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Journey of an Electron through a Circuit V + Top Wire 4 V -- 4V Lose potential in resistor Gain potential in battery _ Bottom Wire 0V Start End Power • Current delivers energy from source to electrical device • Power delivered to device = change in energy of charges time interval P = qV = t Units = (C/s)(J/C) = J/s = Watts (W) • I is the current through the device and V is the voltage across it • For resistors, use V=IR, I=V/R to get: P= P= (and P = IV) • Common unit of electrical energy consumption: kilowatt-hour (kWh) ex. Calculate the resistance of a 40W automobile headlight designed for 12V. ex. A power station delivers 520 kW of power to a factory through 3.0 ohm lines. How much less power is wasted if the electricity is delivered at 50,000 V rather than 12,000V? If we have 2 (or more) circuit devices there are two ways we can wire them. In series: Same current goes through each device. In parallel: Same voltage across each device I= Req V V V = + Req R1 R2 V V= IReq = IReq = I (R1 + R2 ) For series: Req = Req For resistors in parallel: 1 = Req Req = Why is Req smaller than any of the individual resistances for a parallel circuit? Consider two identical resistors (R1=R2) wired in series in a circuit, as shown below. The current in the second resistor is: A) equal to B) half C) smaller than, but not necessarily half the current through the first resistor ex. A 60W and a 100W lightbulb are connected in series across a potential difference of 120V. Find: a) the current in each bulb b) the voltage drop across each bulb c) the power dissipated in each bulb ex. A 60W and a 100W lightbulb are connected in parallel across a potential difference of 120V. Find: a) the current in each bulb b) the voltage drop across each bulb c) the power dissipated in each bulb ex. A 1650 W toaster, a 1090 W iron, and a 1250 W microwave oven are turned on in a kitchen. a) Find the equivalent resistance of the three devices b) Will the 20A circuit breaker open? For the circuit shown in the drawing, what is the ratio of the current I1 in resistance R1 to the current I2 in resistance R2? A) I1 R1 = I2 R2 B) I1 R2 = I2 R1 R2 C) I1 = 1 I2 D) I1 R1 = I2 R1 R2 E) I1 R2 = I2 R1