Current Electricity.notebook. December 17, 2012

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Dennis Rolf Hill
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2 Circuit Diagrams and Assembly 1. Draw a circuit diagram containing a battery, a single throw switch, and a light. 2. Once the diagram has been checked by your teacher, assemble the circuit. Keep the switch open until it has been checked again. 3. Repeat 1 and 2 with a resistor instead of a light. 4. Draw a circuit diagram with both a light and a resistor, and a switch to control each of them individually. After the diagram has been drawn, assemble the circuit to see if it works properly. HW: Read pages 432 to 44. Create a Study Note from these pages, including key terms, processes and explanations. (The point to these notes is to summarize. Keep them brief. Use short forms and organization.) 2

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4 Measuring Current and Voltage To measure current, we use a device called an ( ). It is always connected in line with where we want to measure the current. (It goes directly into the circuit.) To measure voltage, we use a ( ). It is connected around the device for which we want to measure the voltage. 4

5 Reading Analog Meters determine which scale you are using by looking at what terminals the wires are connected to decide how much each 'division' is Examples: A V A V

6 Try this: 1. Draw a circuit diagram with a cell, a resistor, and a light. 2. Now draw a completely different circuit using only the components in part 1. (Note that simply changing the order of the components does not change the circuit at all.) 6

7 Consider the voltage vs current lab that we did earlier. You should have seen that to get more current, you needed to apply more voltage. (The relationship is linear since you need to double one quantity in order to double the other.) What did you notice about the resistance and the slope of the graph? They are the same! This is not a coincidence. Consider a general V vs I graph: When V = V, I = A, therefore the y intercept (b) is zero. V (V) run = I = I rise = V = V I (A) So slope = rise run = V I Since we concluded that the slope is also the resistance (R) then R = V where V is voltage, in volts (V) I I is current, in amperes (A) and R is resistance, in ohms ( ) This is also called Ohm's Law Examples: 7

8 Electrical Power Power is defined as the change in per unit of. Mathematically, where Note that 1 W is 1 J/s. In electricity, however, it is convenient to use other electrical quantities, like current and voltage (which are constant), instead of energy and time (which are continuously changing). Derivation: We know that. However, we also know that, So or Also, or So now Examples: How much current runs through your 24 W TV? How much energy is consumed every hour by a 12 when plugged into a 6. V battery? resistor 8

9 Calculating Cost of Electrical Energy When we pay for electricity, what quantity (V, I, R, P...) are you actually paying for? It is electrical ENERGY that is needed, so that is what we pay for. We know that energy is normally measured in joules (J), but 1 J is a very small amount of energy. (A 1 W light uses 1 J of energy every second!) It is more convenient to use a different unit for energy. Consider this: 1. P = E/t, so E = Pt The energy we use is the product of power and time. When P is in watts (W) and t is in seconds (s), then E is in joules (J). 2. We are billed monthly or bi monthly. In that time, most electrical devices are not on for just a few seconds, but several hours. So instead of measuring time in seconds, we measure it in hours. Also, the power rating for many appliances is measured in kilowatts (kw) as opposed to watts (W). 3. Since E = Pt, if P is in kw, and t is in h, then E is in kilowatt hours (kwh) Note: this is NOT kilowatts per hour The cost of 1 kwh of electricity varies based on where you live and even when you use it! In Ontario we now have time of use billing, where rates are between about 6.3 cents/kwh for "offpeak" times (overnight, weekends), and 11.8 cents/kwh for "peak" times (middle of the day in summer, breakfast and dinner during winter). (Why does the rate need to be different at different times?) The total cost is C = E (in kwh) x rate (in cents/kwh) (divide by 1 to get it into $ from cents Examples: 1. A string of fifty 2 W Christmas lights is turned on for 5 hours a day for 4 full weeks. Assuming an average rate of 9.5 cents/kwh, what is the cost of running these lights for this time? 2. You leave your computer monitor on 24 hours a day, without any energy saving features turned on. The panel on the back of the monitor claims that it draws 1.5 A when on. Assuming an average rate of 7.8 cents per kwh, how much does it cost you to leave it onfor a full year? 9

10 HW: 1. Determine the total cost of running your 9 W hair dryer for 15 minutes a day, for all 19 days of class in the school year. Assume a rate of 1.9 cents/kwh. 2. Complete all three circuits on the hand out. 1

1 of 23. Boardworks Ltd Electrical Power

1 of 23. Boardworks Ltd Electrical Power 1 of 23 Boardworks Ltd 2016 Electrical Power Electrical Power 2 of 23 Boardworks Ltd 2016 What is electrical power? 3 of 23 Boardworks Ltd 2016 Electrical power is the rate at which energy is transferred