1. POWER USAGE IN THE HOME And you!!!!

2. Units of Power are the Watt (= J/s) P = V I J = J C__ s C s Your parents receive an electric bill at the end of each month and the units are kWh (Kilowatt Hour) = 1000 J/s (3600 s/1) = 3.6 x 10 6 J = 3.6 MJ (3.6 million joules) = Energy bill !!!!

3. Hey – that is a lot of energy!!! You said it!!!!! How do we get that much Power into our houses and keep safe????????????????? Boy am I glad you asked!!!!!!!!! Lets draw a parallel to help you visualize the situation..... Think about a water wheel (like the one you drive by in Idaho Springs)...

4. Recall the relationship: P = V I High Voltage Low Current High voltage and low current will provide a fair amount of power – but There is always the threat of getting killed by the high voltage!!!!

5. Recall the relationship: P = V I Low voltage Large current Low voltage and high current will also provide a fair amount of power.... But too much current will start the wires on fire!!!! KE e- = ΔTE wires!

6. So what model should we use?? YES!!!!! Most of Europe uses 240 V (high voltage) and low current...and has too many accidental electrocutions!!!! The U.S. uses 120 V (low voltage) but a higher current...and has more accidental house fires!!! So what is to be done?????? FUSE BOXES (circuit breakers) that limit the current that can pass through the wires of your house – so they do not start a fire!!!

7. How do they work??? What if we wire our circuit in series??? 30 amp fuse box 100 W 1000 W For a single 100 W of P draw using 120 V, the I will =.83A But recall there will be a voltage drop for every appliance That you put in series – so the the 1000 W power draw will require 16.7 A of current if the V available is 60V P = IV

8. Clearly.... The voltage drop requires a large current to allow the appliance to work!!! Since the current needs to be the same through every appliance – smaller appliances will burn out using the high current needed for larger appliances... In no time at all the circuit breaker will be “open” and the current will stop!!! Notice that in series wiring also requires that all the appliances must be on at the same time –or all will be off!!!

9. SO wire in parallel!! 30 A breaker 100 W 1000 W *Each appliance can be turned on or off as needed *Added appliances do not result in a voltage drop *Current draw can be unique to each appliance

10. NOTE How the current draw of the 100 W appliance will still be.83A when using 120 V P = I V so I = P/V = 100/120... So for the entire diagramed circuit the I needed will be.83 +.83 + (8.3 x 3) = 21.5 A so the circuit will remain open!!! And recall that in parallel circuits result in lower resistance (for the entire circuit) when additional branches are added...so extra current will not be a problem for a time...

11. One last thing... If: P = I V and I = V/R then sub in for I... P = V 2 /R so R = V 2 /P For my 100 W bulb using 120 V, it will have 144Ω of resistance My 1000 W bulb will have 14.4Ω of resistance SO in series a total of 207Ω of resistance And in parallel only.15Ω of resistance Smaller R = much safer!!!!!

12. Electrical Power I a)Show that Watts = amps * volts J/S = C/s J/C a)For a house running 120V, how much current is drawn by a 2W clock? 1200W hair dryer?.017 A 10 A a)Why do houses have circuit breakers?

13. Electrical Power II A 6V battery (DC) is attached to a light bulb (R = 3Ω) and motor (R = 9Ω) in series. The wires also have a combined R = 0.5Ω. Calculate: a.I in circuit a.V drop across the bulb and motor a.Power consumed (used) by bulb and motor I = V t / R t = 6/12.5 =.48 A V b = I b R b =.48 (3) = 1.44V V m = I m R m =.48 (9) = 4.38 V P = IV P b =.48(1.44) =.69W P m =.48(4.32) = 2.1 W

14. A 6V battery (DC) is attached to a light bulb (R = 3Ω) and motor (R = 9Ω) in series. The wires also have a combined R = 0.5Ω. D. Number of electrons that leave the battery each second E. Energy carried by each electron I = C/s, Chg. on e- = C/e- therefore I/Chg. on e- = C/s C/e- = e-/s =.48 C/s ÷ 1.6 x 10 -19 C/e- = 3 x 10 18 e-/s Voltage is a measure if EPE.... 6V = 6 J/C (1.6 x 10 -19 C/e-) = 9.6 x 10 -19 J/e-