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Name____________________________________ Date____________ Period___ Assignment #____ TOPIC 20 ELECTRIC CURRENTS Objective: Describe electric current, circuits, and the transfer of energy by means of current electricity. PRODUCING ELECTRIC CURRENT Electric current - electron flow. eLow ---- ---------------- ------------- ------------- ----- ----- High ---------------- ---------------- Need electron pump to keep charge difference. (Battery) AC - alternating current - (no flow - electrons vibrate back and forth). DC - direct current - (electron flow - drift velocity of free electrons in copper is 1.2 x 10-3 m/s) Solar cell (photovoltaic cell) - light energy to electric energy. Generator - kinetic (mechanical) energy to electric energy. CURRENT IN ELECTRIC CIRCUITS Electric circuit is a closed loop. Current is produced by electron pump. + Battery (electron pump) Closed loop Resistor - reduces flow of electrons. - Current THE AMPERE AND ELECTRIC POWER Unit for charge is Coulomb → C. Rate of flow of charge → electric current (I) is measured in Amperes - A. 1A=1C s Ammeter - device that measures current. Electric Power P = IV Power - J/s Watts - W Current - C/s Voltage - J/C A 12 V battery delivers 0.75 A of current to an electric motor connected across its terminals. What is the power rating of the motor? How much energy does the motor use in 7.0 min? OHM’S LAW Almost every conductor offers resistance to curent. Resistance causes a potential difference (voltage) to exist between the ends of a conductor when current passes through it. V = IR R=V I I=V R Resistance - measured in ohms - Ω What is the current through a 20 Ω resistance that has a potential difference of 120 V? DIAGRAMMING ELECTRIC CIRCUITS conductor switch fuse capacitor resistor (fixed) electric connection rheostat (variable resistor or potentiometer) ground lamp + - no electric connection battery series connection dc generator A ammeter V parallel connection voltmeter V 15 Ω A 2A Schematic 30 V - + CONTROLLING CURRENT IN A CIRCUIT Can control current by changing either the voltage applied or the resistance. 2A A I=V R + 60 V 30 Ω - If you want 1 A you can change V or change R. 1A 1A A A + 30 Ω - 30 Ω + 30 Ω 30 V 60 V - Could also use a variable resistor (rheostat or potentiometer). HEATING EFFECT OF ELECTRIC CURRENTS P = IV IR P = I2R P=W t W = Pt W=Q Q = mC∆T Q = I2Rt Electrical energy → Thermal energy Electric heater, stove, and curling iron. A heater has a resistance of 12.0 Ω. It operates on 120.0 V. What is the current through the resistance? What thermal energy in joules is supplied by the heater in 15.0 s? TRANSMISSION OF ELECTRIC ENERGY Transfer from power plant with as little energy loss as possible. 1 km of transmission wire has 0.2 Ω resistance. Can’t change resistance. Plant House 3.5 km Power loss can be figured with P = I R. Need to keep V high and I low to minimize power loss. That is why they are called High Voltage power lines. 2 P=I V → → THE KILOWATT HOUR Electric companies provide energy, not power. Therefore you pay for electric energy, not power. Joule is so small that energy is measured in kWh - kilowatt hours. 1 kWh = (1000 J/s)(3600 s) = 3.6 x 106 J A new color television set draws 2.0 A when operated on 120 V. How much power does the set use? If the set is operated for an average of 7.5 hours per day, what energy in kWh does it consume per month (30 days)? At $0.08 per kWh, what is the cost of operating the set per month?
