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Electric Currents (R) General Physics 2 Current & Resistance 1 Resistors (L) circuit symbol what is the resistance of this resistor? General Physics 2 Try some others. Current & Resistance 2 Electric Power (L) • Power = Energy / time Other useful forms: General Physics 2 Current & Resistance 3 Think-Pair-Share (L) • You buy a 75-W lightbulb in Europe, where electricity is delivered to homes at 240 V. If you use the lightbulb in the US at 120 V (assume its resistance does not change), how bright will it be relative to 75-W 120-V bulbs? [Hint: assume that brightness is proportional to power consumed.] General Physics 2 Current & Resistance 4 Think-Pair-Share (L) • How many kWh of energy does a 550-W toaster use in the morning if it is in operation for a total of 15 min? At a cost of 9.0 cents/kWh, estimate how much this would add to your monthly electric bill if you made toast four mornings per week. General Physics 2 Current & Resistance 5 Power in Household Circuits (L) • • • • Wires heat up at a rate equal to I2R (power) fire hazard if I is too large fuses prevent overloads by breaking circuit short - two wires touching General Physics 2 Current & Resistance 6 Fuses (L) fuses that need to be replaced circuit breakerbreaks circuit with a switch General Physics 2 Current & Resistance 7 Household Circuits (L) • General Physics 2 What is the total current draw by these devices? If the circuit has a 20-A fuse, will the fuse blow? Current & Resistance 8 Alternating Current (L) • DC • • • direct current current is constant AC • • • • alternating current current varies like a sine wave frequency usually 60 Hz (60 cycles per second) Equations are valid when you use RMS values General Physics 2 Current & Resistance 9 Human Nervous System (L) Messages transmitted within nervous system are electrical signals passing along the neuron General Physics 2 Current & Resistance 10 Practice Problems • Determine the equivalent resistance and the current through R1 for the circuits shown. Assume R1 = 10, R2 = 20 , and R1 = 30 , and the battery is 12 V. General Physics 2 Circuits 11