Phys 202A. Lab 7 Batteries, Bulbs and Current

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Coral Baldwin
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1 Phys 202A Lab 7 Batteries, Bulbs and Current Name Objectives: To understand how a voltage (potential difference) results in a current flow through a conductor. To learn to design and wire simple circuits using batteries, bulbs and wires. To learn to use symbols to draw circuit diagrams. To understand how to use the current and voltage probes. To understand current flows at all points in simple circuits. Activity 1: Arrangements that cause a bulb to light. Get one of the circuit kits from the cart. Use a cell, a bulb and a single wire to find some arrangements in which the bulb lights and some in which the bulb does not light. a) Sketch two arrangements in which the bulb lights. b) Sketch below two arrangements in which the bulb does not light. c) Describe as fully as possible what conditions are needed if the bulb is to light, and how these conditions are not satisfied in the arrangements that fail to cause the bulb to light.

2 Activity 2: Other Materials between the battery and the bulb a) Set up the single wire, cell and bulb so that the bulb lights (one of the arrangements you used in activity 1). With the help of your partner, stick in a different objects (one at a time) between the battery and the bulb. Here are some examples of objects you can use: Materials: Paper clips, pencil wood, pencil lead, coins, rubber bands, fingers, air, paper, keys, other materials in the room (there is a box of materials in the classroom for your use). Material Bulb lights? Bulb does not light? The materials that complete the circuit allowing the bulb to light are called conductors while the materials that do not complete the circuit (that prevent the bulb from lighting) are called insulators. Activity 3: How does the cell bulb socket combination work? 1) Examine a bulb carefully. Use a magnifying glass to examine the small bulb. Also examine a big bulb. Draw a diagram showing the anatomy of the bulb: 2) Unscrew the bulb from the socket and examine the socket. Draw a diagram below of the anatomy of a socket.

3 Current is measured with a device called an Ammeter and voltage (potential difference) is measured by a device called a Voltmeter. The instructor will explain how the ammeter and the voltmeter are connected in a circuit. You may know how to do this but you MUST okay your connections with the instructor before you proceed! Measurement of Current Current is measured with an instrument called the ammeter. The ammeter is connected as shown in the figure below: The ammeter is said to be connected in series with the bulb. 1) Connect a single bulb to a two cell battery. a) Do you expect the current to be the same at before and after the bulb? Explain. Connect the ammeter as shown below. Measure the current in the circuit just before the bulb and just after the bulb. Current before the bulb Current after the bulb b) Is charge used up in the bulbs? Explain. 2) Two long bulbs in series (two cells) Connect two long bulbs in series as shown on Figure 1 below. a) How do the brightness of the bulbs compare with each other? b) Do you expect the current to be the same through both the bulbs? Explain Connect the ammeter at the point shown below. Note down the ammeter reading.

4 Fig. 2a) Fig. 2b) Fig 2c) Then connect the ammeter as shown in Figures 2b) and 2c). How do the ammeter readings compare with each other? Ammeter readings: 2a) 2b) 2c) c) What about the current through the battery? Would the battery current be the same current as the current through the bulbs? Explain. d) Can you make out anything about the direction of the flow from your observations? What is the direction of flow of electrons in the circuit? The conventional direction of flow is always taken to be from the positive terminal to the negative terminal of the battery. e) Prediction: Will the current in the circuit increase, decrease or remain the same when you add more bulbs in the circuit? Check your prediction. 3) Two long bulbs in parallel. a) If you connect two long bulbs in parallel with the battery, as shown in the figure below, how would the brightness of the two bulbs compare with each other? b) How would the current in one bulb compare with the current through the other bulb? How does this compare with the current through the battery? c) Measure the current at the points indicated.

5 Summarize in the form of an equation, the relationship between I 1, I 2 and I 3. d) What do you think will happen to the value of the battery current i batt if we increased the number of bulbs in parallel? e) To summarize, what do you think happens to the total resistance of a circuit as the number of parallel branches increases? Explain. f) Unscrew one of the bulbs connected in parallel. Is the other one affected? If the battery is ideal, branches connected directly across it are independent of each other. 4) Two cells connected to one round bulb in series with a parallel pair of round bulbs. a) Predict the brightness of the bulbs when the circuit shown below is closed. Now set up the circuit 3) shown below. Check your prediction and explain your ranking. The current model: Can we conclude that the battery puts out a fixed amount of current which passes through A and then splits evenly between B and C? Using this model then b) Predict how the brightness of bulb A changes when bulb C is unscrewed from its socket.

6 c) Predict how the brightness of bulb B changes when C is unscrewed from the socket. d) Check your predictions - do we need to modify our model? Or can the current model we have developed in this lab explain our observations? Explain.

Batteries n Bulbs: Voltage, Current and Resistance (8/6/15) (approx. 2h)

Batteries n Bulbs: Voltage, Current and Resistance (8/6/15) (approx. 2h) Introduction A simple electric circuit can be made from a voltage source (batteries), wires through which current flows and a resistance,