# Basic circuit question (True/False)

• Sean1218
In summary: Right?In summary, the circuit shows a source of EMF and some identical light bulbs with a switch, S, that can be opened or closed. The questions refer to the situation before and after the opening of S. Based on the equations V=IR and P=I^2R, it can be determined that when S is opened, the potential difference across bulb 4 does not increase (answer 1 is false) and the current through bulb 2 decreases (answer 2 is true). Before the switch is opened, bulb 1 is brighter than bulb 4 and after the switch is closed, bulb 1 is still brighter than bulb 4 (answer 3 is false and answer 4 is false). When
Sean1218

## Homework Statement

The circuit shows a source of EMF and some identical light bulbs. The switch, S, is opened at some point. The questions refer to the situation before and after the opening of S. Answer true or false for each statement.

1. When S is opened, the potential difference across bulb 4 increases.
2. When S is opened, the current through bulb 2 decreases.
3. When S is closed, bulb 1 is brighter than bulb 4.
4. When S is closed, bulb 1 is brighter than bulb 2.
5. When S is opened, bulb 4 becomes brighter.

Circuit: http://i.imgur.com/E9e5Y.gif

V=IR
P=I2R

## The Attempt at a Solution

1. False
2. True
3. False
4. False
5. False

I figured that voltage is constant from the battery, and voltage is the same for each bulb in a parallel circuit, so #1 is False.

Taking away bulb 3 from the circuit means more current for bulb 2 and 3, but it also increases the resistance in the parallel circuit (and thus the total resistance), so the current decreases to compensate. Not sure what the answer would be for #2.

#3 and #4 are the same question as far as I can tell, since bulb 2 and bulb 4 should be identical. I just messed with ratios for the last 3 questions, but I'm not really sure about these.

Any help?

Brightness is proportional to power output, and in this case we care about the amount of current passing through. The current is at a maximum through bulb 1, and this divides into 2*1/2 before the switch is closed and 3*1/3 after the switch is closed.

So the current through bulb 1 is always the greatest of the 4, hence the greatest power (since they are all equal resistances).

But power also depends on resistance, and the resistance is greater in bulb 1. Does this not matter?

I thought you said the bulbs were identical?

Sorry, yea, just confused myself for a second. Any idea for #1, 2 & 5?

1 should be correct.

As for 2, when the switch is closed, the total resistance goes from (1.5)R to (1.333)R. Less resistance means increasing the total current pulled from the voltage source. In fact, you can find the total open-switch current, iopen=V/(1.5R), and the closed-switch current, iclosed=V/(1.333R).

You know that when the switch is open, half the total current goes through bulb 2, and after the switch is closed, 1/3 the total current goes through bulb 2.

So for the currents through bulb 2 before and after switching:

open: ibefore_bulb2=V/(3R) = .333 V/R
closed: iafter_bulb2=V/(4R) = .250 V/R

So the current through bulb 2 decreases when you CLOSE it. Increases when you open it.

## 1. Is a closed circuit necessary for electricity to flow?

Yes, a closed circuit is necessary for electricity to flow. In a closed circuit, the current can flow continuously from the source to the load and back to the source.

## 2. Does the direction of current flow matter in a circuit?

Yes, the direction of current flow does matter in a circuit. Current always flows from the positive terminal of the source to the negative terminal, and this direction should be maintained for proper functioning of the circuit.

## 3. Can a circuit have multiple sources of electricity?

Yes, a circuit can have multiple sources of electricity. These sources can be connected in parallel to increase the overall voltage or in series to increase the overall current.

## 4. Are resistors necessary in a circuit?

Yes, resistors are necessary in a circuit. They help to regulate the flow of current and prevent damage to other components in the circuit.

## 5. Can a circuit have more than one load?

Yes, a circuit can have more than one load. Loads are any components in the circuit that use the electricity, such as light bulbs or motors.

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