Introduction Pre-Lab

Physics 1BL Circuits Winter 2010

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Introduction In this lab you will use the knowledge you have gained about charges and how they move under the influence of electric fields to explore what conditions allow for charges to move around an electric circuit. You will connect wires, light bulbs, and batteries in different configurations to see which form functional circuits. You will determine what kinds of materials are required to form a functional circuit, and you will measure electrical potentials in the circuits you create. Finally you will examine a light bulb and a flashlight to see how they work. Pre-Lab:

1. A negative charge (q = –3.5 Coulombs) is placed above a floor that has a net positive charge. The floor creates a uniform electric field that has a magnitude of 150 N/C.

(a) What is the magnitude and direction of the electric force on the negative charge?

(b) If the charge is initially at a distance 1.25 m above the plane, what will the change in electric potential energy (in Joules) be if it is then moved to a distance 0.75 m above the plane (specify whether this change is positive/negative/zero)?

2. If a proton moved from a location with 5.0 V potential to a location with 10.0 V potential, would its potential energy increase or decrease? If an electron moved from a location with 10 V potential to a location with 5.0 V potential, would its potential energy increase or decrease? Explain.

A: Batteries, Bulbs and Wires

Below are diagrams of 10 different configurations of bulbs, wires, and batteries. Make a prediction about whether or not each configuration will result in the bulb lighting up. Make your prediction by yourself and note your prediction in the space provided on your lab manual. When all the members of the group at your lab table have finished, compare your predictions for each configuration with those of your lab partners. Explain your reasoning and come to a joint conclusion about which circuits will light the bulb. Note your joint predictions on the white board.

Now, with the equipment available in the lab, try each of the 10 configurations shown illustrated below. Use the pieces of bare wire, and hold them in contact with the battery and bulb as the figures show. In each case note if the light bulb comes on. In your lab notebook, draw a diagram of all the configurations that result in the light bulb lighting.

q


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#4 The tip of the bulb touches the positive end of the battery (but not the knob in the middle). A wire touches the metal side of the bulb and the negative end of the battery. Pred: Obs:

#5 A single wire touches the positive end of the battery and the negative end of the battery. The tip of the bulb touches the middle of this wire. Pred: Obs:

#6 The metal side of the bulb touches the positive end of the battery. A wire touches the tip of the bulb and the negative end of the battery. Pred: Obs:

#1 A wire touches the positive end of the battery and the tip of the bulb. Pred: Obs:

#2 The tip of the bulb touches the negative end of the battery. A wire touches the positive end of the battery and the metal side of the bulb. Pred: Obs:

#3 One wire touches the positive end of the battery and the tip of the bulb. A second wire touches the negative end of the battery and the tip of the bulb. Pred: Obs:


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#7 One wire touches the positive end of the battery and the tip of the bulb. A second wire touches the positive end of the battery (not the knob in the middle) and the negative end of the battery. Pred: Obs:

#8 One wire touches the negative end of the battery and the metal side of the bulb. A second wire touches the positive end of the battery and the tip of the bulb. Pred: Obs:

#9 The tip of the bulb touches the negative end of the battery. A wire touches the negative end of the battery and the positive end of the battery. Pred: Obs:

#10 One wire touches the positive end of the battery and the metal side of the bulb. The other wire touches the negative end of the battery and the metal side of the bulb. Pred: Obs:


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Can you think of any other possible configurations? Try at least one different configuration. Does it light the bulb? Make a diagram of the configuration in your notebook and explain why it lights (or does not light) the bulb. Look at the diagrams you’ve drawn in your notebook so far and write your answers to these questions:

• Which part or parts of the battery need to be involved in the circuit in order for the bulb to light?

• Does the wire need to touch the knob on the positive end of the battery, or can the wire touch any part of the positive end?

• Which part or parts of the light bulb must be touched for the light bulb to light?

Now make the circuit shown in Figure 11. Use the special holders provided to make holding the electrical elements less awkward. Get a battery holder, bulb holder, switch and three hook-up wires with plastic sheaths and clips at the ends. Snap the battery into the holder, screw the bulb into its holder and use the hook-up wires to connect the battery, bulb and switch. When the handle of the switch is down, the circuit is said to be “closed”. When the handle of the switch is up, the circuit is said to be “open”. Verify that you can make the bulb light up. The switch, battery and bulb holders you use may Figure 11 not be identical to those shown here but they will be similar and they will perform the same functions. B. Materials of Circuit Elements Does it matter what kind of materials you use to connect the battery to the bulb? In the previous activity, you used wire (bare wire and wire with a plastic sheath). Will any material connecting the battery and bulb in a complete circuit allow the light bulb to light? Construct a circuit similar to the circuit in Figure 12. The iron nail is placed in the circuit. Before you begin, the circuit should be open (i.e. the handle of the switch should be up). Now close the switch.

Does the bulb light? Record your observation in a table.

Open the switch. Remove the nail from the circuit and replace with one of the other items available in Figure 12 the lab. Close the switch. Does the bulb light? Record your observation in your table.


