Parallel Circuits.

Labs written for the CARA Space Explorers, Winter 1994

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This is meant to be handed out to the students.*

Figure 1

First, we will build this series circuit as a review of the concepts, and later we will consider a more complicated system that looks like this:

Figure 2

Note that in the first circuit there is only one path for the current, while in the second circuit there are two paths, one through resistor R2 and one through resistor R3.

resistor color code resistance (ohms) R1 ___________ ____________ R2 ___________ ____________ R3 ___________ ____________These three resistors are going to be used in the same positions marked in Figure 1 and Figure 2, so be sure to keep track of which one is which! Do this by writing in the resistances next to the resistor symbols R1, R2, and R3 on Figures 1 and 2.

voltage of battery: Vbat = _____________ V

Now, build a circuit using the battery and the two resistors R1 and R2, as in Figure 1 (we'll get to resistor R3 later). To avoid running the battery down, we'll keep the switch open until we want to measure something.

voltage resistance current V R I AB VAB = _______ R1 = ______ IAB = _______ BC VBC = _______ R2 = ______ IBC = _______

Claim: the current through R1 (IAB) has to be equal to the current through R2 (IBC), otherwise current would pile up somewhere.

Check: Since you have measured both the resistances (Step 1) and the voltages (step 3), and since you know Ohm's law, you can calculate the currents. Do this, and enter the calculated currents I into the table above. Is the claim verified? That is, does IAB = IBC?

We can also predict the values for VAB and VBC. Your battery has a voltage of Vbat; then,

VAB = Vbat x R1 . (R1+R2) and VBC = Vbat x R2 . (R1+R2)Calculate both of these numbers and fill out the table below:

measured calculated VAB _________ __________ VBC _________ __________

Claim: the combination of the two resistors called R2 and R3 acts like a single resistance R with a value of

R = _____1______ (1/R2 + 1/R3) .Check: Now measure VBC again; if this claim is correct, then

VBC = Vbat x R/(R1+R) .Fill out the table below:

measured calculated VBC _________ __________

Write down an equation that relates I1, I2, and I3:

I1 = _____________ .

Review: if two resistors R1 and R2 are connected in series as in Figure 1, then their combined resistance is

R = R1 + R2 (series)

If two resistors R2 and R3 are connected in parallel as in Figure 2, then their combined resistance is

R = ______1______ (1/R2 + 1/R3) (parallel)## IV. Power

Build a circuit as in Figure 1, except replace the two resistors with a light bulb. If you have been using a 1.5 V battery, use one of the small, round light bulbs. If you have been using a 3 V battery, use one of the larger, oblong bulbs. The light bulb acts just like a resistor: it limits the flow of current from the positive terminal of the battery to the negative terminal. The current heats the filament to a high temperature, so high that it emits light.Heating something up takes energy, which is defined as the ability to do work. The brighter a light bulb looks, the more energy (in the form of heat and light) is being produced per second or per hour. Electrical energy has been converted into heat and light; that is, electrical energy is consumed and heat/light energy is produced. The "Watt" is a unit of a physical quantity called power. Power is the rate of consuming or producing energy, so it has units

power = P = energy/time

The higher the Wattage of a light bulb, the faster it uses electrical energy. To calculate the power that is consumed/radiated by your light bulb, use the formula

P = V x I

where V is the voltage of your battery and I is the current flowing in the circuit. Remembering that Ohm's Law is

V = I x R ,

write down an expression for the power P in terms of just I and R.

P = _____________

If one of your resistors used in the previous Steps was getting hot, then you were putting too much power into it.

Important Disclaimers and Caveats

Go back to the Chicago Winter 1994 Electronics home page.