Electric Currents R General Physics 2 Current Resistance

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 k. Wh 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/k. Wh, 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 I 2 R (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 is constant AC • • alternating 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 R 1 for the circuits shown. Assume R 1 = 10 , R 2 = 20 , and R 1 = 30 , and the battery is 12 V. General Physics 2 Circuits 11