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20.4 Electric Power and Energy

- Understand the physics of resistance in a wire.
- The wire's resistance can be changed to see how they affect it.

- Many people have electricity.
- There are power transmission lines.
- We think of lightbulbs in terms of their power ratings.
- We will compare a 25-W bulb with a 60-W bulb.
- The 60-W bulb needs to draw more current to have a better power rating.
- The resistance of the 60-W bulb must be lower than that of the 25-W bulb.
- If we increase power, we also increase voltage.
- When a 25-W bulb that is designed to operate on 120 V is connected to 120 V, it briefly glows and then burns out.

- The potential difference the charge moves through is expressed as, where is the charge moved and is the voltage.

- The product of current times voltage is electric power.
- The units of watt are familiar to power users.
- Power has units of joules per second, or watt, since the SI unit for potential energy is the joule.
- You can charge a cell phone or other electronic device from an auxiliary power outlet in a car.
- The circuit can deliver a maximum power if these outlets are rated at 20 A.
- Electric power may be expressed as kilovolt-amperes in some applications.

- To see the relationship of power to resistance, we combine the two laws.

- The first equation is always valid and the other two can only be used for resistors.
- The power supplied by the voltage source and the power dissipated by the Resistor are the same in a simple circuit.

- There are three different expressions for electric power.
- The lower the resistance, the greater the power delivered.
- The effect of applying a higher voltage is greater than expected.
- When the voltage is doubled to a 25-W bulb, its power almost triples to 100 W, burning it out.
- The power of the bulb would be exactly 100 W, but it would be higher at higher temperatures.

- We can find the power by knowing the voltage and current.
- We can find the power with the knowledge of the voltage and resistance.

- The 30 W dissipated by the hot light.
- When it's cold, the 411 W is higher.
- As the bulb's temperature increases, the initial power decreases.

- There are several ways to find the current when the bulb is cold.

- The cold current is higher than the steady-state value of 2.50 A, but it will quickly decline as the bulb's temperature increases.
- As a device comes on, most circuit breakers are designed to tolerate very high currents for a short time.
- Special "slow blow" fuses are required in some cases, such as with electric motors.

- The higher your electric bill is, the more electric appliances you use.
- The fact is based on the relationship between power and energy.
- You pay for the use of energy.

- The energy used by a device is 20.34.
- The longer the lightbulbs are on, the greater their use.

- It is easy to estimate the cost of operating electric appliances if you know their power consumption rate in kilowatts, the time they are on in hours, and the cost per kilowatt-hour for your electric utility.

- It is possible to convert kilowatt-hours to joules.

- Reducing the time of use is one way to reduce the electrical energy used.
- This will result in a reduced impact on the environment.

- Some of the fastest ways to reduce electrical energy in a home or business are improvements to lighting.
- 20% of a home's use of energy goes to lighting, while 40% goes to commercial establishments.
- The fluorescent lights are four times more efficient than the incandescent lights.
- A 15-W bulb has the same brightness and color as a 60-W bulb.
- A bent tube inside a globe or a spiral-shaped tube is connected to a standard screw-in base that fits standard incandescent light sockets.
- They last up to 10 times longer because of the less heat transfer.
- The significance of an investment in such bulbs is addressed in the next example.
- The new whiteLED lights are 5 times longer than the old ones and are twice as efficient.
- Their cost is still high.

- The relationship is useful in many different ways.
- The power level and duration of your activity are related to the amount of energy you use in exercise.
- Power level and time are related to the amount of heating by a power source.
- The power and time of exposure are related to the radiation dose.

- The energy used in kilowatt-hours and the cost per kilowatt-hour are used to find the operating cost.

- The total cost will be $7.20 for 1000 hours.

- The electricity cost will be $7.20/4 since the CFL uses 15 W and not 60 W. The investment cost will be a tenth of the bulb cost for that time period of use, or 0.1 dollars per pound.
- For 1000 hours, the total cost will be $1.

- Even though the initial investment is higher, it is cheaper to use the CFLs.
- The increased cost of labor that a business must include for replacing the incandescent bulbs more often has not been figured in.

- Take a look at the total wattage used in the rest rooms.

- Most of the examples dealt with so far have constant voltage sources.
- The current is a constant once it is established.
- It is the state of the circuit.
- The majority of well-known applications use a time-varying source.
- The circuit is known as an alternating current circuit if the source varies frequently.
- Commercial and residential power serves a lot of our needs.
- Around the world, the AC voltages and frequencies are different.

- A simple resistance circuit uses the voltage and current in phase.
- AC sources have different frequencies and peak voltages.

- There is a potential difference between the terminals of the AC source.
- The expression is given by.

- Figure 20.17 shows a schematic of a simple circuit.

- The current in the resistor is the same as the driving voltage.
- As the current repeatedly goes through zero, the fluorescent light bulb's Resistor will light up and dim 120 times per second.
- If you wave your hand back and forth between your face and a fluorescent light, you will see a stroboscopic effect.
- The power is variable because the light output fluctuates.
- The power is supplied.

- Their product is non- negative and fluctuates between zero and, since the voltage and current are in phase here.
- The average power is.

- The 60-W light bulb in your desk lamp has an average power consumption of 60 W.

- The areas above and below the line are equal, but it can also be proven using trigonometric identities.

- To get a root mean square, the quantity is squared, its mean is found, and the square root is taken.
- The average value for AC is zero.

- Most household electricity is 120 V AC, which means it is 120 V. The 1.0-kW microwave oven consumes a lot.
- The equivalent DC values for a simple resistive circuit are the rms and average values.

- The equations for power are similar to those for DC, but rms and average values are used for AC.

- 120 V is 60.0 W and we can find the peak power from the average power.

- The AC voltage can change from 170 V to and back 60 times a second.
- A constant 120 V is the equivalent DC voltage.

- Peak power is the amount of power at a time.

- The power swings from zero to 120 W one hundred twenty times per second, and the power averages 60 W.

- AC is the most common power-distribution system.
- In most parts of the world, the 120-V AC is used for homes and on the job, but the power is transmitted at much higher voltages.
- It is cheaper to build a few large electric power-generation plants than it is to build many small ones.
- It is important that energy losses are minimized when sending power long distances.
- High voltages can be transmitted with less power losses than low ones.
- The user's voltage is reduced for safety reasons.
- AC is used in most large power distribution systems because it is easier to increase and decrease it than DC.

- To reduce power loss in the transmission lines, power is distributed over large distances.
- The power plant's voltages are stepped up by passive devices called transformers, which can be found in many places around the world.
- The transformer reduces the voltage transmitted for safe use.

- We can find the current flowing from and then the power dissipated in the lines by taking the ratio to the total power transmitted.

- To find the current, we rearrange the relationship.

- The power dissipated in the lines is found by knowing the current and resistance.

- An acceptable loss is one-fourth of a percent.
- If 100 MW of power had been transmitted at 25 kV, a current of 4,000 A would have been required.
- This would result in a power loss of 16.0% in the lines.
- The lower the voltage, the more current is needed.
- Lower-resistance lines can be built, but they require larger and more expensive wires.
- There would be no loss in the transmission lines if the lines were economically produced.
- There is a limit to current in superconductors.
- AC is used in most large-scale power distribution systems because it is easier to raise and lower than high voltages.

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