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Electricity Part 2: Electric Current Electric current is the amount of charge moving past a point Maxwell Demon counting the charges passing a given point Definition of current I = current Q = amount of charge that passes point. t = time for charge to pass by. Q I t Units of Current Q Coulomb I t sec 1 Ampere (Amp) = 1 Coulomb/second Electric currents only flow in wires. 1. True 2. False 50% 1 50% 2 Examples of Currents Solar wind interacting with the earths magnetic field 1) Coronal Mass Ejection 2) Aurora from space 3) Aurora from ground CME animation Aurora from space Aurora effects 3 electron beams in a color TV Tokamak Fusion Experiments JET discharge We usually think of currents in wires Opening switch Simple circuit When a charged particle passes through the battery, it gains energy. When the particle passes through the light bulb it gives up the energy as heat. Ohm’s Law V=IR V= Voltage of the Battery. I=current in circuit. R=Resistance in the bulb/resistor. (Depends on materials and geometry.) Units of Resistance R=V/I (volts/amps) By definition, 1Ohm = 1 volt/amp, or 1=1V/A. Log Ride Analogy Water circuit analogy Example problem How many amps of current would flow in a light bulb that has a resistance of 60 if it is connected to a 12 V battery. I V/R 12V I 0.2 A 60 Power in a circuit When Charge Q passes through the battery it gains an amount of energy E=(Q)V (This is the amount of work the battery does on the charge.) If the charge takes an amount of time t to pass through the battery, the battery supplies a power of (does work at a rate of) E Q P V t t But Q I t Thus P IV The power supplied by the battery must be dissipated in the resistor. We also know the V=IR. Power dissipated in resistor P IV I ( IR) P I R 2 Which type of light bulb has a larger resistance, a 100W bulb or a 60W bulb? 25% 25% 25% 2 3 25% 1. The 100 W bulb 2. The 60 W bulb 3. They will have the same resistance 4. None of the above 1 4 Example Calculation What is the resistance and how much current flows through a 100 W bulb? Note: The wattage on a bulb is its power output and assumes that you will use it in the US where the voltage in 110 V. I P /V I 100W 0 . 91 A 110V RV/I R 110V 0.91 A 121 Redo the calculations for a 60 W bulb I P /V I 60W 110V 0.55 A RV/I R 110V 0.55 A 200