Investigation 13

Electric Circuits

In this investigation, we construct a model for electric current that we can use to predict and explain the behavior of simple electric circuits.

I.Complete circuits

A.Obtain a battery, a light bulb, and a one piece of wire. Connect these in a variety of ways. Sketch each arrangement below.

Arrangements that do light the bulb / Arrangements that donot light the bulb

You should have found four different arrangements that light the bulb. Examine the arrangements that light and do not light the bulb. State the requirements that must be met in order for the bulb to light.

B.When a student briefly connects a wire across the terminals of a battery, the wire feels warm. Why does the wire get warm?

The student finds that the wire seems to be equally warm at points 1, 2, and 3. Based on this observation, what might you conclude about the amount of current flow at each place in the wire?

C.Light a bulb using a battery and a single wire. Observe and record the brightness of the bulb when objects made out of various materials are inserted into the circuit. Try materials such as paper, coins, pencil lead, eraser, your finger, etc.

What is similar about the objects that let the bulb light?

D.Carefully examine a bulb. Two wires extend from the filament of the bulb into the base. You probably cannot see into the base, however, you should be able to make a good guess as to where the wires are attached. Describe and draw where the wires attach. Explain based on your observations in parts A to C.

On the basis of the observations that we have made, we will make the following assumptions:

1.When an unbroken path exists from one terminal of the battery, through the rest of the circuit, and then back to the other terminal of the battery, we have a complete and continuous path for the flow of electrons. We call this flow electriccurrent.

2.The brightness of a bulb can be used as an indicator of the amount of current through that bulb: the brighter the bulb, the greater the current.

We will use these assumptions when examining and deducing the characteristics of various circuits.

II.Bulbs in series

(In this section and in section III, use two batteries in the holder as your source.)

Set up a two-bulb circuit with identical bulbs connected one after the other as shown. Bulbs connected in this way are said to be connect in series.

A.Compare the brightness of the two bulbs with each other. (Pay attention only to large differences in brightness. You may notice minor differences because two “identical” bulbs are, in fact, not quite identical.)

Use the assumptions we have made in developing our model for electric current to answer the following questions:

1.Is the current the same through both bulbs? How do you know?

2.Do you think that switching the order of the bulbs might make any difference in the brightness of the bulbs? Check your answer.

3.If you were to unscrew one of the bulbs from the circuit, what do you think will happen? Did it? Why?

B.Compare the brightness of each of the bulbs in the two-bulb circuit with that of a bulb in a single-bulb circuit. (A quick way to connect a single-bulb circuit is to disconnect the wire from point A, and attach it to point B. The upper bulb is no longer “in the circuit.” There is no complete path for current to flow.)

Use the assumptions we have made in developing our model for electric current to answer the following questions:

1.How does the current through a bulb in a single-bulb circuit compare with the current through the same bulb when it is connected in series with a second bulb in a two-bulb circuit?

2.What does your answer to question 1 imply about how the current coming from the battery in a single-bulb circuit compares to the current coming from the battery in a two-bulb series circuit? That is, is the current coming from the battery in a single-bulb circuit greater than, less than, or the same as the current coming from the battery in a two-bulb series circuit?

C.We may think of a bulb as causing an impeding effect (or resistance) to the current in the circuit.

1.Thinking of the bulb in this way, would adding more bulbs in series cause the impeding effect (or totalresistance) to increase, decrease, or remain the same as before.

2.Formulate a rule for predicting how the current coming from the battery would change (i.e., whether it would increase, decrease, or remainthesame) if the number of bulbs connected in series were to be increased or decreased.

III.Bulbs in parallel

Set up a two-bulb circuit with identical bulbs so that their terminals are connected together as shown. Bulbs connected together in this way are said to be connected in parallel.

A.Compare the brightness of the bulbs in this circuit. Are they the same brightness? (Remember that minor differences do not matter.)

1.From your observation of the brightness of the bulbs, what can you conclude about the amount of current through each bulb?

2.If you were to unscrew one of the bulbs from the circuit, what do you think will happen to the other bulb? Why?

B.Compare the brightness of each of the bulbs in the two-bulb circuit with that of a bulb in a single-bulb circuit. (When you unscrew one bulb from the circuit, you now have a single-bulb circuit.)

1.Did the brightness of the remaining bulb in the single-bulb circuit change (remember substantially) when the first bulb was unscrewed? What does this say about the current through the remaining bulb now in comparison to the current through it before?

2.Is the brightness of each bulb in the two-bulb parallel circuit greaterthan, lessthan, or equalto that of a bulb in a single-bulb circuit?

3.How does the amount of current coming from abattery connected to a single bulb compare to the current coming from a battery connected to a two-bulb parallel circuit? (More current delivered, less current delivered, or the same amount of current delivered?)

C.Based on your observations above, formulate a rule for predicting how the current coming from the battery would change (i.e., whether it would increase, decrease, or remainthesame) if the number of bulbs connected in parallel were increased or decreased.

What can you infer about the total resistance of a circuit as the number of bulbs connected in parallel is increased or decreased?

IV.Circuits – Conclusions

Circuits consisting of bulbs (resistances) can be connected to batteries in series or in parallel (or in any combinations of series and parallel).

As we connect more and more bulbs in series to a battery, we know that the total resistance of the

circuit ______, and the current delivered by the battery ______.

As we connect more and more bulbs in parallel to a battery, we know that the total resistance of the circuit ______, and the current delivered by the battery ______.

V.Application of These Ideas to Your Home Electrical Circuits

The batteries in the circuits described above act very much like the voltage sources supplied to your home by the electric company, except that the batteries deliver DC and the electric company delivers AC. This voltage supplies the energy needed to power the electrical devices in your home. The bulbs in the above investigation act like the electrical devices in your home.

A.From what you found in this investigation, do you think that the electrical devices in your home are connected in series or parallel? How do you know?

B.Why is it dangerous to connect too many lights and electric devices to one electrical circuit?

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