Samuel J. Ling; William Moebs; Jeff Sanny

Conceptual Questions

9.1 Electrical Current

1.

Can a wire carry a current and still be neutral—that is, have a total charge of zero? Explain.

2.

Car batteries are rated in ampere-hours $(A \cdot h)$ . To what physical quantity do ampere-hours correspond (voltage, current, charge, energy, power,…)?

3.

When working with high-power electric circuits, it is advised that whenever possible, you work “one-handed” or “keep one hand in your pocket.” Why is this a sensible suggestion?

9.2 Model of Conduction in Metals

4.

Incandescent light bulbs are being replaced with more efficient LED and CFL light bulbs. Is there any obvious evidence that incandescent light bulbs might not be that energy efficient? Is energy converted into anything but visible light?

5.

It was stated that the motion of an electron appears nearly random when an electrical field is applied to the conductor. What makes the motion nearly random and differentiates it from the random motion of molecules in a gas?

6.

Electric circuits are sometimes explained using a conceptual model of water flowing through a pipe. In this conceptual model, the voltage source is represented as a pump that pumps water through pipes and the pipes connect components in the circuit. Is a conceptual model of water flowing through a pipe an adequate representation of the circuit? How are electrons and wires similar to water molecules and pipes? How are they different?

7.

An incandescent light bulb is partially evacuated. Why do you suppose that is?

9.3 Resistivity and Resistance

8.

The IR drop across a resistor means that there is a change in potential or voltage across the resistor. Is there any change in current as it passes through a resistor? Explain.

9.

Do impurities in semiconducting materials listed in Table 9.1 supply free charges? (Hint: Examine the range of resistivity for each and determine whether the pure semiconductor has the higher or lower conductivity.)

10.

Does the resistance of an object depend on the path current takes through it? Consider, for example, a rectangular bar—is its resistance the same along its length as across its width?

Pictures are a schematic drawing of a resistance object with the long side of the length R and the short side of the length R prime. In the left picture, current flows along the long side; in the right picture, current flows along the short side.

11.

If aluminum and copper wires of the same length have the same resistance, which has the larger diameter? Why?

9.4 Ohm's Law

12.

In Determining Field from Potential, resistance was defined as $R \equiv \frac{V}{I}$ . In this section, we presented Ohm’s law, which is commonly expressed as $V = I R$ . The equations look exactly alike. What is the difference between Ohm’s law and the definition of resistance?

13.

Shown below are the results of an experiment where four devices were connected across a variable voltage source. The voltage is increased and the current is measured. Which device, if any, is an ohmic device?

Figure is a plot of current versus voltage. For A, current originally increases with voltage, then saturates and remains the same. For B, current linearly increases with voltage. For C current increases with voltage at a growing late. For D current decreases with voltage approaching zero.

14.

The current I is measured through a sample of an ohmic material as a voltage V is applied. (a) What is the current when the voltage is doubled to 2V (assume the change in temperature of the material is negligible)? (b) What is the voltage applied is the current measured is 0.2I (assume the change in temperature of the material is negligible)? What will happen to the current if the material if the voltage remains constant, but the temperature of the material increases significantly?

9.5 Electrical Energy and Power

15.

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 the power companies transport electric power using this method?

16.

Your electric bill gives your consumption in units of kilowatt-hour (kW $\cdot$ h). Does this unit represent the amount of charge, current, voltage, power, or energy you buy?

17.

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

18.

An immersion heater is a small appliance used to heat a cup of water for tea by passing current through a resistor. If the voltage applied to the appliance is doubled, will the time required to heat the water change? By how much? Is this a good idea?

9.6 Superconductors

19.

What requirement for superconductivity makes current superconducting devices expensive to operate?

20.

Name two applications for superconductivity listed in this section and explain how superconductivity is used in the application. Can you think of a use for superconductivity that is not listed?