Electric CurrentElectric Currentand Ohm’s Lawand Ohm’s Law

Sec 20.2Sec 20.2

Electric CurrentElectric Current

electric current – continuous flow of electric current – continuous flow of electric chargeelectric charge

measured in ampere (A) – 1 coulomb per measured in ampere (A) – 1 coulomb per secondsecond

two typestwo typesdirect current – charge flows one waydirect current – charge flows one way

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

alternating current – charges vibrate back and alternating current – charges vibrate back and forth around a fixed positionforth around a fixed positionType of current in householdsType of current in households

Conductors and InsulatorsConductors and Insulators

conductor – material through which charge conductor – material through which charge can easily flowcan easily flowexamples: metals, salt water, gases in examples: metals, salt water, gases in

fluorescent lightfluorescent lightconductors transmit charge because they conductors transmit charge because they

have free electrons in their structurehave free electrons in their structure insulator – material through which charge insulator – material through which charge

cannot easily flowcannot easily flowExamples: plastic, foam, rubber, woodExamples: plastic, foam, rubber, wood

Challenge QuestionChallenge Question

Q: If a lightening storm occurs, you are Q: If a lightening storm occurs, you are relatively safe in a car. Is this because of relatively safe in a car. Is this because of the car’s conducting properties or the car’s conducting properties or insulating properties?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 never reaches you.electric current never reaches you.

ResistanceResistance

opposition to the flow of electronsopposition to the flow of electronsmeasured in ohms (measured in ohms (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 length, thickness depends on material’s length, thickness and temperatureand temperatureLonger wire increases resistanceLonger wire increases resistanceThicker wire decreases resistanceThicker wire decreases resistance Increasing 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 thickA 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 charges to flowfor charges to flowbatteries, solar cells and generators are all batteries, solar cells and generators are all

sources of electrical energysources of electrical energycharges flow from areas of high potential charges flow from areas of high potential

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

Voltage (continued)Voltage (continued)

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

potential difference = the difference in potential difference = the difference in electrical potential energy of two places in electrical potential energy of two places in an electric fieldan electric fieldmeasured in volts (V)measured in volts (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 circuit first discovered by Georg Ohm (1826)first discovered by Georg Ohm (1826)

Voltage = current x resistance or V = I x RVoltage = 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 the resistance constant, what happens to the current?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 lower from increased sweating. Moist skin has a lower resistance than dry skin. As the resistance of the resistance than dry skin. As the resistance of the skin goes down, the current will increase which skin goes down, the current will increase which is detected by the meter.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 bird, A: In order for the current to flow through the bird, their needs to be a potential difference. When their needs to be a potential difference. When both feet are on the same line there is no 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 hanging current flows. You also would be safe hanging on the power line as long as you were not on the power line as long as you were not touching anything else. If you touch the line and touching anything else. If you touch the line and touch another object or the ground there would touch another object or the ground there would then be a potential difference and you would be then be a potential difference and you would be electrocuted.electrocuted.