Physics 2.2 lab 7 emf and terminal voltage PDF

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Experiment 8: Study of a Source of emf and Terminal Voltage

Due Date: 03/28/2018 Objective

To study a source of electromotive force (emf) by using the potentiometer method and distinguish between the emf and terminal voltage of the source of the emf.

Experimental Apparatus ● ● ● ● ● ● ● ● ● ● ●

Slide-wire potentiometer Standard dry cell Unknown sell Tap water cell Galvanometer Resistance box Ammeter Voltmeter Connecting wires Double-throw switch Low voltage DC power supply

Theory To have a steady current in a conductor, we need a path that forms a closed loop or Complete circuit. But the path cannot consist entirely of resistance. In a resistor, charge always moves in the direction of decreasing potential energy. There must be some part of the circuit where the potential energy increases. The problem is analogous to an ornamental water fountain that recycles its water. The water squirts out of openings at the top, cascades down over the terraces and spouts, and collects in a basin in the bottom. A pump then lifts the water back to the top for another trip. Without the pump, the water would just fall to the bottom and stay there. In an electric circuit there must be a device somewhere in the loop in which a charge travels "uphill," from lower to higher potential, despite the fact that the electrostatic force is trying to push it from higher to lower potential. The influence that makes charge move from lower to higher potential is called electromotive force. This "force" is what does the "work" that raises the potential of the charges flowing in the circuit. Every complete circuit with a steady current must include some device that provides electromotive force. The symbol (E) is also used to signify emf. Batteries, electric generators, solar cells, thermocouples, and fuel cells are examples of sources of emf. All such devices convert energy of some form (mechanical, chemical, thermal, and so on) into electrical energy and transfer it into the circuit where it is connected. An ideal source of emf maintains a constant potential difference between its terminals, independent of the current through it. We define electromotive force quantitatively as the magnitude of this potential difference and denote it as emf. During the Experiment we will later see how the real-life sources of emf differ in their behavior from this idealized model.

Procedure:

Part 1 1. Set up the circuit provided in the lab manual 2. Turned on the power supply and adjusted the voltage so that the ammeter reads about .1 – .2A. 3. Recorded the measurements for the standard battery. When the switch was in position A, we obtained the balance of the galvanometer. Recorded the Ls. 4. Measured for the unknown cell. We closed the switch in the position B and obtained the balance of the galvanometer. Recorded the Lx, 5. Replaced the unknown battery in the circuit given in the lab manual, with the tap water cell, which was composed of Zn and Cu Electrodes. And we repeated step 4.

Part 2 1. Measured the resistance of the voltmeter using the LCR meter and then recorded the reading. 2. Measured the terminal voltage, of the unknown battery with the voltmeter and used the circuit shown in figure 8.4 in the lab manual, then we recorded the terminal voltage. 3. Repeated the step 2 for the tap water.

Data

Questions 1. What is required for a sustained current flow in an electrical circuit? -Electromotive force 2. What is the difference between a terminal voltage and an emf? -EMF is the electromotive force. It is the source of potential difference that creates current. The terminal voltage is the voltage at the terminals of an emf source. It is less than the emf because of internal resistance. 3. A battery, which emf is 1.8 V, is connected to a voltmeter that records 1.5 V. If the resistance of the voltmeter is 200 Ω, what is the internal resistance of the battery? -r=((−V)/V)R=((1.8−1.5)1.5)×200= 40Ω 4. Why is the null reading (zero reading) necessary for the measurement of an emf? -So that you can compensate from deviation at rest. 5. Which terminal of the tap water is positive, Zn or Cu? -Copper

Conclusion In conclusion, we were able to study the source of electromotive force (emf) by using the potentiometer method and distinguish between the emf and terminal voltage of the source of the emf. Throughout the process of this lab we were able to get our data and values easily. However, in the process on the lab report I noticed that one of our numbers didn’t agree with one of the

rules, which is that the terminal voltage should be less than the emf because of internal resistance. The terminal voltage measured by us was greater than emf, which by definition is wrong. Our internal resistance for this part was negative, and that is due to the terminal voltage being greater than emf....