Transcript Voltage
Introduction to Physical Science Monday, Wednesday, Thursday Tom Burbine [email protected] Quiz on Thursday • • • • • • • • • Coulomb’s Law Electric Potential Electric current Electrical resistance Ohm’s Law Voltage = current x resistance Power = current x voltage Series Circuit Parallel Circuit • http://hilaroad.com/camp/projects/lemon/lemon_b attery.html • 1 lemon can generate approximately 0.83 Volts • Electric potential (volt) = electric potential energy charge • A conductor is a material which contains movable electric charges (electrons) • Zinc-iron nails and copper are good conductors of electricity • Insulators are non-conducting materials with fewer mobile charges (electrons), which resist the flow of electric current Voltage is like water pressure • Voltage is analogous to a water pressure difference • A high pressure difference (higher voltage) will result in a faster electron flow Current • Electric current, measured in amperes, is the rate of flow of electric charge • One ampere is the rate of flow of one coulomb of charge (6.24 x 1018 electrons) per second Electrical Resistance • The electrical resistance is a measure of its opposition to the passage of an electric current • Resistance = Voltage Current • The resistance of a wire depends on three factors: the length of the wire, the cross-sectional area of the wire, the resistivity of the material composing the wire The resistance would be greater for a longer wire, less for a wire of larger cross sectional area, and depends upon the material out of which the wire is made • Electrical resistance of a conductor is a measure of how difficult it is to push the charges along Ohm’s Law • Current = voltage resistance Units: • 1 Ampere = 1 volt 1 ohm • Usually written as • • • • V = IR V is Voltage in Volts (V) I is Current in Amperes (A) R is Resistance in Ohms () Example • How much current flows through a lamp with a resistance of 60 when the voltage across the lamp is 12 V? Example • How much current flows through a lamp with a resistance of 60 when the voltage across the lamp is 12 V? • V = IR • I = V/R = 12 V/ 60 = 0.2 A • What is the resistance of a toaster that draws a current of 12 A when connected to a 120-V circuit? • What is the resistance of a toaster that draws a current of 12 A when connected to a 120-V circuit? • V = IR • R = V/I = 120 V/ 12 A = 10 • Why do you get more of a shock when you touch a battery with wet hands than dry hands? Example • Why do you get more of a shock when you touch a battery with wet hands than dry hands? • I = V/R • Resistance of hands soaked in salt water = 100 • Resistance of dry hands = 100,000 • Lower resistance equals higher current Series Circuits • Electric current passes through a single pathway • Total resistance is the sum of the resistances • Current is equal to the Voltage divided by the sum of the resistances • A break in the path results in an open circuit Series Circuits Series Circuits • V = I(R1 + R2 + R3 + …) Series Circuits • If one lamp burns out, the current through the other lamps cease. All lamps go out • Adding more lamps reduces the current through the circuit • Adding more lamps reduces the brightness of each lamp Parallel Circuits • The Voltage is the same across each lamp • Current divides along the parallel branches Parallel Circuits Parallel Circuits • If one of the lamps in a parallel circuit burns out, the other lamps are unaffected • However, the total current in the circuit will decrease Parallel Circuits • If you add another lamp, the brightness of the other lamps is unchanged • However, the total current will increase Parallel Circuits • Overloading is when you add too many devices in parallel • The total current increases too much • The wire can overheat • Series Parallel Power • Power = current x voltage • Power = charge x energy time charge Units: • Watts = amperes x volts • 1 kilowatt = 1,000 Watts = energy time • power = energy/time • energy = power x time • 1 kilowatt-hour = amount of energy consumed in 1 hour at the rate of 1kilowatt • If electric energy equals 25 cents/kilowatt-hour, how much would it cost to operate a 100-W bulb for 10 hours? • If electric energy equals 25 cents/kilowatt-hour, how much would it cost to operate a 100-W bulb for 10 hours? • cost = rate x power x time • cost = (25 cents/kilowatt-hour)(0.1 kW)(10 hours) • Cost = 25 cents Example • Lamp at 120 Watts is operating on a 120 Volt line, what current does it draw? Example • Lamp at 120 Watts is operating on a 120 Volt line, what current does it draw? • Power = current x voltage • current = power/voltage = 120 Watts/120 Volts • current = 1 ampere Any Questions?