## University of California, San Diego Physics 1b - Thermal Physics & Electromagnetism

 H. E. Smith Spring 2000  Physics 1B - Tutorial #7 You will be provided with light bulbs, batteries and clip wires. You should work in pairs where possible. Make sure each student makes each circuit. You may make your notes on this sheet or separate pages as you wish. Please leave the equipment in good condition for the next tutorial session.

## I. Complete Circuits

• A. Light a bulb using a single battery and a single wire. Observe and record the behavior (i.e., brightness) of the bulb when objects made out of various materials are inserted into the circuit. (Try materials such as paper, coins, pencil lead, eraser, your finger etc.)
• What is similar about most of the objects that let the bulb light?

 B. Carefully examine a bulb. Two wires extend from the filament of the bulb into the base. You probably cannot see into the base, however, you should be able to make a good guess as to where the wires are attached. Describe where the wires attach. On the basis of the observations that we have made, we will make the following assumptions: A flow of charge exists in a complete circuit from one terminal of the battery, through the rest of the circuit, back to the other terminal of the battery, through the battery and back around the circuit. We call this flow electric current. (Of course, what you are seeing is the luminous power output of the bulb, which is related to the power consumed and to the current by P=VI). For identical bulbs, the bulb brightness can be used as an indicator of the amount of current through that bulb: the brighter the bulb, the greater the current. Starting with these assumptions, we will develop a model that we can use to account for the behaviour of simple circuits.

## II. Bulbs in Series

Set up a two-bulb circuit with identical bulbs connected one after the other as shown. Bulbs connected in this way are said to be connected in series.
 A. Compare the brightness of the two bulbs with each other. (Pay attention only to large differences in brightness. You may notice minor differences if two "identical" bulbs are, in fact, not quite identical.) Use the assumptions that we have made in developing our model for electric current to answer the following questions. Is current "used up" in the first bulb, or is the current the same through both bulbs? Do you think that switching the order of the bulbs might make a difference? Check your answer. On the basis of your observations alone, can you tell the direction of the flow through the circuit? Two bulbs in series

• B. Compare the brightness of each of the bulbs in the two-bulb circuit with that of a bulb in a single-bulb circuit. Use the assumptions that we have made in developing our model for electric current to answer the following questions.
1. How does the current through the bulb in a single-bulb circuit compare with the current through the same bulb when it is connected in series with a second bulb? Explain.
2. What does your answer to question 1 imply about how current through the battery in a single-bulb circuit compares to the current through the battery in a two-bulb series circuit? Explain.
• C. We may think of a bulb as presenting an impediment, or resistance, to the current in the circuit.
1. Thinking of the bulb in this way, would adding more bulbs in series cause the total impediment to the flow, or total resistance, to increase, decrease, or stay the same as before?
2. Formulate a rule for predicting how the current through the battery would change (i.e., whether it would increase, decrease, or remain the same) if the number of bulbs connected in series were increased or decreased.

## III. Batteries in Series

Using the 2 bulbs in series add an extra battery in series with the first so that their voltages both act in the same direction. Draw the circuit you have created.

• A. Compare the brightness of the 2 light bulbs with 2 batteries in series to their brightness with one battery only. Should the brightness of each bulb in your 2 battery/2 bulb circuit be the same as in the 1 battery/1 bulb circuit?

• B. Briefly connect the 2 batteries to a single light bulb. (Just touch the terminal). What do you see?

• C. Turn one of the batteries around. What happens?

## IV. Bulbs and Batteries in Parallel

 Set up a two-bulb circuit with identical bulbs so that their terminals are connected together as shown. Bulbs connected together in this way are said to be connected in parallel. A. Compare the brightness of the bulbs in this circuit. What can you conclude from your observation about the amount of current through each bulb? Describe the current in the entire circuit. Base your answer on your observations. In particular, how does the current through the battery seem to divide and recombine at the junctions of the two parallel branches? Two bulbs in parallel

• B. Is the brightness of each bulb in the two-bulb parallel circuit greater than, less than, or equal to that of a bulb in a single-bulb circuit? Disconnect one bulb and check your answer.
1. How does the amount of current through a battery connected to a single bulb compare to the current through a battery connected to a two-bulb parallel circuit? Explain, based on your observations.

• C. Formulate a rule for predicting how the current through the battery would change (i.e., whether it would increase, decrease, or remain the same) if the number of bulbs connected in parallel were increased or decreased. Base your answer on your observations of the behavior of the two-bulb parallel circuit and the model for current. What can you infer about the total resistance of a circuit as the number of parallel branches is increased or decreased?

• D. With both bulbs connected in parallel add a second battery in parallel. Does this affect the brightness of the bulbs? If the 2 batteries had slightly different voltages (maybe one is a little flat) would you expect the bulbs to change a little in brightness as the second battery is added?

• E. How would you compare the current through each battery with the current through a single battery? Would the 2 batteries in parallel be able to light the bulbs for a longer period of time than a single battery?

## V. More Complicated Circuits

 A. The circuit at right contains three identical bulbs and a battery. You may use 2 batteries in series to make a higher voltage. Connect and disconnect a wire to act as a switch. Predict the relative brightness of the bulbs in the circuit with the switch closed. Explain. Predict how the brightness of bulb A changes when the switch is opened. Explain. B. Predict the relative brightness of bulbs B1, B2, and B3 in the circuits shown below. (A dashed box has been drawn around the network of circuit elements that is in series with each of these bulbs.) What does your prediction imply about the relative current through the batteries? Explain. Put together the circuits so that you can check your answers. Resolve any conflicts between your answers and your observations. C. Before setting up the circuit shown at right: Predict the ranking of the currents through the battery and each bulb (iBat, i1, i2, and i3). Explain. Set up the circuit and check your predictions. If your observations and measurements are not consistent with your predictions, resolve the inconsistencies.   Physics 1B Home Tutorials

Gene Smith