Electrical Power

Goal of the class Differentiate between direct current and alternating current. Relate electric power to the rate at which electrical energy is converted to other forms of energy. Question of the Day: What factors affect the resistance of a piece of wire? Previous answer: No previous question  Previous question: What factors affect the resistance of a piece of wire?

Question Hair dryers, microwaves, stereos, and other appliances use electric power when plugged into your outlets. What is electric power? Is electric power the same as the power discussed in the chapter “Work and Energy?” Do the utility companies bill your household for power, current, potential difference, energy, or something else? What do you think is meant by the terms alternating current (AC) and direct current (DC)? Which do you have in your home? You may wish to build a simple circuit with a battery or two, a bulb, and a switch with connecting wires. You could use this circuit as a focal point for the discussion of resistance. Ask students to think about what resistance might mean and how it would apply to the different components they see (bulb, wire, insulator on the wire, switch, and so on).

Direct Current Batteries use chemical energy to give electrons potential energy. Chemical energy is eventually depleted. Electrons always flow in one direction. Called direct current (DC) Electrons are repelled from the negative terminal. As a result, they have high PE and flow toward the neutral plate. Eventually, as electrons build up on the plate, the PE of electrons on the plate equals that of the battery, and the flow stops. Similarly, electrons flow from the neutral right plate to a lower PE, the + terminal of the battery. Another explanation is that repulsion builds up between electrons on the left and right plates. This repulsion drives the electrons on the right plate to the battery terminal. + -

Alternating Current Generators change mechanical energy into electrical energy. Falling water or moving steam Electrons vibrate back and forth. Terminals switch signs 60 times per second (60 Hz). Called alternating current (AC) AC is better for transferring electrical energy to your home. Electron flow switches so rapidly that we do not see light bulbs flicker as the current passes though zero twice each cycle.

Energy Transfer Is the electrical potential energy gained, lost, or unchanged as the electrons flow through the following portions of the circuit shown: A to B B to C C to D D to A Explain your answers. After students consider the different segments of the circuit, show the graph of PE on the next slide, followed by the Visual Concept on slide 6 to reinforce the concept. Make sure they explain why they believe energy is gained or lost in the wires, bulb and battery.

Energy Transfer A to B (unchanged) B to C (lost in bulb) C to D (unchanged) D to A (gained in battery) After students consider the different segments of the circuit, show the graph of PE on the next slide, followed by the Visual Concept on slide 6 to reinforce the concept. Make sure they explain why they believe energy is gained or lost in the wires, bulb and battery.

Electric Power Power is the rate of energy consumption (ΔPE/Δt ). For electric power, this is equivalent to the equation shown below. SI unit: joules/second (J/S) or watts (W) Current (I) is measured in amperes (C/s). Potential difference (ΔV) is measured in volts (J/C). Substitute using Ohm’s law (V = IR) to write two other equations for electric power. The Student Edition shows how the equation given here is derived from the definition of power as W/t or PE/t. You may wish to challenge students to work from the definition to derive the equation shown on the slide. This will reinforce the concept that we are using the same definition of power as that used in the chapter “Work and Energy.” For the last bullet point, students should come up with P = I2R and P = V2/R with simple substitutions.

Practice Problems A toaster is connected across a 120 V kitchen outlet. The power rating of the toaster is 925 W. What current flows through the toaster? What is the resistance of the toaster? How much energy is consumed in 75.0 s? Answers: 7.7 A, 16 , 6.94  104 J For problems, it is a good idea to go through the steps on the overhead projector or board so students can see the process instead of just seeing the solution. Allow students some time to work on problems and then show them the proper solutions. Do not rush through the solutions. Discuss the importance of units at every step. Problem solving is a developed skill and good examples are very helpful. Q = Current x time = 0.83 C/s x (3600 s) = 2988 C = 3.0 x 103 C (3.0 x 103 C) x (6.25 x 1018 electrons/C) = 1.9 x 1022 electrons R = V/I = 120 V / 0.83 A = 144.6  = 1.4 x 102 

Household Energy Consumption Power companies charge for energy, not power. Energy consumption is measured in kilowatt·hours ( kw·h). The joule is too small. A kw·h is one kilowatt of power for one hour. Examples of 1 kw·h: 10 light bulbs of 100 W each on for 1 h 1 light bulb of 100 W on for 10 h 1 kw·hr = 3 600 000 J or 3.6 x 106 J Find the cost of 1 kw•h from your power company. It is likely to be a around $0.10. This provides an opportunity to discuss the cost of leaving appliances turned on for extended periods of time.

Electrical Energy Transfer Transfer of energy from power plants to your neighborhood must be done at high voltage and low current. Power lost in electrical lines is significant. P = I2R Power lines are good conductors but they are very long. Since power companies can’t control the resistance (R), they control the current (I) by transferring at high voltage. Alternating current can be changed from low voltage/high current and back using transformers. This allows transmission over large distances at high voltage/low current to reduce power loss. Close to its destination, the energy is converted back to low voltage/high current.

National Grid The equivalent equations for PEelectric are: PEelectric = 1/2C(V)2 PEelectric = Q2/2C

Question Hair dryers, microwaves, stereos, and other appliances use electric power when plugged into your outlets. What is electric power? Is electric power the same as the power discussed in the chapter “Work and Energy?” Do the utility companies bill your household for power, current, potential difference, energy, or something else? What do you think is meant by the terms alternating current (AC) and direct current (DC)? Which do you have in your home? Power is the rate of energy consumption, just as we defined it in the chapter “Work and Energy” and with the same units (J/s or W). Utilities do not bill homes for power but instead bill for the total energy used each month. Power companies use units of kw•h (1 kw•h = 3,600,000 J). With DC, electrons always flow in one direction. With AC, electrons vibrate back and forth. AC is used in homes, and DC is used in batteries.

Homework Please complete questions on page 616 Q 47, 66, 70, 73, 78