Physics Gang Signs Review

1. If your were a positively charged particle headed toward the Earth’s equator from outer space which way would the Earth’s magnetic field force you? B-field =

v =

q =

so the Force =

2. If you were moving toward the Earth at either pole then what would happen?

• NOTHING. . .the direction of the velocity and the B-field are parallel to each other so NO FORCE is felt!!

(This is what causes the Northern and Southern Lights.)

• Electrons from solar wind interact with the nitrogen and oxygen in the atmosphere anywhere from 20 to 200 miles above the surface of the Earth

COLORS of auroras:O2 = Green (up to 150 miles) Red (above 150 miles from surface)

N2 = Blue (up to 60 miles) Purple (above 60 miles from surface)

Michael Faraday

• The discovery that an electric current in a wire produced magnetism was a turning point in history.

If a Current creates a Magnetic Field ...

IB

... Can a Magnetic Field cause electric current?

I

Faraday’s Contribution

• Discovered the electric current could be produced in a wire by simply moving a magnet in or out of a wire coil.

• No battery or other voltage source was needed.

• The amount of voltage produced depends on how quickly the magnet passes through coil of wires.– It doesn’t matter if it’s the magnet moving

through the coil or visa versa.

Electromagnetic Induction

• ANY CHANGE in a MAGNETIC FIELD, will create an ELECTRIC CURRENT

• Inducing voltage by changing the magnetic field around a conductor is electromagnetic induction.

Faraday’s Law

Faraday’s Law says that you can create (induce) more voltage by

1) moving the B-field faster2) moving it into and out of more coils of wire

**the amount of current produced by electromagnetic induction depends not only on the induced voltage but the resistance of the coil and circuit to which it’s connected.**

Generator and AC current• A GENERATOR uses electromagnetic induction, it turns

mechanical energy into electrical energy.• As the coil enters the magnetic field, voltage is induced one

direction, as the coil leaves the magnet, voltage is induced the other direction.– This produces current that travels one direction in a wire, and then

switches directions as the magnet leaves the coil = AC current

Transformers• Uses Faraday’s Law and ferromagnetic properties of an iron

core to raise and lower AC voltages.• Used to transfer electric energy from one circuit to another.

How it works

• Current passes through primary, as a result, current passes through the secondary.

• Iron core guides B-field lines from primary through the secondary.

• Voltage of primary is passed to EACH loop of secondary.– Twice as many loops, twice the voltage produced in secondary– Three times the loops, three times the voltage produced

Equation

• Relationship between the voltages is with respect to the number of turns of the wire.

s

p

s

p

s

p

I

I

n

n

V

V

Energy transferred

• The coil transfers energy from one coil to another• The rate at which energy is transferred is POWER

– The primary gives no more power than the second uses (COE)

REMEMBER:

P = VI

IN TRANSFORMERSPIN = POUT

VIIN = VIOUT

Step Up and Step Down transformers• Transformers are also used to raise and lower the

voltage to meet certain operating requirements• The ratio between the number of turns in the primary

and the secondary is called the Turns Ratio

SECONDARYPRIMARY

I1 I2

SECONDARYPRIMARY