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# The potato - a source of EMF

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Introduction

Data Handling

Physics B (Advancing Physics)

Brief: In this coursework I will need to do the following

• Understand how the data came about
• Analyse the given data using my knowledge of physics
• Evaluate and draw conclusions from the graphs

Investigating a Potato as a source of EMF.

Introduction: In this coursework I will be investigating the EMF produced by a potato cell (“a potato cell” in the sense of an electrical device, not a cell from a potato.)

The potato cell generates an “electromotive force” (EMF) by electrical reactions that take place between the chemicals inside the potato and the zinc electrode connected to it.

But how does it work?

The Zinc electrode reacts with the acid inside the potato and a transfer of electrons takes place between the zinc electrode and the acid. The zinc is oxidized to Zn2+ and the acid, whose active ingredient is positively charged hydrogen (H+), is reduced to hydrogen gas (H2).

Zn + 2H+ Zn2++ H2

Oxidation: Zn Zn2++ 2e-

Reduction: 2H+ + 2e- H2

The above redox reaction will happen with or without a copper electrode present in the potato but the copper electrode is used to draw the negative electrons from the cell towards it so that a current can be made to go through the external circuit. Therefore the potato will become a potato cell and a source of EMF.

Presumably the higher the acidic content of the potato the more acid there is to react and so the greater the power of the potato cell.

Middle

Terminal p.d. (V)

I (mA)

Voltmeter resolution (V)

Ammeter resolution (mA)

Ohmmeter resolution (Ω)

5.03

0.41

0.08

0.02

0.02

10

3.48

0.36

0.10

0.02

0.02

10

2.56

0.31

0.12

0.02

0.02

10

1.79

0.26

0.14

0.02

0.02

10

1.30

0.22

0.16

0.02

0.02

10

0.776

0.16

0.18

0.02

0.02

1

0.466

0.12

0.20

0.02

0.02

1

0.228

0.07

0.22

0.02

0.02

1

0.0033

0.02

0.24

0.02

0.02

0.1

 Electrodes 2cm apart Rload (kΩ) Terminal p.d. (V) I (mA) Voltmeter resolution (V) Ammeter resolution (mA) Ohmmeter resolution (Ω) 4.36 0.44 0.10 0.02 0.02 10 3.05 0.38 0.12 0.02 0.02 10 2.41 0.34 0.14 0.02 0.02 10 1.77 0.29 0.16 0.02 0.02 10 1.346 0.26 0.18 0.02 0.02 1 1.091 0.22 0.20 0.02 0.02 1 0.664 0.16 0.22 0.02 0.02 1 0.314 0.10 0.24 0.02 0.02 1 0.1650 0.06 0.26 0.02 0.02 0.1

Graphs for Terminal p.d. (V) against current (I) to find EMF and internal resistance: I (A) I (mA) Terminal p.d. (V) 0.00008 0.08 0.36 0.00010 0.10 0.28 0.00012 0.12 0.22 0.00014 0.14 0.10 0.00016 0.16 0.02

y = -4300x + 0.71

V = - rI + EMF

EMF: 0.71 Volts

Internal resistance: 4300Ω I (A) I (mA) Terminal p.d. (V) 0.00008 0.08 0.41 0.00010 0.10 0.36 0.00012 0.12 0.31 0.00014 0.14 0.26 0.00016 0.16 0.22 0.00018 0.18 0.16 0.00020 0.20 0.12 0.00022 0.22 0.07 0.00024 0.24 0.02

Conclusion

Conclusions:

I expected the EMF to stay the same throughout the experiment but it did not. I now understand that the EMF would decrease because the potato is being discharged like a normal cell.  Like a normal cell the potato cell will eventually stop working all together. This however, does not explain why the EMF decreased from 8-4cm and then increased from 4-2cm. I do not know what order the results were recorded in but from my understanding it would make sense for the data to have been collected in the following order: 8cm, 2cm and then 4cm.

As the distance between the electrodes increase

• EMF stays more or less the same depending on how long the cell has been running. The EMF will eventually run out and the potato cell will be discharged.
• The internal resistance increases as there is more starch and actual potato for the electrons to get though which requires more energy.
• The p.d. decreases because the internal resistance increases • I can see how The Maximum Power Theorem could relate to my set of data however, because of the inaccuracy of the curve due to the lack of results and the difficulty in calculating the turning point I cannot assume that it is true.

Bibliography

Websites used:

• http://www.ndt-ed.org/EducationResources/HighSchool/Electricity/voltage.htm
• http://www.practicalphysics.org/go/Experiment_164.html
• http://www.antonine-education.co.uk/Physics_AS/Module_3/Topic_4/topic_4.htm
• http://en.wikipedia.org/wiki/Electromotive_force
• http://www.electro.patent-invent.com/electricity/inventions/maximum_power_theorem.html
• http://www.wisc-online.com/objects/index_tj.asp?objID=DCE9904

Books used:

• Chemistry in Context
• Advanced Physics text book – Steve Adams & Jonathon Allday
• Advancing physics text book.

Other:

• Advancing Physics CD ROM (Chapter 2)

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