9.5 Electrical Energy and Power

Electric motors have a reasonably high efficiency. A 100-hp motor can have an efficiency of 90% and a 1-hp motor can have an efficiency of 80%. Why is it important to use high-performance motors?

Is the efficiency of the various light bulbs the only consideration when comparing the various light bulbs?

Common household appliances are rated at 110 V, but power companies deliver voltage in the kilovolt range and then step the voltage down using transformers to 110 V to be used in homes. You will learn in later chapters that transformers consist of many turns of wire, which warm up as current flows through them, wasting some of the energy that is given off as heat. This sounds inefficient. Why do t…

Resistors are commonly rated at $\frac{1}{8} \text{W}$, $\frac{1}{4} \text{W}$, $\frac{1}{2} \text{W}$, 1 W and 2 W for use in electrical circuits. If a current of $I = 2.00 \text{A}$ is accidentally passed through a $R = 1.00 \Omega$ resistor rated at 1 W, what would be the most probable outcome? Is there anything that can be done to prevent such an accident?

A $20.00 -\text{V}$ battery is used to supply current to a $10 -\text{k} \Omega$ resistor. Assume the voltage drop across any wires used for connections is negligible. (a) What is the current through the resistor? (b) What is the power dissipated by the resistor? (c) What is the power input from the battery, assuming all the electrical power is dissipated by the resistor? (d) What happens to the energy dissipated by the resistor?

A heater is being designed that uses a coil of 14-gauge nichrome wire to generate 300 W using a voltage of $V = 110 \text{V}$. How long should the engineer make the wire?

The power dissipated by a resistor with a resistance of $R = 100 \Omega$ is $P = 2.0 \text{W}$. What are the current through and the voltage drop across the resistor?

A physics student has a single-occupancy dorm room. The student has a small refrigerator that runs with a current of 3.00 A and a voltage of 110 V, a lamp that contains a 100-W bulb, an overhead light with a 60-W bulb, and various other small devices adding up to 3.00 W. (a) Assuming the power plant that supplies 110 V electricity to the dorm is 10 km away and the two aluminum transmission cables…