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Electricity – Potential, Power, and Resistance Drill #25 Quote: A goal is a dream with a deadline. ~Napoleon Hill Objective: TAKS Warm-up Potential, Power, Resistance Fun w/ Electricity TAKS question: In animal cells like the one shown above, which organelle is used for storing water? F Nucleus G Mitochondrion H Ribosome J Vacuole What is an electric field??? • It is an area of influence around a charged object. • Positively charged objects within that field will experience an electrical force. ++ + + + + ++ ++ + Electric Field F + Test Charge F -- - -- --- - - - Electric Field Electric Potential Energy • Review – Potential energy = mgh • Based on height – Charges naturally move from higher to lower • To move a charge against natural direction in relation to electric field = WORK! • Same applies to electricity! (related to placement of charge in electric field) Electric Potential • Dependent solely on location of charge… – E potential = PE Q (charge) + positive - negative Circuitry • To light a bulb, two different circuits are at work! – External (wiring) – Internal (battery) • Movement of charge from PE to PE • The Electric Potential Difference = V final – V initial = Work = ∆PE charge charge Voltage = Volts = 1 Joule energy / Coulomb of charge Electric Current • The rate at which electrical charges flow through a wire or conductor is called electric current. Electrical current = charge passing through a given area / time I = Δq/Δt SI Unit = C/s = Amperes (A) Practice 1. A current is said to exist whenever _____. a. a wire is charged b. a battery is present D. c. electric charges are unbalanced d. electric charges move in a loop 2. The diagram at the right depicts a conducting wire. Two cross-sectional areas are located 50 cm apart. Every 2.0 seconds, 10 C of charge flow through each of these areas. The current in this wire is ____ A. Current is the ratio of charge to time. The quantity of charge passing through a cross section in 2 seconds is 10 C. The ratio of charge to time is I = Q / t = ( 10 C) / ( 2 s) = 5 C/s = 5 Ampere Current • Positive charges = carriers of charge – In most cases! • Current = direction that positive charges move in… – Electrons move in opposite direction Electrical Power • Rate at which energy is added/removed from a circuit Power = Work = energy consumed Time Time • Measured in wattage (W) – Do not confuse with W for work! Wattage = 1 Joule sec Electrical Power • Power = rate at which energy is added/removed from a circuit by a battery or a load. • Current = rate at which charge moves within a circuit. • Electric potential difference = potential energy difference per charge between two points. Then……. Practice Determine the ... • a. ... current in a 60-watt bulb plugged into a 120-volt outlet. • b. ... current in a 120-watt bulb plugged into a 120volt outlet. • c. ... power of a saw that draws 12 amps of current when plugged into a 120-volt outlet. • d. ... power of a toaster that draws 6 amps of current when plugged into a 120-volt outlet. • e. ... current in a 1000-watt microwave when plugged into a 120-volt outlet. Practice • a. The current in a 60-Watt bulb plugged into a 120-Volt outlet is 0.5 A. I = P / V = (60 W) / (120 V) = 0.5 A • b. The current in a 120-Watt bulb plugged into a 120Volt outlet is 1.0 A. • I = P / V = (120 W) / (120 V) = 1.0 A • c. The power of a saw that draws 12 amps of current when plugged into a 120-Volt outlet is 1440 W. • P = V • I = (120 V) • (12 A) = 1440 W • d. The power of a toaster that draws 6 amps of current when plugged into a 120-Volt outlet is 720 W. • P = V • I = (120 V) • (6 A) = 720 W • e. The current in a 1000-Watt microwave when plugged into a 120-Volt outlet is 8.3 A. Resistance • Resistance - the tendency for a material to oppose the flow of electrons - All materials have some electrical resistance. - Resistance depends on length, crosssectional area, material, and temperature Ex. Making wires thinner, longer, or hotter increases the resistance. Resistors • Resistors provide a specified amount or resistance to a conductor. • Used to regulate the amount of current in a conductor. Ohm’s Law • Ohm’s law - the current in a circuit (I) equals the voltage difference (V) divided by the resistance (R). R = V/I • Resistance is measured in ohms (). Sample Problem The resistance of a steam iron is 19.0 Ω. What is the current in the iron when it is connected across a potential difference of 120 V? R = V/I V= 120 V R= 19 Ω I = V/R= 120 V / 19 Ω = 6.32 A Electrical Power revisited…. • Electrical Power is the rate at which electric charge converts electrical potential energy to non-electrical forms of energy. P = IΔV = I²R = ΔV²/R SI Unit = watt (W) Sample Problem An electric space heater is connected across a 120 V outlet. The heater dissipates 1320 W of power in the form of electromagnetic radiation and heat. Calculate the resistance of the heater. P = ΔV²/R P = 1320 W V= 120 V R = ΔV²/P = (120 V)²/ 1320 W = 10.9 Ω Electric Power • Electric companies measure energy consumed in kilowatt-hours (kW •h) Electric Power How much does it cost to operate a 100.0 W light bulb for 24 h if electrical energy costs $0.080 per kW•h? Total Energy Used = (0.100 kW)( 24 h) = 2.4 kW•h Cost = (2.4 kW•h)($ 0.080/ kW•h) = $0.19