In one second, do fewer electrons leave the bulb than enter the bulb? Does the e

Question

In one second, do fewer electrons leave the bulb than enter the bulb? Does the electric potential energy of electrons change while inside the bulb? The students have an adjustable power source, insulated wire, lightbulbs, resistors, switches, voltmeters, ammeters, and other standard lab equipment. Assume that the power supply and voltmeters are marked in 0.1 V increments and the ammeters are marked in 0.01 A increments. Describe an experimental procedure that could be used to answer questions (1) and (2) above. In your description, stale the measurements you would make and how you would use the equipment to make them. Include a neat, labeled diagram of your setup. Explain how data from the experiment you described can be used to answer question (!) above, Explain how data from the experiment you described can be used to answer question (2) above. A lightbulb is nonohmic if its resistance changes as a function of current. Your setup from is to be used or modified to determine whether the lightbulb is nonohmic. How, if at all, does the setup need to be modified? What additional data, if any, would need to be collected? How would you analyze the data to determine whether the bulb is nonohmic? Include a discussion of how the uncertainties in the voltmeters and ammeters would affect your argument for concluding whether the resistor is nonohmic.

Explanation / Answer

Hi,

In this case we have two questions we need to answer, and to do that we are asked to design an experiment to collect some data.

Assuming we have all the material we can create a circuit as follows:

First we have the power source, then we put an ammeter (we assume that every ammeter used in this case has a resistance equal to cero, so it will not affect the circuit). After that, we put the lightbulb then we place another ammeter and finally we put the resistor All this elements will be connected om series.

Between the the lightbulb, the first ammeter and the second one we put the terminals of a voltmeter. This new equipment will be in parallel with the lightbulb.

With the previous circuit we will be able to:

- Measure the current before and after it passes through the lightbulb.

- Measure the voltage before and after the lightbulb.

- Take different measurements of both the current and the voltage as we can change the energy output of the power source.

(a) The procedure to use the previous setup will be as follow:

1. Turn on the power source and adjust its output to the minimum value.

2. Wait for 5 seconds and take note of the measure of the current reported by each ammeter and the measure of voltage reported by the volmeter.

3. Divide the range (maximum - minimum) of power output the power source can supply between an integer number.

4. Increse the output of energy in the power source by a an n fraction of the range.

5. Repeat steps 2 and 4 until you reach the maximum value.

6. Turn off the power source.

The result of this experiment will be a data of voltage and current with n meditions in different conditions, due to the variation in the supplied power.

(b) In theory there should be no difference in the current before and after the bulb. This means that the same number of electrons that enter the bulb should be exiting it at any time. This experiment should prove that if the values indicated by each ammeter are equal or at least very similar (in this case the uncertainty of the ammeter should be consider).

On the other hand we know that the bulb works thanks to a resistor inside it. The resistor transforms the electric potential energy into heat and light, so the electric potential energy should decrease. The theory can be proved by this experiment as well. If the theory is right the value of the voltage (an inderect measure of the electric potential energy) should be different than cero (probably negative, as we are going from a point of higher potential to a point of lower one). If the voltage were cero, this would mean that both points, before and after the bulb, have the same potential.

(c) I think that the setup of the previous part is perfectly able to handle an experiment to answer this new question. Besides, I consider that with the data obtained in the previous part we are ready to give an answer. The only think we should do is to add both values of current (the one given by each ammeter) and calculate the mean value.

(d) The data collected before was basically a group of values of voltage and mean current. We could draw a graphic of the voltage as a function of the current. If the bulb is ohmic said data will be succesfully adjusted by a straight line that will cross the origen. However, if the bulb is nonohmic then the data should be adjusted by a function of any other kind.

The uncertainty of the equipment used would have an effect in this case, as it could caused a greater dispersion of the data. However, this effect should be small.

I hope it helps.

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