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Unformatted text preview: Lab 9: Capacitors and Inductors - behavior of RC circuits and RL circuits 1 Introduction An RC circuit contains a resistor and a capacitor. Similarly, a circuit with a resistor and an inductor is an RL circuit. When these circuits are connected to a DC power supply (such as a battery), the current through the circuits and the potential difference be- tween the terminals of the circuit elements will vary with time. During this experiment, you will study the basic behavior of an RC circuit and an RL cir- cuit. In particular, you will study how the voltage across the plates of a capacitor (also known as a con- denser) changes with time, first when the capacitor is being charged, and then when it is being discharged. You will measure the time constant associated with this change in voltage, and use it to understand the rules for how capacitors combine in series and paral- lel circuits. Finally, you will study how the potential difference across the terminals of an inductor changes with time when an RL circuit is connected to or dis- connected from a power supply. EXERCISES 1-12 PERTAIN TO THE BACK- GROUND CONCEPTS AND EXERCISES 13-18 PERTAIN TO THE EXPERIMENTAL SECTIONS. 2 Background Consider the circuit shown in figure 1 in which a power supply, with a potential difference V between its ter- minals, is connected across a resistor R . Assume that the switch has been open for a long time. Exercise 1a: What is the electric potential at points A, B, C, D, E and F? Exercise 1b: What is the potential across the resis- tor (between points D and E)? Exercise 1c: Immediately after the switch is closed, what is the electric potential at points A, B, C, D, E and F? Exercise 1d: What is the electric potential difference across the resistor? Exercise 1e: A long time after the switch has been closed, what is the electric potential at points A, B, C, D, E and F? What is the electric potential difference across the resistor? Now check your answers with your TA before proceeding further. A parallel plate capacitor is easy to understand. It is made up of two metal plates separated by a small air gap. When the plates are connected to a battery, opposite charges build up on the plates. The amount of charge that can be stored is directly proportional to the potential difference between the plates of the capacitor. The capacitance C (farads) is the propor- tionality constant relating the charge Q (coulombs) stored on the plates, and voltage V (volts) between the plates. This relationship is given by, Q = CV (1) After the capacitor is “charged up” (a long time af- ter the switch is closed) no current flows through the circuit. The voltage drop across R (recall V = IR ) is zero and the potential difference between the plates of the capacitor is equal to the potential difference be- tween the plates of the battery. The capacitor is now fully charged....
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