Ch 25

The Question: If an electric current in a wire produces a magnetic field, is the reverse true? …i.e. Does a magnetic field produce a current?

The Answer: Let’s try it out!• If magnet is stationary, _________

• If magnet is moving in, _________ _________________________

•If magnet is moving out, ________ _____________________________________________

•If the poles of the magnet are reversed, ___________________

no currentcurrent

is produced

current is produced in opposite direction

current changes direction

Additional Observations:• If 2 magnets are used (twice the magnetic field strength), the current will _________.

• If the magnet is moved more slowly, the current will be ________.

• If the magnet is continuously moved back and forth, ___________ ________ is created!

To summarize all of our observations:• When there is a _________ magnetic field around a conductor (the coil of wire), a ________ is induced in the conductor.

• The amount of current induced is related to the strength of the ______________ and the _____ at which the field changes.

double

smaller

alternating current

changingcurren

t

magnetic field

rate

Michael Faraday, in England, in 1831 was the first to discover this phenomenon, termed ELECTROMAGNETIC INDUCTION.Faraday’s Law quantifies the phenomenon:

The induced voltage (which ultimately can produce a current) in a coil is proportional to the product of:

• The _____ at which the magnetic field strength ________ within the coil, and the

•________________in the coil.

ratechanges

Number of loops

An application of Electromagnetic Induction:

OK… so NOT the “more than meets the eye” kind, but the electrical transformer….

Purpose of Transformers: Used to change or “transform” the voltage in a circuit.

How do they work? Demo: A pair of coils are placed side-by-side. One is connected to a battery (DC) and one is connected to an ammeter.

When the switch is closed (or opened) in the primary coil, __________________________ occurs in the secondary coil

a tiny, BRIEF surge of current

But, WHY?• When the switch closes, the current increases from zero. This ________ current produces a ____________________ around the primary coil.

•This changing magnetic field is “felt” by the nearby secondary coil, which according to Faraday’s Law, will induce a _________ in the secondary coil, (which then produces a current) The effect is enhanced (larger current produced in secondary) if an iron core is added…. DEMO

changingchanging magnetic

field

voltage

The key to obtaining a current in the secondary coil is to establish a changing magnetic field in the primary coil. Instead of using a battery (DC) and switching the current on/off repeatedly (a little impractical!!) to accomplish this, _____________________ in the primary coil is used.

Why do transformers use AC, and not DC?

If DC was used in the primary coil, the output would be __________________!

alternating current

Nothing (zero Volts)

So… HOW does the voltage change?If we assume 100% efficient transformers…

Now, if the two output loops are joined in series, the voltages _____.

Since the secondary coil had more windings (loops), the secondary voltage was ________. This is called a _________ transformer.

sum

higherStep-up

Did we just “create” electrical energy?

____________!! Energy is always _________. Since POWER is the _____ at which energy is consumed, POWER is then, also, conserved….

secondaryprimary PP (Assuming 100% efficient transformer)

For a “Step-Up” transformer..… so… as Voltage ↑, Current ___.

Of course NOTconserved

rate

SSPP VIVI SSPP VI IV

↓

SSPP VI IV

Step-Up vs. Step-Down Transformers

Step-Up

_____ Windings on Secondary

Coil

Secondary

Voltage __

Step-Down

_____ Windings on Secondary

Coil

Secondary

Voltage __

The secondary voltage is proportional to the number of secondary windings according to…

S

P

V

V Where…

NP = # of windings on primary coil

NS = # of windings on secondary coil

More

↓

↑Less

SSPP VI IV

S

P

N

N

Simulation at http://micro.magnet.fsu.edu/electromag/java/transformer/index.html

A Model of a transformer: Demo…

“Exploded” View

Assembled View

• 1 V AC applied to inner (primary) coil produces ___ V AC on outer (secondary) coil

• PREDICTION: 3 V AC applied to inner (primary) coil will produce ___ V AC on outer (secondary) coil, because the secondary coil must have ____ times the number of windings as the primary coil.

Iron core (won’t work without it!)

Note: If the input voltage is applied to the outer coil, it becomes the primary, and the transformer is a step-______.

13

3913

down

1 : 13 winding

ratio!

Electrical Power DistributionA Typical Scenario…

With the use of transformers, electrical power is sent across long transmission lines at ______ voltages. WHY? If the voltage is high, the current will be relatively ____, so there will be less power loss in the lines.

HIGH

low

In the 1880’s, there was a debate/ battle between two great scientists – Nikola Tesla and Thomas Edison over whether to use AC or DC for electric power distribution.

Tesla: Advocate

for AC

Edison: Advocate

for DC

_______ won because with ____, a transformer can be used to change the voltage. ____ voltage can’t be “transformed” to a higher voltage which would be necessary to reduce the power/heat loss in the distribution lines. Edison’s answer to that problem? “Just build a power station every mile or so”!!

“The War of the Currents”

Tesla ACDC

Sign at Niagara Falls, Summer 2008

Example: A 2 power line supplies power to a small subdivision at 8000 V and 100 Amps.

• Calculate the total power supplied to the subdivision.

RIPlost2

First, derive a useful formula…

• Calculate the power loss in the distribution line.

__________ IIVP RI RI 2

WPlost 000,20(2.5% of the supplied power would be lost.)

• If a transformer was used to step-up the voltage by a factor of 10 (to 80,000 V), calculate the new power loss in the distribution line.

If V ↑ by 10 times, I ______________________.

Or… SSPP VIVI VIVA S 000,808000100

AIS 10

I ↓ by 10 times, but the power loss was reduced by a factor of ____. WHY?

↓ by 10 times … I= 10A

100