Chapter 18 Electric Currents. Loop Rule The sum of all the potential differences around a circuit...

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Chapter 18 Electric Currents

Transcript of Chapter 18 Electric Currents. Loop Rule The sum of all the potential differences around a circuit...

Page 1: Chapter 18 Electric 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.

Chapter 18Electric Currents

Page 2: Chapter 18 Electric 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.

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.

Page 4: Chapter 18 Electric 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.

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.

Page 5: Chapter 18 Electric 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.

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.

Page 6: Chapter 18 Electric 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.

Electric Batteries

• The only battery with an acceptable name is Energizer.

• A “rechargeable” battery is a lie.

Page 7: Chapter 18 Electric 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.

Stop

• Finish Lab• Paper Throw

Page 8: Chapter 18 Electric 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.

Hudson River

Page 9: Chapter 18 Electric 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.

Seneca River

Page 10: Chapter 18 Electric 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.

St. Lawrence River

Page 11: Chapter 18 Electric 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.

Niagara River

Page 12: Chapter 18 Electric 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.

Oswego River

Page 13: Chapter 18 Electric 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.

Current in a Wire• In a river:

• How much water/time• In a circuit:

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

Page 14: Chapter 18 Electric 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.

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?

Page 15: Chapter 18 Electric 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.

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?

Page 16: Chapter 18 Electric 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.

Whitewater Rafting

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

Page 17: Chapter 18 Electric 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.

Ohm’s Law

• Proportionalities

Page 18: Chapter 18 Electric 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.

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?

Page 19: Chapter 18 Electric 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.

Stop

• Paper Throw

Page 20: Chapter 18 Electric 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.

Plinko

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

Page 21: Chapter 18 Electric 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.

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.

Page 22: Chapter 18 Electric 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.

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.

Page 23: Chapter 18 Electric 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.
Page 24: Chapter 18 Electric 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.

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.

Page 25: Chapter 18 Electric 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.

STOP

• Throw paper

Page 26: Chapter 18 Electric 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.

Sample Question

Page 27: Chapter 18 Electric 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.

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)

Page 28: Chapter 18 Electric 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.

Sample Problem

• Calculate the resistance of a 40W automobile headlight designed for 12V.

Page 29: Chapter 18 Electric 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.

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?

Page 30: Chapter 18 Electric 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.

Conventional Current

Page 31: Chapter 18 Electric 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.

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.

Page 32: Chapter 18 Electric 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.

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.

Page 33: Chapter 18 Electric 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.

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.

Page 34: Chapter 18 Electric 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.

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.

Page 35: Chapter 18 Electric 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.

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.

Page 36: Chapter 18 Electric 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.

HW Due Friday

• Ch 18: 5, 9, 11, 13, 20, 34, 37

Page 37: Chapter 18 Electric 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.

STOP

• Throw paper