Physics 152 V 1 + V 2 = V. Parallel & Series Circuits Electric Power. Announcements. Wednesday, April 4, 2007

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1 ics Wed Apr.04. Announcements Parallel & Series Circuits Electric Power Wednesday, April 4, 2007 Help sessions W 9-10 pm in NSC 119 Masteringics WU #18 due Mon., April 9 Hwk #4 due Wed., Apr. 11 WU #19 due Fri., April 13 Exam - Thurs., April 12 A 3.0 A current is observed to flow in the circuit at right. What is the resistivity of carbon? Worksheet Problem # = 1, 1 2, 2 2 So, what happens after the battery is connected to this circuit? Knight (2004) Worksheet Problem #2 1 2 = 1 1, 1 2, 2 2 So, what happens after the battery is connected to this circuit? 1 = 2 due to conservation of charge! 1

2 = 1 2 = 1 2 = ( 1 2 ) We can construct an equivalent circuit with a single resistor... eq 1, 1 2, 2 = eq = 1 2 = 1 2 = ( 1 2 ) eq = 1 2 = eq esistors in series ADD. eq So, what happens after the battery is connected to this circuit? The current divides, with part of it going through 1 and part of it going through 2. Worksheet Problem #3 The potential difference across 1 and 2 must be the same! (Just like capacitors in parallel!) eq < 1, 2... The current flowing through a circuit with resistors connected in parallel will be much greater than the current through a series circuit of the same resistors. This is why it is NOT a good idea to plug too many appliances into the same electrical outlet! Eventually, the breaker will trip! = 1 2 conservation of charge = 1 1 = We can construct an equivalent circuit with a single resistor... eq = eq 2

3 = / eq eq = eq 1 2 = 1 1 = = / 1 2 = / 2 = 1 2 = = eq 1 2 esistors in parallel ADD NESELY. esistors in parallel add inversely. Capacitors in parallel simply add. esistors in series simply add. Capacitors in series add inversely. What is the equivalent resistance of this combination of resistors? Worksheet Problem # Battery esistor 2 Capacitor Ground What happens in a series circuit if one of the resistors burns out? 1 2 = 0 When one element fails, the circuit is broken; the current ceases to flow; and all the other elements (in the same branch of the circuit) stop functioning as well. Old Christmas tree lights used to be wired in series. When one bulb burned out, the whole string of lights went dark. Figuring out which bulb had failed could be quite a chore! 3

s there any time you might want to use a series circuit? s there any time you might want to use a series circuit? Well, a fuse box or circuit breaker box is a good place to use a series circuit.

4 s there any time you might want to use a series circuit? s there any time you might want to use a series circuit? Well, a fuse box or circuit breaker box is a good place to use a series circuit. When the appliances in your house draw too much current, a potential fire hazard exists. (Too much current leads to too much joule heating of the wires.) Circuit breakers and fuses are designed to fail when the currents through them become too large. By sending power into your house through a circuit breaker or fuse, you prevent large currents from entering the house. The circuit breaker (fuse) is wired in series with the electrical outlets in the house. The outlets are actually wired in parallel with each other. Breaker Box Outlets with Appliances Breaker Box Outlets with Appliances No Current! When the breaker is tripped, current ceases to flow to the outlets connected to that circuit in the house. We use parallel circuits to wire our houses, offices, dorm rooms, etc., etc., etc. n the parallel circuits, in our houses, each outlet gets the same voltage. f one element in the circuit fails, the others will continue to receive current. Why do we want our outlets wired in parallel? Breaker Box Current flows Outlets with Appliances Unlike in the series circuit, the failure of one element in the parallel circuit does not disable the entire circuit. 4

5 Worksheet Problem #5 We define power to be the amount of work done in a given amount of time... What does POWE mean in electrical circuits? ab = Power is the change in energy per unit time. P = E t a b c d cd = - = - bc = da = 0 a b c d As we move a charge q from a to b, it gains potential energy equal to U = q 0 We assume that the wires have no internal resistance. 0 P = E t = q t = a b Power is the change in energy per unit time. 0 c d f we look at infinitessimally small bits of charge in similarly small time intervals, we can define an instantaneous power: P = de dt = dq dt = P = [P] = [] [ ] [P] = A P = E t [P] = [ E] / [ t] [ P ] = J / s = Watt P = 2 [P] = [] 2 [] = A 2 5

So, when the electric company sends you a bill at the end of the month, what are they charging you for? Well, what are the units of the quantity for which they bill you?

6 So, when the electric company sends you a bill at the end of the month, what are they charging you for? Well, what are the units of the quantity for which they bill you? Kilowatt-Hours 10 3 W - Hours 10 3 (J/s) Hours 10 3 (J/s) Hours (3600 s/hour) 3.6 X 10 8 J Worksheet Problem #6 An electrical heater with an internal resistance of 8 is attached to a 50.0 potential difference. What is the power rating of the heater? Worksheet Problem #7 Worksheet Problem #8 6

Electricity Test Review

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