Purpose

In this experiment you will study the basic concepts of electric potential difference (voltage) and electric current. This experiment is designed to help you understand how voltage and current are divided up when circuit components are combined in series and parallel. It will also allow you to develop facility in wiring simple circuits.

Apparatus

Equipment includes: power supply, batteries, small light bulbs, light bulb sockets, resistors, and multimeters. In this lab, Bulb #1 refers to the light bulb with the spherical glass top and Bulb #2 refers to the light bulb with the cylindrical glass top.

Procedure

Introduction to DC Circuits

Examine the two different light bulbs in your setup. The bulb filament (the very thin conductor that glows) is made of tungsten, a metal that does not melt until reaching a very high temperature. A glowing tungsten wire would rapidly oxidize and burn up in air, so there is a vacuum or an inert gas such as argon inside the bulb. The reason the filament glows when current passes through it is that not all the electrical power (energy/second) dissipated in the bulb in converted to heat energy (as is the case in most resistive elements), but some of it is converted to the light energy that you see. Screw each light bulb into the base plate holders provided, and use the multimeter to measure the resistance of each bulb.

What are the two resistances:

R_{bulb #1}__________

R_{bulb #2}__________

Turn on the power supply. Using the multimeter in its voltage measuring position, measure the voltage of the output of the power supply.

V_{power supply}______

Power Dissipated in a Resistive Circuit

For each bulb set up the simple circuit as illustrated in Figure 1.

Which bulb is brighter?
Why is that bulb brighter? Let’s consider this question mathematically. The brightness of a light bulb is directly proportional to the power (energy per second) dissipated by the filament. However, our perception of brightness is in reality roughly proportional to the square root of the power.[1] That is, if a bulb appears twice as bright, it is likely to actually be emitting four times as much energy. In a bulb, some energy will be dissipated in the form of heat (which is how most resistive elements dissipate energy), while some will be dissipated in the form of light energy that you can see. Remember that .

Measuring the Resistance of Elements in a Circuit

Directly measuring the resistance of objects in a working circuit is impossible (try it out with your multimeter if you want!). Sometimes, an easier way to measure the resistance of elements in a circuit is to measure the voltage across the element and the current running through the element. You have already measured the load voltage which, because there is only one resistive element (the light bulb) in the circuit, it is the same as the voltage drop across the light bulb. (According to Kirchhoff’s Law the total voltage drop around a circuit must be zero. If the power supply raises the voltage 3 volts, the resistive element has to use 3 volts.)

What is the current flowing through each bulb? Remember that the current must be measured with the multimeter in series with the light bulbs in the circuit.
I_{bulb #1}__________

I_{bulb #2}__________
Knowing the voltage across each bulb and the current flowing through each light bulb, what are the actual resistances of the bulbs?
R_{bulb #1}__________

R_{bulb #2}__________
How did the resistances change from your first measurement of the resistance of the bulbs not connected to the circuit? What does this tell you about the resistive nature of tungsten?

Power Dissipated in a Resistive Circuit (Part 2)

Knowing the resistance of each light bulb, the voltage across each bulb, and the current through each bulb, calculate the power dissipated by each bulb using the three different equations from question four. Please show your work.
Are these results consistent with your observation of the relative brightness of each bulb? Why?

Lamps in Series

The connection for two bulbs in series is shown in Figure 2. Connect the circuit in figure 2 using two Bulb #1s.

From your observation of the brightness of the lamps, what can you conclude about the current through each lamp compared to the case of the single bulb and battery circuit? Is this consistent with your answer to question two on the pre-lab? Please explain.
Please sketch the circuit in Figure 2 below and label the current (I₁, I₃, I₂, etc) in each
branch of the circuit. Measure the current into and out of each of the lamps using the ammeter. Comment on the three values you measure: (1) between the positive power terminal and a bulb, (2) between the two bulbs, and (3) between the bulb and the ground power terminal. Do the numbers you observe agree with your conclusions in question 10? Please explain.

Next measure the voltage across each of the lamps separately, using the multimeter as a voltmeter. Also measure the voltage across the power supply. What can you say about the way the voltage is divided across these identical lamps in series?

With the circuit of Figure 2, try taking an extra wire and connecting it across the two terminals of one lamp. This is known as a short circuit. What happens to that lamp? What happens to the other lamp? Is this consistent with your prediction from your pre-lab? Please explain.

Next replace a Bulb #1 with a Bulb #2.

Measure the voltage drop across the two different light bulbs. Does their sum add up to the voltage across the batteries?

Lamps in Parallel

The connection for two lamps in parallel, connected to the power supply, is shown in Figure 3. Connect the circuit in Figure 3 using two identical Bulb #1s.

Please sketch the circuit in Figure 3 below and label the current (I₁, I₃, I₂, etc) in each
branch of the circuit. Measure the current into and out of each lamp and the current out of the power supply. How doe these currents appear to be related? Do the numbers you observe agree with Kirchhoff’s current law? Please explain.

Measure the voltage across each of the lamps separately and the voltage across the batteries. What can you say about the potential difference across lamps in parallel?

Add another Bulb #1 in parallel with the others.

What happens to the brightness of the other bulbs? Is this expected?

Add a Bulb #2 in parallel with the other bulbs.

What happens to the brightness of the other bulbs? Why?
Can you make a general statement about the independence of parallel branches of a circuit connected to a constant voltage source?

Series-Parallel Circuits

Figure 4

Assemble the circuit in Figure 4 using Bulb #1s.

For Figure 4, what did you predict in the pre-lab would happen to the brightness of the three light bulbs when a fourth light bulb is added in parallel with lamp B and C? Try it! Did reality match your predictions?

Now add a fifth light bulb, also in parallel with B and C.

What happens to the brightness of the bulbs? How is this situation different from questions 18 and 19?