Electric CurrentElectric Currentand Ohm’s Lawand Ohm’s Law

Sec 20.2Sec 20.2

Electric CurrentElectric Current

electric ______ – continuous flow of electric ______ – continuous flow of electric chargeelectric charge

measured in _______ (A) – 1 coulomb measured in _______ (A) – 1 coulomb per secondper second

two typestwo types_________ current – charge flows one way_________ current – charge flows one way

Found in devices like flashlights and calculatorsFound in devices like flashlights and calculators

_________ current – charges vibrate back _________ current – charges vibrate back and forth around a fixed positionand forth around a fixed positionType of current in householdsType of current in households

Conductors and InsulatorsConductors and Insulators

conductor – material through which charge can conductor – material through which charge can easily floweasily flow examples: ___________________________ examples: ___________________________

______________________________________________________________ conductors transmit charge because they have conductors transmit charge because they have

free electrons in their structurefree electrons in their structure ___________ – material through which charge ___________ – material through which charge

cannot easily flowcannot easily flow Examples: ___________________________Examples: ___________________________

Challenge QuestionChallenge Question

Q: If a lightening storm occurs, you are Q: If a lightening storm occurs, you are relatively _______ in a car. Is this relatively _______ in a car. Is this because of the car’s conducting because of the car’s conducting properties or insulating properties?properties or insulating properties?

A: You are safe because of the car’s A: You are safe because of the car’s conducting properties. If lightening strikes conducting properties. If lightening strikes the car, the metal conducts the electricity the car, the metal conducts the electricity around the outside of the car and to the around the outside of the car and to the ground. Since you are inside the car, the ground. Since you are inside the car, the electric current _________ reaches you.electric current _________ reaches you.

ResistanceResistance

opposition to the ______ __ ________opposition to the ______ __ ________measured in _______ (measured in _______ (ΩΩ))occurs because electrons moving through occurs because electrons moving through

a conductor collide with other electrons, a conductor collide with other electrons, ions and atomsions and atoms

depends on material’s ______, thickness depends on material’s ______, thickness and _____________and _____________Longer wire increases resistanceLonger wire increases resistanceThicker wire decreases resistanceThicker wire decreases resistanceIncreasing temperature increases resistanceIncreasing temperature increases resistance

Quick CheckQuick Check

Determine which of the following in eachDetermine which of the following in each

pair would have the lower resistance:pair would have the lower resistance:a.a. A wire 10 mm thick or 5 mm thick A wire 10 mm thick or 5 mm thick

________________

b.b. A spoon at 25A spoon at 25°C or at 30°C ___________°C or at 30°C ___________

c.c. A A fluorescent bulb 20 cm long or 120 cm fluorescent bulb 20 cm long or 120 cm long.long. ______________ ______________

VoltageVoltage

need source of electrical energy in order need source of electrical energy in order for _________ __ _______for _________ __ _______batteries, solar cells and generators are all batteries, solar cells and generators are all

sources of electrical energysources of electrical energycharges flow from areas of _____ charges flow from areas of _____

potential to areas of ____ potential in potential to areas of ____ potential in much the same way as objects fall from much the same way as objects fall from areas of high gravitational potential areas of high gravitational potential energy to low gravitational potential energy to low gravitational potential energyenergy

Voltage (continued)Voltage (continued)

electrical potential energy of a charge electrical potential energy of a charge depends on its _______ in an electric depends on its _______ in an electric field just like the gravitational potential field just like the gravitational potential energy of an object depends on its height energy of an object depends on its height above the earthabove the earth

__________ ________ = the difference __________ ________ = the difference in electrical potential energy of two places in electrical potential energy of two places in an electric fieldin an electric fieldmeasured in ________ (V)measured in ________ (V)also called voltagealso called voltage

Ohm’s LawOhm’s Law

relationship between current, resistance relationship between current, resistance and voltage in a circuitand voltage in a circuitfirst discovered by Georg Ohm (1826)first discovered by Georg Ohm (1826)

______ = current x resistance or V = I x R______ = current x resistance or V = I x R

Ohm's Law SimulationOhm's Law Simulation

Summary of SimulationSummary of Simulation

1.1. If you increase the voltage and keep the If you increase the voltage and keep the resistance constant, what happens to resistance constant, what happens to the current?the current?

2.2. If you increase the resistance and keep If you increase the resistance and keep the voltage constant, what happens to the voltage constant, what happens to the current?the current?

Challenge QuestionChallenge Question

Q: A simple lie detector consists of an electric Q: A simple lie detector consists of an electric circuit, one part of which is part of your body. A circuit, one part of which is part of your body. A sensitive meter shows the current that flows sensitive meter shows the current that flows when a small voltage is applied. How does this when a small voltage is applied. How does this technique indicate that a person is lying?technique indicate that a person is lying?

A: When people lie their skin tends to get moist A: When people lie their skin tends to get moist from increased sweating. Moist skin has a from increased sweating. Moist skin has a lower resistance than dry skin. As the lower resistance than dry skin. As the resistance of the skin goes down, the current resistance of the skin goes down, the current will increase which is detected by the meter.will increase which is detected by the meter.

Challenge QuestionChallenge Question

Q: If power lines have high voltages and are so Q: If power lines have high voltages and are so dangerous, why don’t birds get electrocuted dangerous, why don’t birds get electrocuted when they land on the lines?when they land on the lines?

A: In order for the current to flow through the A: In order for the current to flow through the bird, their needs to be a potential difference. bird, their needs to be a potential difference. When both feet are on the same line there is When both feet are on the same line there is no potential difference between their feet and no potential difference between their feet and no current flows. You also would be safe no current flows. You also would be safe hanging on the power line as long as you were hanging on the power line as long as you were not touching anything else. If you touch the line not touching anything else. If you touch the line and touch another object or the ground there and touch another object or the ground there would then be a potential difference and you would then be a potential difference and you would be electrocuted.would be electrocuted.