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Ohm's Law: Overview and Objectives

Background

Before learning about Ohm's Law, it helps to become familiar with the power supply and gel box. The electrode at which electrons enter the gel box from the power supply (along the black wire) is called the cathode, and is negative (-). The electrode at which electrons leave the box and re-enter the power supply (along the red wire) is called the anode and carries a positive charge (+). The flow of electrons sets up a potential energy difference between the electrodes. This is known as electrical potential, and is measured in volts. It establishes an electric field through which the ions in the gel box fluid migrate. The migration of ions in the fluid creates electrical current which is measured in milliamperes (milliamps) which measures the rate of flow of electrons. Resistance, measured in ohms, indicates how easily the current can flow through the gel box fluid. The higher the resistance, the harder it is for the ions to migrate across the box.

Ohm's Law describes the relationship between voltage, current, and resistance, named after Georg Ohm, a German physicist born in the 18th century. Using a gel box and power supply, it is possible to determine Ohm's Law. This can be done by keeping the resistance constant while varying the voltage, and measuring the resulting current. The data from that process can be plotted on a graph and then compared with other graphs to find the equation which corresponds. The same process can also be used by keeping the voltage constant while varying the resistance and measuring the resulting current.

Purpose

This laboratory explores the concept of Ohm's Law and teaches analytical skills by having students derive Ohm's Law from their observations.

Materials per team

          power supply sodium chloride solution [1M]     rack for NaCl, if needed    
          gel box 300 mL distilled water beaker for used tips
          p1000 micropipet/tips     beaker for measuring water paper towels/Kimwipes

  • You can substitute other measuring instruments (droppers, pasteur pipets, regular pipets). Volumes are approximate.
CAUTION
  • Study the gel box. It is designed so that when the box is opened, the electrodes are disconnected.

  • Slide the lid of the gel box off by using the small finger hold "mounds" on the top of the lid.
    DO NOT, repeat, DO NOT PULL ON THE WIRE LEADS TO REMOVE THE TOP!!!

  • Since any wet surface can become conductive, it is advisable NOT to touch any part of the apparatus (gel box, wires) while the power supply is on. This is especially important if the outside of the box is wet, or if your hands are wet.


Procedure     Activity Sheet



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