Section 3 – Electric Current Models

Experiment 3.1 A) Set up a simple circuit with a single battery and bulb. We will call this the “indicator bulb.” B) Add another bulb in series. BEFORE you connect the wires, predict the relative brightness of the bulbs. Are you correct? C) Add a third bulb in series. BEFORE you connect the wires, predict the relative brightness of the bulbs. Will the bulbs change brightness? Write your prediction down in your notes.

Experiment 3.1 C) con’t. How does the brightness of bulb A compare to the indicator bulb? What about to the 2 bulb series? Do you think a bulb may have current through it but be too dimly lit for you to see it?

Experiment 3.1 D) You have seen what happens to the indicator bulb (Bulb A) when more bulbs are added in series to the circuit. What can you infer about the current through the indicator bulb (hence through the battery) as each bulb is added in the circuit? E) We may think of a bulb as presenting an obstacle, or resistance, to the current in the circuit. Thinking of the bulb in this way, how you would expect the total obstacle to the flow, or total resistance, to be affected by the addition of more bulbs in series?

Experiment 3.1 F) Use the line of reasoning suggested above to summarize the results of this experiment as a rule that you can include in your model for electric current. The rule should allow you to predict whether the current through the battery increases or decreases as the total resistance in the circuit is increased or decreased.

Exercise 3.2 Suppose you have a closed box in a circuit as shown. You observe a certain brightness in the indicator bulb A. Imagine that someone makes a change in the electrical elements inside the box. You do not see what has been done. However, you observe that the indicator bulb becomes brighter. What can you conclude about the current through the battery? Explain how bulb A is serving as an indicator bulb in this circuit. What can you conclude about the total resistance in the circuit? Has it increased, decreased, or stayed the same? Explain.

Experiment 3.3 A. Set up a circuit consisting of a battery and two bulbs, A and B, as show on the board. A will be in the indicator bulb. B. Now add another bulb, C, in parallel with bulb B as shown. BEFORE you wire it, predict the relative brightness of the bulbs. Write down your prediction. Wire the bulbs and record your findings.

Exp. 3.3 C In this experiment you have observed what happens to an indicator bulbs when a bulb is added in parallel with another bulb in the circuit. What can you infer about the current through bulb A (and hence through the battery) as bulb C is added in parallel with bulb B? Refer to exercise 3.2 where you had a circuit containing a black box. What do you conclude happens to the total resistance of the circuit when bulb C is added in parallel with bulb B?

3.3 C con’t, D Use the results of Exercise 3.2 and this experiment to devise a rule for how the current through the battery changes when a bulb is added in parallel to another bulb. D) Restate the rule you devised in Exp. 3.1 in which you discussed how adding bulbs in series to a circuit affects the resistance. You should be able to show that the rule you have just devised for bulbs added in parallel is consistent with your rule from Experiment 3.1

The results of Experiment 3.3 may seem puzzling. One way to think about the changes in current when the bulbs are connected in parallel is to consider the additional pathways available to the current after is passes through bulb A. Not only is the pathway through the original bulb B still available, but other pathways become available when additional bulbs are connected in parallel with bulb B.

Exercise 3.4 Student 1 “Adding bulbs to a circuit increases the total resistance. There is a bigger obstacle to the current so less flows.” Student 2 “Adding bulbs to a circuit may increase or decrease the total resistance. It all depends on how you add them. If you add them in parallel, you give the current more pathways, so the total resistance is less.”

As part of Experiment 2.6, we observed that the brightness of a single bulb connected directly across the battery does not change significantly when a second bulb is connected in parallel to the battery. In the next experiment, we investigate the changes in brightness of bulbs as additional parallel branches are connected across the battery.

Experiment 3.5 A. Set up a single bulb circuit. Connect two bulbs in series and add this combination as a branch in parallel with the single bulb as shown. Is there a significant change in brightness of the single bulb when the second branch is connected? Is there a significant change in brightness of the single bulb or the two bulbs in the second branch when a third branch with three bulbs in series is connected?

Exp. 3.5 B Predict what will happen in the following cases. After you have written down your prediction, check your answers. What will happen to the brightness of bulb E when bulb A is unscrewed? What will happen to the brightness of bulb B and to the brightness of bulb D when bulb E is unscrewed? Explain.

Experiment 3.5 C We speak of parallel branches as being independent of one another when changes in one branch do not significantly affect the other branch. Do parallel branches of bulbs across a battery appear to be dependent or independent of one another?