Pre-LAB 4 Assignment Fundamentals of Circuits II: Currents ...

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Fundamentals of Circuits II: Currents & Circuits v 0.3 Objectives • To understand current in a circuit where a battery lights a bulb. • To understand series connections in an electric circuit. • To understand the relationship between the current in all parts of a series circuit. • To understand the meaning of parallel connections in an electric circuit. • To understand the relationship between the currents in all parts of a parallel circuit. • To begin to understand the concept of resistance. Overview In the previous lab you found that a bulb only lights when there is an electric current passing through the bulb. You also found that the bulb must be connected to a battery in a complete, continuous circuit in order for there to be a current. Specifically, there must be a continuous connection from the positive terminal of the battery, through the connecting wire to the bulb, through the bulb, through the connecting wire to the negative terminal of the battery and through the battery. Recall that you measured the current both as it entered current elements like a battery or a bulb and when it left. From these measurements you discovered that the electric current flowing out of a device is equal to the current flowing into it. These devices did not seem to “use up” any current which passed through. Current it seems doesn’t just disappear. This suggests that electrical current is the flow of some conserved quantity i.e., electric charge. In addition, by measuring voltage in circuits with one bulb and with two bulbs you observed that the potential difference (voltage) across a battery (if one can neglect its internal resistance) is fairly constant whether it is connected to one light bulb or two. In other words, the voltage of a battery is a property of the battery and not of the circuit. By contrast, you found that adding a second bulb in series to the first lead to a decrease in the current. Circuit elements like bulbs provide a resistance to electrical currents. The total resistance of a circuit determines the current when the circuit is connected to a battery. The larger the resistance, the smaller the current. In this lab we will consider more complicated circuits in order to extend and reinforce what we have learned previously. You will be comparing currents at different points in these circuits by making measurements using current probes and by comparing the brightness of bulbs. This lab has been adapted from the Real Time Physics Active Learning Laboratories [?]. The goals, guiding principles and procedures of this labs closely parallel the implementation found in the work of those authors [?, ?, ?]. Investigation 1: Current in Series Circuits In the spirit of scientific discovery, you will make a series of predictions about currents in various circuits. Afterwords, you will compare these predictions with measurements and observations. If you find that your experimental measurements and/or observations disagree with your predictions you should identify which if any of the assumptions which lead you to your PHYS-204:Physics II Laboratory 2
Fundamentals of Circuits II: Currents & Circuits v 0.3 initial prediction have been shown to be false. Subsequently, you should develop and adopt new concepts which are consistent with the results of your measurements. Our fist activity requires the following equipment: • Computer with Logger-Pro software • Lab Pro interface • two current probes • Two 1.5 V D cell battery with holder • connecting wires • alligator clips • Two #14 bulbs in sockets • contact switch Note: the battery holder, light bulbs and contact switch are part of the Pasco circuit board. B + − A + − C (a) (b) Figure 1: Two different circuits: (a) a battery with a single bulb and (b) a battery with two bulbs. All three bulbs are identical. Prediction 1.1 Consider the two circuits shown in Fig. 1. Predict the relative brightness of the three bulbs shown in Figs 1(a) and 1(b) from brightest to dimmest. In making this prediction you should assuming that the batteries and bulbs have identical characteristics. (Recall that the voltage across a battery is essentially independent of the number of bulbs it is connected to.) In your prediction you should identify which bulbs, if any, are equal in brightness. Hint: Helpful symbols are > ”is greater than”, < ”is less than”, = ”is equal to,” for example B > C = A. PHYS-204:Physics II Laboratory 3
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Pre-LAB 4 Assignment Fundamentals of Circuits II: Currents ...

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