Suppose you have a closed box with some type of resistance in a circuit as shown

Question

Suppose you have a closed box with some type of resistance in a circuit as shown. You observe a certain in the indicator bulb A. Someone makes a change in the electrical elements in the box, but you can't see what he did. However, you observe that the indicator bulb becomes brighter. 1. What can you conclude about the current through the battery? 2. What can you conclude about the total resistance in the circuit? Has it increased, decreased or stayed the same? Set up a circuit as shown to the right. Bulb A will be our indicator bulb. Now add another bulb of the same type, bulb C, in parallel with bulb B as shown in the next diagram. 1. Before you close the switch, use your model for electric current to predict the relative brightness of the bulbs. Write your prediction down. Do you expect bulb A to change? 2. Close the switch. How does the brightness of bulb A change, if at all, when another bulb is added to the circuit in parallel with bulb B? 3. 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? 4. Refer to part IVE above to the circuit containing the black box. Using the same reasoning, what do you conclude happens to the total resistance in the circuit when bulb C is added in parallel with bulb B? 5. Restate the rule you devised in part IV D 3, which involved a series circuit. You should be able to show that the rule you have just devised for parallel circuits is consistent with the rule you devised in IV D 3. Explain your rule below.

Explanation / Answer

F)
1)
YES

2)
Brightness of the Bulb A will Increase !!

3)
Let the resistance of each bulb be R.
Initial Resistance = R + R = 2R

After adding bulb parallel to B,
Req = R + 1/ (1/R + 1/R) = 3R/2

Iintial = V/2R = 0.5 R
Ifinal = 2*V/3R = 0.666 V/R

So we see Current in the circuit will increase after we add another Bulb !!!

4)
We see total resistance of the circuit has increased after adding Bulb !!!

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