Physics 2306 Experiment 6: Direct Current (DC) circuits, Part 2 Objective • •
To learn to apply the concept of potential difference to explain the action of batteries in a circuit and to understand how potential difference is distributed in different parts of series and parallel circuits. To understand what distinguishes an ohmic device from a non-ohmic device.
Required background reading Young and Freedman, sections 25.2, 25.3, 26.1
Introduction In the last lab, you explored currents at different points in series and parallel circuits. You saw that in a series circuit, the current is the same through all elements. You also saw that in a parallel circuit, the current divides among the branches so that the total current through the battery equals the sum of the currents in each branch. You have also observed that when two or more parallel branches are connected directly across a battery, making a change in one branch does not affect the current in the other branch(es), while changing one part of a series circuit changes the current in all parts of that series circuit. You have seen that connecting light bulbs in series results in a larger resistance to current flow and therefore a smaller current, while a parallel connection results in a smaller resistance and larger current (for a given battery voltage).
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In this lab, you will first examine the role of the battery in causing a current in a circuit. You will then compare the potential differences (voltages) across different parts of series and parallel circuits. Based on your previous observations, you probably associate a larger resistance connected to a battery with a smaller current, and a smaller resistance with a larger current. You will explore the quantitative relationship between the current through a resistor and the potential difference (voltage) across the resistor. When the conditions are such that this ratio is constant over a range of V and I, this relationship is known as Ohm’s law. You will also look at some circuit devices that do not obey Ohm’s law (called non-ohmic devices).
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Name___________________ ID number_________________________ Lab CRN ________________ Lab instructor_______________________ Ph 2306 Experiment 6 Pre-lab assignment (complete and turn in at the beginning of your lab session) 1. A resistor has four colored stripes in the following order: orange, orange, brown and silver. What is the resistance of the resistor and its tolerance?
V . The resistance in general varies with the type of I material and other factors such as temperature. When the conditions are such that the resistance R is constant over a range of V and I values we say that the device is “ohmic”; that is it obeys Ohm’s law. a. Give an example of an ohmic device. 2. Resistance is defined as R =
b. Give an example of a non-ohmic device. Explain (in simple terms) why it is nonohmic.
3. How does a diode behave in a circuit? (Your answer should focus on how its behavior is different for positive and negative voltages.)
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B + _
A
+ _ C
+ _
D
+ _ E
4. In the circuit above, all the batteries and light bulbs are identical. Rank the relative brightness of all five bulbs using >,
