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Electric Current The study of electric charges in motion Potential Difference (V) • Charge flows from higher potential to lower potential until electric potential equilibrium is reached. • When electric potential is zero (equilibrium), current flow stops. Example: Charge Distribution • What is the total charge of three conducting spheres with charges of 6q,-1q, and 0q qtotal 6q (1q) 0q qtotal 5q A 6q B -1q C 0q Example: Charge Distribution cont. • What is the final charge distribution if sphere A and B touch? A 6q 2.5q B -1q 2.5q C 0q Example: Charge Distribution cont. • What is the final charge distribution if sphere B and C touch? A 2.5q B 1.25q 2.5q C 0q 1.25q Example: Charge Distribution cont. • What is the total charge of three conducting spheres qtotal 2.5q 1.25q 1.25q qtotal 5q A 2.5q B 1.25q C 1.25q Electric Current • Electrical charge in motion is electric current. • Current is measured by counting the amount of charges that pass a given point per second Symbol I variable current Unit A (amp) 1 Cs 1A Comparing a DC circuit to Flow of Water http://faraday.physics.utoronto.ca/IYearLab/Intros/DCI/Flash/WaterAnalogy.html •What component in the electrical circuit is equivalent to the pump in the animation? Direct Current (DC) • DC stands for direct current. Here current only travels in one direction. Current vs. Time (DC) 1.2 Current (A) 1 0.8 0.6 0.4 0.2 0 0 1 2 Time (s) 3 4 Alternating Current (AC) • AC stands for alternating current. Here current switches directions at a set frequency. Current vs. Time (AC) 1.5 Current (A) 1 0.5 0 -0.5 0 1 2 -1 -1.5 Time (s) 3 4 Alternating Current (AC) • AC stands for alternating current. Here current switches directions at a set frequency. Voltage • Current flows only when there is a potential difference (V) • Voltage sources can sustain a potential difference • Voltage in a circuit is analogous to pressure in a water hose. J C 1V 1 OFF ON Voltage Source Voltage Sources • There are different types of devices that can provide a source of voltage. • Sources that provide DC include: – Batteries – DC Power Supplies – DC Generators • Sources that provide AC include: – Electrical Outlets – Alternators – Power Inverters Electric Circuits • Electric current flows very well if there is a complete loop for charge to flow. This is called an electrical circuit. • Circuits often contain various elements, giving it practical use. Examples of circuit elements include: – Switches – Sources of Resistance (Such as a Light) – Meters Circuit Symbols • Each circuit element has its own symbol. • Common circuit symbols are shown below. Wire Battery A Conductor of Current Switch Source of DC Charge Flow Ammeter Opens and Closes Circuits Resistor Measures Current Voltmeter Provides Resistance to Current Flow Measures Voltage More Circuit Symbols • Here are some additional circuit symbols that you may see. Capacitor Diode Stores Charge on Plates Potentiometer Variable Resistor Only Allows Current to Flow One Way Junction All Four Wires Connect AC Source Provides AC Current Ground Drains Excess Charge Buildup Crossing Wires Only Cross and do not Connect. Electrical Resistance • Every circuit contains some resistance to current flow. • This is due to imperfections in the crystalline lattice structure of the conductor. • Imagine the lattice structure below is the atoms in a wire. • Notice how the moving electrons experience resistance. • The variable for resistance is R. • The unit for resistance is the Ohm (symbol W). Ohm’s Law • The current in a circuit is directly proportional to voltage and inversely proportional to resistance. 1 I R IV • This relationship is known as Ohm’s Law. Voltage (V) V R I Resistance (W) Current (A) Sample Problem (Ohm’s Law) • A toaster is connected to a 120V outlet and draws 3A of electrical current. • What is the resistance of the toaster? V R I 120V R 3.0 A R 40W Power (Watt W) Power measures the rate at which energy is transferred. • Thermal energy P=I2R – Wasted or unwanted energy during transmission Power and Electricity • There are also two useful equations that relate power to electrical quantities. P VI PI R 2 • Notice how current is squared in the second equation. Increasing current in a circuit drastically increases the power consumed. • High current wires generate heat. • This is why electricity transferred over large distances is at high voltage and not high current. Otherwise, the power losses would be very wasteful. Resistance of power lines is .2 ohms per Km How much power is lost during transmission if a home 3.5Km from the power plant cooks on a stove that draws 41A? R 2( wires )*.2W / km(3.5km) R 1.4W P I 2R P 412 A(1.4W) p 2350W How can energy loss be reduced? Either reduce I or R R is a fix physical property P VI High voltage lines transmit power between 500,000V750,000V to reduce energy loss. Original loss P 2350W 2350W I 750, 000V I .0031A Ploss (.0031A) 2 (1.4W) 5 Ploss 1.34 x10 W Sample Problem (Electrical Power) • What is the power consumed when the toaster in on for 45s ? Given V 120V I 3.0 A P VI P 120V 3.0 A P 360W E P E Pt t E 360W 45s E 16, 200 J The Kilowatt Hour • Did you ever read an electric bill? • You are charged for the number of kilowatt hours used during the month. • This is energy, not power, because: E Pt 1kW =1000W kWh Time (h) Power (kW) • The amount of kilowatt-hours gets multiplied times a rate to find the overall energy cost. C RE Cost ($) Rate Energy (kWh) $ kWh How Much Energy is a kWh? • We all know that the SI unit for energy is the Joule (J). • How many Joules are there in one kilowatt hour? 1kW 1000W 1000 J s 1h 3600s E Pt E 1000 J s 3600s E 3, 600, 000 J or E 3.6 10 J 6 Sample Problem (KWh) • During the winter, an electric heater runs 8 hours every day over the course of a month (30 days). • The power consumed by the heater is 1200W. • How many kWh of energy are consumed? • If the rate is $0.11 per kWh, then what is the cost to operate the heater? 1200W 1.2kW t 8h 30 240h E 1.2kW 240h E 288kWh C RE $ C 0.11 kWh 288kWh C $31.68 Example: Charge Distribution cont. • Sphere B is twice as large as sphere A, what will be the charge distribution after they touch? A 5q A 3.33q B 5q 6.67q Electric Circuit & Energy Conversion Video Clip Electron flow & Current video clip