Answers to Questions
1. Assume that a battery rating is given in ampere-hours (A-h). What quantity is being measured?
Charge. The ampere is a charge per unit time, and the hour is a time, so the product is charge. 1 Ampere-hour of charge is 3600 Coulombs of charge.
2. When you operate a flashlight, what is being used up: battery current, voltage, energy, power, or resistance?
Energy. As charges leave the battery terminal, they have a relatively high potential energy. As they move through the flashlight bulb, they lose potential energy. The battery uses a chemical reaction to replace the potential energy of the charges, by lowering the battery’s chemical potential energy. When a battery is “used up”, it is unable to give potential energy to charges.
3. One terminal of a car battery, usually the negative, is said to be connected to "ground." Since this cannot be the actual ground, what is meant by this expression?
The negative terminal is connected to the metal chassis, frame, and engine block of the car. Therefore all voltages used for electrical devices in the car are measured with respect to the car’s frame. Compare this with how we measured potential energy earlier.
4. Can a copper wire and an aluminum wire of the same length have the same resistance?
Resitance is given by R = rL/A.
If the ratio of resistivity to area is the same for both the copper wire and the aluminum wire, then the resistances will be the same.
5. Assume you have a rectangular solid made of carbon. It has sides of length a, 2a, and 3a. How would you connect the sires from a battery in order to obtain
a. The least resistance
b. The greatest resistance
Resitance is given by R = rL/A.
Thus, to minimize the resistance, you should have a small length and a large cross-sectional area. Likewise, to maximize the resistance, you should have a large length and a small cross-sectional area.
6. The equation P = V2/R indicates that the power dissipated in a resistor decreases if the resistance is increased. The equation P = I2R implies that the opposite is true.
Explain why there is no contradiction.
To say that P = V2/R indicates a decrease in power as resistance increases implies that the voltage is constant.
To say that P = I2/R indicates an increase in power as resistance increases implies that the current is constant.
Only one of those can be true for any given situation.
If the resistance changes and the voltage is constant, then the current must also change.
Likewise, if the resistance changes and the current is constant, then the voltage must also change.
7. You may have noticed that light bulbs almost always fail just as they are turned on and not after they have been on for a period of time. Why does this happen?
When a light bulb is first turned on, it will be cool and the filament will have a lower resistance than when it is hot. This lower resistance means that there will be more current through the bulb while it is cool. This momentary high current will make the filament quite hot. If the temperature is too high, the filament will vaporize, and the current will no longer be able to flow in the bulb.
8. A 15-A fuse blows repeatedly. Why is it dangerous to replace this fuse with a 25-A fuse?
You will then be allowing more current to flow than it was designed to handle - possibly resuling in a fire.