Chapter 18Electric Currents

Loop Rule• The sum of all the potential differences around a circuit must be zero.

• If you get 10 Volts at the battery…you have to give up 10 volts somewhere around the circuit.

Lightbulb Lab• Do it.• http://www.youtube.com/watch?v=8ve23i5K334

Electric Batteries• A battery generates a potential

difference by producing a separation of charge.

• This separation of charge occurs from chemical energy being transformed into electrical energy.

• In our example of our single cell "battery", the acid solution dissolves the Zinc electrode, which leaves behind two electrons upon the electrode as it dissolves, thus negatively charging it.

Electric Batteries• A standard every day "dry cell"

battery, works off the same principle as our "wet" battery above, only it uses an electrolyte paste instead of a acidic liquid.

• Batteries do work on charges, causing them to separate.

• Batteries are a source of potential difference.

Electric Batteries• The only battery with an

acceptable name is Energizer.• A “rechargeable” battery is a lie.

Stop• Finish Lab• Paper Throw

Hudson River

Seneca River

St. Lawrence River

Niagara River

Oswego River

Current in a Wire• In a river:

• How much water/time• In a circuit:

• Current is measured in Amperes (A)• Ampere = Coulomb/second

Practice Problem

• 5 Coulombs of charge pass by a certain point in a wire in a period of 2 seconds. What is the current in this section of wire?

Practice Problem

• 5 x 10 20 electrons pass by a certain point in a wire in a period of 3 seconds. What is the current in this section of wire?

Whitewater Rafting• http://www.youtube.com/watch?v=28wbI73IVvQ• The filament in a light bulb is similar to the rapids in a river.• How?

Ohm’s Law• Proportionalities

Sample Problem• A small flashlight bulb draws 300 mA from its 1.5 V battery.

• What is the resistance of the bulb?• How would the current change if the battery was 1/3rd as big?

Stop• Paper Throw

Plinko• http://www.youtube.com/watch?v=E7DKxe_m1AM• Things we could change about the board?• Act it out

Microscopic View of Charges• The potential difference (V) in a battery

sets up an electric field.• Electrons are forced in one direction by

the electric field.• The amount of stuff electrons run into,

resistance (R), determines the current (I) in the circuit.

Resistance in a Wire• The unit for Resistance is Ohms (Ω)• Resistance in a wire is:

–Directly proportional to the resistivity “rho” (Ω * m)– How bad of a conductor the wire is.

–Directly proportional to the length “l” (m)– How much wire there is.

– Inversely proportional to the cross sectional area “A” (m2)– How thick the wire is.

Resistance Practice Problems

• Determine the overall resistance of a 100-meter length of copper wire with a cross sectional area of .1 m2.

• Determine the overall resistance of a 2-meter length of nichrome wire with a radius of 2 cm.

STOP• Throw paper

Sample Question

Electrical Power• Mechanical Power was how much energy we transformed in a certain

amount of time. Electrical Power is the same thing. Derive:

• Power is measured in Watts.• Current is measured in Amps (C/s)• Potential difference is measured in Volts (J/C)

Sample Problem• Calculate the resistance of a 40W automobile headlight designed for

12V.

Sample Problem• How much power does my roommate Brian’s bedroom light generate

if it draws 0.5 amps of current and is plugged into a 120 Volt wall socket?

• He leaves his light on 40 hours a week. If electricity costs $0.12/kWh, how much less money should I pay in rent each month?

Conventional Current

Conventional Current Flow• Much like the E field lines, current travels from positive to negative.• This is our convention even though protons do not move.

Why do we Physics?• So our house doesn’t burn

down.• Fuses and circuit breakers are

designed to “switch off” when too much current passes through.

Why do we Physics?• So you can get places.• Maglev trains literally levitate

ontop of the track.• Work is being done to create

“superconducting” materials which have no resistance.

• This will make maglev trains incredibly cheap and efficient.

Why do we Physics?• To help people better (and make

boatloads of cash)• Superconductors are used in

MRI’s, as well as SQUID’s.• Each of these can be used to

create images of brainwaves.• SQUID’s are currently being used

to search for oil.

Why do we Physics?• So you can be alive…and help

other people be alive.• Your nervous system works using

charged ions that create potential differences.

• Those potential differences send electrical impulses to different parts of your body which allow you move, react and function.

HW Due Friday• Ch 18: 5, 9, 11, 13, 20, 34, 37

STOP• Throw paper