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Chapter 7B

Measuring currentWe measure current as it flows through an ammeter or a Multimeter.

Ammeter – measures Amperes (current) I

Ohmmeter – measures Ohms (resistance)

Voltmeter – measures Volts (pressure) v

Multimeters measure all three.

Be careful! Meters are damaged by too much current.

Your meter may contain a circuit breaker or fuse. Protect circuits from too much current.

If meter doesn’t work, then circuit breaker or fuse may have acted.A circuit breaker can be reset, but a fuse must be replaced.

Electricity in your house

House and school electricity:

alternating current (AC)

Current alternates direction,back and forth.

Happens 60 times per second

60 Hertz

http://www.bbc.co.uk/schools/gcsebitesize/physics/electricity/mainselectricityrev8.shtml

Why? AC power lets electricity travel longer distances, with less resistance. Less loss of power, more efficient.

“Edison’s Miracle of Light” – PBS television special

Animation of a circuit with AC, and with DC http://www.pbs.org/wgbh/amex/edison/sfeature/acdc.html

Animation of AC power generation. http://www.pbs.org/wgbh/amex/edison/sfeature/acdc_insideacgenerator.html

AC Generator in action – See how an AC current is madeMolecular Expressions Electricity and Magnetism IntroductionNational High Magnetic Field Laboratory, Florida State Univ.

http://micro.magnet.fsu.edu/electromag/java/generator/ac.html

In this unit we mostly study batteries. These produce direct current (DC), not AC.We need to know the difference between the two.

7.3 Resistance

Conductors: Allow charge to easily flow.Most metals are good conductors. Nearly all plastics are not.

Insulators: Do not allow charge to flow. Glass and nearly all plastics are good insulators.

Semiconductor: More difficult for charges to flow through than conductors, but easier to flow through than insulators.Lets you have fine control of e- flow.

Computer chips, LED’s and some kinds of lasersare made from semiconductors.

No such thing as a pure conductor or insulator.

All materials have some resistance to flow of charges.

All materials can allow some charge to flow, if voltage is high enough.

Direct Current (DC) Circuit Water Analogyhttp://hyperphysics.phy-astr.gsu.edu/hbase/electric/watcir.html

If R is high then little I flows. Any devices that use electrical energy has some R.

If you string more light bulbs together, R increase and I decreases

Ohm – unit of resistance - measured in Ohms. Written with Greek letter omega ( Ω )Named for German physicist Georg Ohm (1787-1854)

Physical UnitSymbol

quantity ________________ Current Amps I

Resistance Ohms Ω

Voltage Volts V

If v then I If v then I

If r then I If r then I

Definition: If a circuit has a resistance of 1 ohm (1 Ω), then a current of 1 amp flows, when a voltage of 1 volt is applied.

V = IR 1 Volt = (1 Amp)(1 Ohm)