9.5 Electrical energy and power  (Page 6/8)

The life spans of the three types of bulbs are significantly different. An LED bulb has a life span of 50,000 hours, whereas the CFL has a lifespan of 8000 hours and the incandescent lasts a mere 1200 hours. The LED bulb is the most durable, easily withstanding rough treatment such as jarring and bumping. The incandescent light bulb has little tolerance to the same treatment since the filament and glass can easily break. The CFL bulb is also less durable than the LED bulb because of its glass construction. The amount of heat emitted is 3.4 btu/h for the 8-W LED bulb, 85 btu/h for the 60-W incandescent bulb, and 30 btu/h for the CFL bulb. As mentioned earlier, a major drawback of the CFL bulb is that it contains mercury, a neurotoxin, and must be disposed of as hazardous waste. From these data, it is easy to understand why the LED light bulb is quickly becoming the standard in lighting.

Summary of relationships

In this chapter, we have discussed relationships between voltages, current, resistance, and power. [link] shows a summary of the relationships between these measurable quantities for ohmic devices. (Recall that ohmic devices follow Ohm’s law $V=IR$ .) For example, if you need to calculate the power, use the pink section, which shows that $P=VI$ , $P=\frac{V^2}{R}$ , and $P=I^{2}R$ .

Which equation you use depends on what values you are given, or you measure. For example if you are given the current and the resistance, use $P=I^{2}R$ . Although all the possible combinations may seem overwhelming, don’t forget that they all are combinations of just two equations, Ohm’s law $\left(V=IR\right)$ and power $\left(P=IV\right)$ .

Summary

• Electric power is the rate at which electric energy is supplied to a circuit or consumed by a load.
• Power dissipated by a resistor depends on the square of the current through the resistor and is equal to $P=I^{2}R=\frac{V^2}{R}$ .
• The SI unit for electric power is the watt and the SI unit for electric energy is the joule. Another common unit for electric energy, used by power companies, is the kilowatt-hour (kW $·$ h).
• The total energy used over a time interval can be found by $E={\displaystyle \int Pdt}$ .

Conceptual questions