Conceptual Questions

ssm Solution is in the Student Solutions Manual.

| 1 | The drawing shows a circuit in which a light bulb is connected to the household ac voltage via two switches, S1 and S2. This is the kind of wiring, for example, that allows you to turn a carport light on and off from either inside the house or out in the carport. Explain which position A or B of S2 turns the light on when S1 is set to (a) position A and (b) position B.

| 2 | When an incandescent light bulb is turned on, the tungsten filament becomes white hot. The temperature coefficient of resistivity for tungsten is a positive number. What happens to the power delivered to the bulb as the filament heats up? Does the power increase, remain the same, or decrease? Justify your answer.

| 3 | ssm Two materials have different resistivities. Two wires of the same length are made, one from each of the materials. Is it possible for each wire to have the same resistance? Explain.

| 4 | Does the resistance of a copper wire increase or decrease when both the length and the diameter of the wire are doubled? Justify your answer.

| 5 | One electrical appliance operates with a voltage of 120 V, while another operates with 240 V. Based on this information alone, is it correct to say that the second appliance uses more power than the first? Give your reasoning.

| 6 | Two light bulbs are designed for use at 120 V and are rated at 75 W and 150 W. Which light bulb has the greater filament resistance? Why?

| 7 | Often, the instructions for an electrical appliance do not state how many watts of power the appliance uses. Instead, a statement such as “10 A, 120 V” is given. Explain why this statement is equivalent to telling you the power consumption.

| 8 | The drawing shows a circuit that includes a bimetallic strip (made from brass and steel, see Section 12.4) with a resistance heater wire wrapped around it. When the switch is initially closed, a current appears in the circuit, because charges flow through the heater wire (which becomes hot), the strip itself, the contact point, and the light bulb. The bulb glows in response. As long as the switch remains closed, does the bulb continue to glow, eventually turn off permanently, or flash on and off? Account for your answer.

Question 8

| 9 | The power rating of a 1000-W heater specifies the power consumed when the heater is connected to an ac voltage of 120 V. Explain why the power consumed by two of these heaters connected in series with a voltage of 120 V is not 2000 W.

| 10 | ssm A number of light bulbs are to be connected to a single electrical outlet. Will the bulbs provide more brightness if they are connected in series or in parallel? Why?

| 11 | A car has two headlights. The filament of one burns out. However, the other headlight stays on. Draw a circuit diagram that shows how the lights are connected to the battery. Give your reasoning.

| 12 | In one of the circuits in the drawing, none of the resistors is in series or in parallel. Which is it? Explain.

| 13 | ssm You have four identical resistors, each with a resistance of R. You are asked to connect these four together so that the equivalent resistance of the resulting combination is R. How many ways can you do it? There is more than one way. Justify your answers.

| 14 | Compare the resistance of an ideal ammeter with the resistance of an ideal voltmeter and explain why the resistances are so different.

| 15 | Describe what would happen to the current in a circuit if a voltmeter, inadvertently mistaken for an ammeter, were inserted into the circuit.

| 16 | The time constant of a series RC circuit is . Verify that an ohm times a farad is equivalent to a second.

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