1. Chapter 34 Notes Electric Circuits

2. Electric Current Intro Voltage is an “electric pressure” That can produce a flow of charge, or current, within a conductor The flow is restrained by the resistance it encounters When the flow takes place along one direction, it’s called direct current (DC) When it flows to and fro, it is called alternating current (AC) The rate at which energy is transferred by electric current is power

3. Flow of Charge and Electric Current Charge flows through a conductor until there is a common potential, i.e. a common voltage. Voltage provides the “electric pressure” to move electrons between the 2 ends of a wire. Charges will only flow through a circuit if there is a voltage across the circuit. Sources of continuous voltage A battery Electromagnetic induction part 1 part 2part 1 part 2 Chemical cells Solar cells Thermocouple Piezoelectric cell

4. Current and Voltage Sources To maintain a continuous current there must be a voltage source (an “electric pump”) to provide a sustained potential difference. Current is the flow of electric charge. In solids, only electrons are free to move. In fluids, + and - ions may move as well (like the electrolyte in a car battery). Examples of energy suppliers (providing a steady flow) were discussed on the previous slide Electric current is measured in amperes (A) the flow of 1 coulomb of charge per second

5. Current (I) and Voltage (V) sources An alternating potential difference of 120 volts is supplied to each outlet in your home. Remember: Electric Potential (Volts) = Electric Potential Energy (Joules)/Amount of Charge (Coulombs) 120 Joules of energy is supplied to each Coulomb of charge that is made to flow in the circuit.

6. Electric Resistance (R) The amount of current depends on both voltage and resistance. (Ohm’s Law) Resistance is a measure of how “easily” the electrons can flow through a conductor. The resistance of a wire depends on: The conductivity of the material its made from (how well it conducts) Its thickness (thick have less resistance than thin) Its length (long wires have more resistance than short (optical vs coaxial audio cables) ) And, its temperature. The higher the temperature, the more resistance (why there’s fans in our electronics or our phones get hot when playing games). Resistance can become zero at very low temp, near absolute zeroabsolute zero

7. Measuring Resistance Electric resistance is measured in Ohms (  ). Current in a circuit is directly proportional to the voltage impressed across the circuit, and is inversely proportional to the resistance of the circuit Ohm’s Law: Current = Voltage/Resistance Current is measured in Amperes (A). An older symbol for ampere is the “amp.” 1 ampere = 1 volt/ohm The greater the resistance, the less the current Resistors are used to regulate current inside electric devices, such as radios.

8. Ohm’s Law Practice What is the resistance of an electric griddle that draws 12 amps of current when connected to a 120 V circuit? How much current is drawn by a lamp that has a resistance of 100 ohms when a voltage of 50 V is impressed across it?

9. Fill in the blank

10. Electrical Transmission DC, direct current, current in one direction (a battery) AC, alternating current, electrons move in both directions Electrical transmission using AC is more efficient than DC. High voltage electricity is more efficiently transmitted than low voltage electricity. Easier to increase/decrease AC voltage using transformers than DC voltage.

11. Power The rate at which electric energy is converted into another form is called electric power. Power = current x voltage P = I x V Power measured in Watts (W) A 60 watt device uses 60 joules of energy each second Easy to do calculations

12. Calculating Electrical Energy usage The electric power rating (in watts) of an appliance is usually listed on the appliance Use the power rating to calculate how much energy it uses for the time that it is on Example A toaster has a power rating of 1200W. If it takes 1 minute to toast a slice of bread, how much energy does the toaster use? Energy = power (watts) x time (seconds) 1200W x 60s 72,000 watt-seconds or joules This energy could run a 60W light bulb for more than 20 minutes 72,000J is a huge number and can scare consumers so electric companies use something different

13. Calculating Electrical Energy Usage (cont) Electric companies use the kilo-watt hour (kWh) Still calculated by multiplying power by time But power rating is expressed in kWh and time of usage in hours You can convert a power rating in watts to kilowatts by dividing by 1000W/kilowatt The 1200W toaster uses 1.2 kW Measure time in hours The toaster takes 1 minute/60min = 0.017 hours Calculate electrical energy used as follows Energy = Power x time 1.2 kW x 0.017 h = 0.02 kWh

14. Power examples How much power is used by a calculator that operates on 8V and 0.1A? If it is used for one hour, how much energy does it use? Will a 1200W hair dryer operate on a 120V line if the current is limited to 15A by a fuse? Can two hair dryers operate on this line? Assignment

15. Electricity Formulas Formulas: V = I R I = V/R R= V/I P = IV E = Pt Variable Unit (abbreviation) Voltage (V) Volts (V) Current (I) Amps (A) Resistance (R) Ohms (Ω) Power (P) Watts (W)

16. Electric Bills The electric company charges you for the energy you use each month. If they used the standard unit, Joules, they would have to charge you for millions of them. This would scare people. So they invented a larger unit of energy, the kilowatt-hour, that is 3,600,000 times larger than a Joule.

17. Kilowatt-hour Joules are watts x seconds. If you use a 100-W bulb for 30 hours (108,000 s) you use 10,800,000 Joules of energy. E = Pt = 100 x 108,000 = 10,800,000 J Kilowatt-hours are kilowatts x hours. If you use a 100-W bulb (0.100 kW) for 30 hours you use 3 kilowatt-hours of energy. E = Pt = 0.1 x 30 = 3 kWh

18. Which Bulb is Cheaper?

20. How much would it cost to run this bulb for 1000 hours?

21. First, how much did the bulb itself cost for 1000 hours.

22. Next, calculate the cost of the energy to run it.

23. Total Cost $0.66 for the bulb $1.65 for the electricity $2.31 total for 1000 hours of operation

24. Power in appliances 61” Flat screen 2 hr 30 min/day

25. Power in appliances Blu Ray 2 hr 23 min/day (length of Skyfall)

26. Power in appliances Receiver for DTS surround sound 2 hr 23 min/day (length of Skyfall)

27. Power in appliances Nintendo Wii 45 min/ day

28. Power in appliances Microwave 1 min 30 sec/ day