Background

Desktop computers and other electronic devices make use of many fundamental laws of electricity and magnetism discovered over the past two centuries. Combinations of various components--resistors, capacitors, diodes, etc.-- cause different effects in an electric circuit. How is an electric circuit designed? What factors in an electric circuit determine how bright a bulb glows? How do different arrangements of light bulbs in circuits affect how the bulbs shine? In this experiment you will explore the fundamentals of electric circuits, by learning how to arrange bulbs in circuits to achieve different effects. You will also develop a model to predict the amount of current flowing through different parts of an electric circuit.

Purpose

1.) To investigate different arrangements of light bulbs in electric circuits.

2.) To generate a model that you can use to predict bulb brightnesses in simple electric circuits.

Materials

battery, D cell	battery holders (2)
bulbs (6)	bulb holders
wires, stripped ends (16)	bolts (2)
broken battery	broken capacitor

Your initial "working" model will be: A bulb's brightness depends on the amount of current flowing through the bulb.

As you assemble each circuit below, observe the relative bulb brightnesses (the large brightness differences, not the small ones). As you construct each circuit, test and modify the initial model so that you can predict bulb brightnesses in all of the electric circuits (Circuits 1 - 8).

1.) Assemble the two circuits shown below (Circuits 1 and 2). Compare the bulb brightnesses, and discuss how to modify or add to the "working model". Your model should allow you to predict the current flowing through both the bulbs and the batteries.

2.) Add a second bulb in series in Circuit 1 to make Circuit 3. Before closing the switch, use your model to predict the bulb brightnesses. Add a third bulb to your series circuit to make Circuit 4. Before closing the switch predict the bulb brightnesses. What happened to the brightness of bulb A as more bulbs were added "in series" to your circuit? Does the current through the batteries decrease or increase as the total resistance in the circuit is increased?

3.) Based on your observations in Circuits 1-4, discuss your "working model" for predicting bulb brightness with your group. Record your revised "working model". Your model should allow you to predict whether the current increases or decreases as the total resistance of the circuits increases or decreases.

4.) Leave Circuit 3 assembled for comparison, and build Circuit 5 as shown on the right below. Before closing the switch in Circuit 5, use your "working" model to predict the separate bulb brightnesses of A, B and C in Circuit 5.

5.) How does the brightness of bulb A in each of the circuits above indicate the amount of current flowing through the batteries? How did adding bulb C in parallel with B change the current through the battery ? What happens to the total resistance of the circuit when bulb C is added in parallel with bulb B? If necessary, revise your "working model". You should be able to predict bulb brightnesses in circuits with both series and parallel components.

6.) Assemble Circuit 1 and Circuit 6. Predict whether the addition of a parallel branch to Circuit 1 will change the brightness of bulb A. Test your prediction.

How is the parallel branch in Circuit 6 different from the one in Circuit 5? Predict what will happen to bulbs A, B and C when you add a third "parallel" branch to Circuit 6 to make Circuit 7 below. Test your prediction.

Use your model to predict what will happen to E's brightness when you unscrew bulb A. Check your prediction. When bulb E is unscrewed, predict what will happen to the brightnesses of bulbs B and D. Check your prediction.

Do parallel branches of bulbs across a battery depend on each other or do they act independently in a circuit?

7.) Use your model to predict the relative brightnesses of all of the bulbs in Circuit 8. Test your predictions. Predict what will happen to A and B when you unscrew bulb C or D or E. Test your predictions, and revise your "working model", if necessary. How do the parallel branches in Circuit 8 differ from Circuit 7 ?

8.) Set up the circuit below. Before closing the switch, predict the brightness of bulb F in Circuit 9 compared to F in Circuit 7. Test your prediction. What will happen to the brightness of E when A is unscrewed? What will happen to the brightness of B and D when bulb E is unscrewed?

9.) If time permits, build a six-bulb circuit of your own design. Draw the circuit diagram and use your model to predict the relative brightnesses of each of the bulbs in your circuit.

Your Report for Experiment 8 should consist of a Discussion section only (no Summary, Introduction, etc.)

Include the following:

1. Describe your final model for predicting relative bulb brightnesses in circuits. Be specific about describing how the bulb arrangements in parallel and series branches of a circuit affect the current flowing through both the bulbs and the battery. Give examples, and include figures to illustrate your model.

2. Give an example of the six bulb circuit of your own design, and include its circuit diagram. Use your model to predict the relative bulb brightnesses in your six-bulb circuit. Explain your reasoning for your predictions.