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Repeat with all the other items provided and any other items you wish to test. Include a bare metal wire and a plastic covered wire. Hook to the bare ends of the plastic covered wire. What happens? Try connecting to the plastic covering instead of the bare ends. What happens?

Is there anything similar about the items that allow the bulb to light? Materials that allow the bulb to light are called conductors. Materials that do not allow the bulb to light are called insulators. Hook up the circuit with the battery and bulb. Close the switch so the bulb lights up. Look closely at the bulb. Which part of the bulb is actually glowing? You should have access to a larger light bulb which has had the protective glass removed. Examine this bulb closely. On the picture, write in whether or not you think each labeled part of the bulb is made of conducting or insulating material. Test your ideas using your results above. Which part glows? What was the function of the glass cover before it was removed?

Draw a diagram of a battery and a bulb connected by two wires. Show in your bulb diagram how you think the wires touching the side and bottom of the bulb connect to the filament support wires. Trace the path of conductors between the battery and the bulb. Figure 13 C. How does the battery need to be connected? The evidence from experiment A suggests that one side of the bulb needs to be connected to the positive end of the battery, and also that the other side of the bulb needs to be connected to the negative end of the battery.

Do the two sides of the bulb need to be connected to the positive and negative ends of the same battery? Consider the configuration in figure 14. Do you think the bulb will light? Explain your reasons. Get two batteries, two wires and a bulb. Hook up the arrangement shown above. Does the bulb light up? Why do you think the bulb behaves this way? Describe three ways you can add one wire (only one wire, not more) to this arrangement to get the bulb to light. Figure 14


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Connecting wire Battery (positive end is the longer line) Switch Light bulb

Figure 15

D. Schematic diagrams of circuits: Drawing circuits with realistic pictures for the electrical components takes some time and drawing skill to complete. It is customary to use symbols for circuit elements. A wire is drawn as a line connecting two circuit elements. Symbols for switches, light bulbs and batteries are shown in figure 15.

Draw the circuit diagram for the circuit of figure 11 using these symbols. E. How does a flashlight work? Examine the flashlight with your group. Take it apart and figure out how it works. Draw a picture of the various electrical parts of the flashlight and show how they are connected together. Draw a circuit diagram. Write a few sentences to explain how the flashlight works – that is, what actually happens when you slide the switch and the light goes on. Draw your sketch on a discussion board and display it so other groups can see it. When they are ready, walk around the room and look at the other groups’ drawings. If you find a diagram much different from your own, discuss those differences with the other group(s).

F. Measuring Electric Potential Differences in a circuit. An electrical “multi-meter” is an instrument that can measure many useful properties that are required to analyze circuits (e.g. voltage, current, resistance etc). You will need to use these meters in several of the Physics 1B labs. For this lab we will use only one meter function, that of potential difference. Check the picture of the meter we will use on the next page. You will note that the meter has 2 probes that plug into terminals on its front surface. When the dial is set to “voltage” the meter measures the electrical potential difference between the tips of the two probes. (Use the 2V scale to start. This is a few clicks counterclockwise from the “OFF” position). The multi-meter reading implicitly assumes the “negative” or black colored probe is at 0V (zero volts). You cannot measure an “absolute potential” with this device. Start with the components of the circuit shown in figure 11 (i.e. battery, bulb, wires and switch). Do not “wire up the circuit”, leave the components separate. Measure the potential difference across each element. Record your observations. Measure the battery voltage the conventional way with the black probe on the negative end of the battery and


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An electrical multi-meter

the red probe on the positive end. Then reverse the meter (black probe to positive end battery etc). What do you expect the meter to read when reversed? What does it read?

Now wire up the circuit from figure 11 (battery, bulb and switch all connected with clip on wires, switch closed, bulb on). Touch the negative terminal (black) of the meter to the negative end of the battery, and touch the positive terminal of the meter (red) to the positive end of the battery. Record the meter reading. Does touching the meter wires to the battery ends change the bulb brightness?

Next move the red wire from the meter to the connector on the light bulb holder (the one connected to the plus side of the battery). • Record the potential at this point with reference

to the black probe which is still connected to the negative end of the battery.

• Next move the red wire to the other side of the light bulb, then to each side of the switch, and finally around to the negative side of the battery. Record all these data in a table.

• Draw a circuit diagram (again) for this configuration. Mark “0V” next to the negative terminal of the battery. Write the observed voltage on the diagram at all the points you measured.

• You have several different “circuit elements”, battery, wires, bulb, switch. Which have a measurable potential difference across them?

• Repeat with the switch “open” (i.e. the bulb is off). Compare potentials across the bulb with the brightness of the bulb.

Finally find a second battery in a holder. With more wires, connect the positive end of the original battery to the negative end of the new battery. Then connect the positive end of the new battery to one terminal of the bulb holder as before. • Does the bulb still light? Compare its brightness now to its brightness in the single

battery circuit. Comment on any differences. • Using the multi-meter measure potential differences across each of the circuit

elements. Record these in a table. • Draw the circuit diagram for this circuit and record the voltages on the diagram as

you did above. • Compare potentials across the bulb with the brightness of the bulb.

Conclusion:

1. Please do a conclusion write up for the section specified by your TA.

Documents

Introduction Pre-Lab