Expt 8 Act 3 and 5 & Applications

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Experiment 8: Series/Parallel Circuit Elements Laboratory Report Joe Mari Isabella Caringal, Rowena Chiang, Khrista Maria Evangelista, Berthrand Martin Fajardo Group #3 Department of Biological Sciences College of Science, University of Santo Tomas España, Manila Philippines Abstract

1. Introduction

2. Theory

3. Methodology

4. Results and Discussion

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Activity 1 Activity 2 Activity 3 Electromotive Force of Cell (E) Known Resistance (R) Current (I) Internal Resistance (r)

8.30 V 27 x 101 ± 5% Ω 0.03 A 6.67 Ω

Table 3. Data for computing the internal resistance of a cell.

As mentioned, the electromotive force of a cell represents the maximum potential difference between two electrodes of a cell [1]. On Table 3, the voltmeter read 8.30 V when connected to an external resistance (R) listed in the table. The old battery used in the experiment has built up enough internal resistance (r) that is why its electromotive force has decreased to its present value instead of 9 V. Internal resistance of the battery has been affected by its size, chemical properties, age, temperature, and discharge current [2]. This leads to loss of a useful amount of power when the battery is used. To compute for the internal resistance of the cell in the experiment, the following equation is used:

r=

E−IR I

where E is the electromotive force, I is the electric current, and R is the known resistance which is obtained during Activity 1. Substituting the values:

r=

8.30 V −0.03 A (270 Ω) =6.67 Ω 0.03 A

Activity 4

Activity 5 Parts of a Galvanic/Voltaic Cell

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Figure 1. Galvanic Cell [4].

Electrolytic cells and galvanic cells are different in the sense that the former consumes electrical energy while the latter produces electrical energy. They operate in both spontaneous redox reactions. Two separate half-cells compose the galvanic cells that permit the flow of electrons with electrons being released during oxidation and with electrons being accepted during reduction. Due to the electron flow, the entire cell generates an electrical current. The figure above shows a typical cell that uses zinc and copper as electrodes. The cathode serves as a site where reduction occurs and the anode as a site where oxidation occurs. A salt bridge separating the two electrodes usually contains an electrolyte. It allows the flow of current through the cell but it does not mix the contents of the two half-cells. Consequently, the voltmeter above the cell is used to measure the electrical potential difference that results during the process of electron flow generating an electrical current. 5. Conclusion

6. Applications 1. State the laws for series and parallel combination of resistances. Were these laws verified in your experiment? For series wiring, the same electric current (I) runs through each device while the voltage (V) and resistance (R) change. The total resistance and voltage are determined by adding the individual resistances and voltages, respectively. On the other hand, parallel wiring infers that the same voltage (V) is applied across the device while the electric current (I) and resistance (R) change. The total current is

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equal to the sum of the individual currents and the reciprocal of the total resistance equals the sum of the reciprocals of the individual resistances. These rules were verified in the experiment by measuring the individual current, voltage, and resistance in each set of circuits using a multimeter. 2. You have 4 identical resistors, each with a resistance of 5 ohms. Determine all possible resistances that you may get using all four resistors. In a series circuit, all four resistors would equal 20 Ω by adding the individual resistances. Placing all four resistors in a parallel circuit, the total resistance is 5/4 Ω or 1.25 Ω. Putting 2 of them in series (10 Ω) and another 2 in series (10 Ω) and then putting both strings in parallel, a net resistance of 5 Ω is obtained (10/2 Ω). Putting 3 in series (15 Ω) and the other one in parallel, 3.75 Ω is obtained (15x5/15+5). Putting 2 in series (10 Ω) and in series with the parallel combination of the last two (2.5 Ω), a resistance of 12.5 Ω is obtained (10Ω + 2.5Ω). Placing 2 in parallel (2.5 Ω) and 2 in series (10 Ω) and putting both in parallel, 2 Ω is obtained (2.5x10/2.5+10). Finally, placing 3 resistors in parallel (1.67 Ω) and the fourth in series with the parallel, 6.67 Ω is obtained (1.67Ω + 5Ω). 3. The human body is a good conductor, being almost 70% water. A dry skin has a resistance as high as 104-106 ohms. However, when the skin is wet, the resistance drops to 1000 ohms or less. Why? Relate this fact to the operation of a lie detector. Within the body, bones and fats have the greatest resistance while the nerves and muscles have the least resistance. However, majority of the body’s resistance can be attributed to the skin or the dead, dry cells of the epidermis or outer layer of the skin. This is because they are very poor conductors. If the skin becomes moist or wet, they also become good conductors and the resistance lowers greatly. Moreover, aside from acting as a resistor, the skin can also serve as a capacitor if placed in contact with a metal. The lie detector uses this concept as a useful tool. When the person being tested tells a lie and then begins to sweat, his/her body resistance lowers and is detected by the machine. 4. Compare the human circulatory system to an electric circuit. An electric circuit is in many ways similar to the circulatory system. The arteries, veins, and capillaries, which compose the circulatory system, act as wires in a circuit. As blood vessels deliver blood to the different parts of the body, the circuit wires carry electric current to the entire electrical system. Just as the heart pumps blood and provides as the force or pressure to circulate it around the body, the battery or generator produces voltage that drives the current throughout the circuit. 5. Are household circuits normally wired in series or in parallel? Why?

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Normally, household circuits are wired in parallel because it would be impractical to use a series connection. If one socket is not used or is turned off, the circuit would be open and all the other sockets would not work. Unlike when using parallel circuit, turning off one socket would mean that other sockets could still be used. Furthermore, using a series wiring would apply the same current to all appliances inside the household and not all appliances require the same amount of current. 6. Biomedical Application. Discuss the working principle of ventricular defibrillator. “Defibrillation is a medical technique used to counter the onset of ventricular fibrillation (VF), a common cause of cardiac arrest, and pulseless ventricular tachycardia, which sometimes precedes ventricular fibrillation but can be as dangerous on its own.” Electrical shock is used to reset the electrical state of the heart so that it may beat to a rhythm controlled by its own natural pacemaker cells. Applying electrical shock will depolarize the entire electrical conduction system of the heart and goes back to its normal rhythm, terminating ventricular arrhythmia. [3] 7. References [1] Electromotive Force. (n.d.). Science Waterloo. Retrieved September 9, 2013, from www.science. uwaterloo.ca/~cchieh/cact/c123/emf.html [2] Fitzpatrick, R. (2007, July 14). Emf and Internal Resistance. Retrieved September 9, 2013, from farside.ph.utexas.edu/teaching/302l/lectures/node57.html [3] Defibrillation. (2013). Science Daily. Retrieved September 9, 2013, from www.sciencedaily.com/ articles/d/defibrillation.htm [4] Lower, S. (2010). Electrochemistry 2: Galvanic Cells and Electrodes. Retrieved September 9, 2013, from http://chemwiki.ucdavis.edu/Analytical_Chemistry/Electrochemistry/Electrochemistry_2%3A_ Galvanic_cells_and_Electrodes