mechanical energy to electrical energy

• (picture on the next slide)

Transformers

• A transformer works by inducing a changing magnetic field in one coil, which induces an alternating current in a nearby second coil

• It changes the voltage of the energy• Only AC current can be transformed• The voltage is either stepped up or stepped

down depending on the ratio of coils of the primary to the secondary

Transformers (Cont.)

• If the secondary has a hundred times as many turns as the primary, then a hundred times as much voltage will be induced

• With a transformer, energy is conserved. The power used in the secondary is supplied by the primary.

• If the voltage is increased, the current is decreased

Multiple Choice Questions

• 1. According to Faraday’s Law, how can an electric current be produced?

a. spinning a magnet b. wrapping a wire c. rubbing a magnet on another object d. moving a magnet through coiled wire

• 2. Increasing the number of loops of wire that move in a magnetic field ________ the induced voltage and the current in the wire.

a. decreases b. increases c. does not affect d. eliminate

• 3. The force that you exert on the magnet _________ the distance that you move the magnet is your input work.

a. multiplied by b. divided by c. subtracted from d. added to

• 4. A generator converts ______ energy to _______ energy.

a. mechanical, electrical b. electrical, mechanical c. thermal, mechanical d. electrical, radiant

• 5. A motor converts ______ energy to _______ energy.

a. mechanical, electrical b. electrical, mechanical c. thermal, mechanical d. electrical, radiant

• 6. What does a transformer change? a. current b. resistance c. voltage d. power

• 7. In a transformer, if the secondary has half as many turns as the primary, then _______ as much voltage is induced in the secondary.

a. one fourth b. four times c. twice d. half

• 8. If 120V is used to power a toy electric train that operates at 6V then a ______ transformer should be used that has a primary to secondary turns ratio of ________.

a. step up, 20/1 b. step up, 1/20 c. step down, 20/1 d. step down, 1/20

• 9. What is the main benefit of AC compared to DC?

a. it has the ability to transform b. it is alphabetically superior c. DC is more widely used d. it alternates

• 10. If the first set in a transformer produces 10V with a current of 6 amps and the second half a transformer has 5 amps, what is the voltage in the second half of the transformer?

a. 1V b. 5V c. 10V d. 12V

Answers

• 1. D• 2. B• 3. A• 4. A• 5. B• 6. C• 7. D• 8. C• 9. A• 10. D