Determination Of Ohm’S Law From Physical Data
Time: This activity should take one period for students to complete data collection, create graphs, and find best-fit functions using Excel.
Level: Physics classes that are studying electric circuits for the first time.
OverviewYou or your students will set up a basic electric circuit, consisting of a power supply, ammeter, resistors, and wiring. Using this circuit, students will follow a hand-out to conduct two sets of measurements and plot those data. • First, while keeping the resistance constant, students will vary the voltage and observe its effect on electric current measured with an ammeter. Students should collect enough different voltages so they can produce a graph. This graph of electric current as a function of voltage should be linear. • Second, students will vary the resistance while keeping voltage constant, and measure the effect of resistance on electric current. Again, enough different resistances should be used to get points for a graph of current as a function of resistance. With a power law fit, this should be current ~ 1/resistance. Students will make two separate plots of current vs. voltage and current vs. resistance and determine the best-fits. The plots will suggest the relationship between current, voltage and resistance, and can be used to justify Ohm’s law, I = V/R.
Students in many science classes do not get many opportunities to use raw data to make plots, fit data, and extract laws or equations from those data. This activity is meant for students to do just that, much as experimentalists do at the professional level.
Students will learn how to make simple circuits, as well as measure the basic quantities associated with electronics.
Students will also learn a useful relationship in Ohm’s law, and be aware of why it is setup the way it is through direct observation and measurement.
Students should be able to make graphs using Excel or other preferred software, and find best-fit equations to data (such as with Excel).
Students should know what a basic circuit consists of, and should be familiar at least with the concepts of what resistance and currents are, so the understand what they are measuring.
This activity can and should be done before students are exposed to basic circuitry principles; the goal is for students to ‘discover’ Ohm’s law.
An electric circuit can be thought of as a racetrack for electrons. In order for the electrons to move through a material, it must get pushed in one direction. This is done with an electric field, which creates forces on electric charges such as electrons. But to have an electric field present either in space or between two points of a wire, there must be a voltage difference, ∆V, between those two points (which has a distance ∆r. The strength of the electric field is given by E = ∆V/∆r.
How do we set up a voltage difference for a circuit? This is what the battery or power supply does. It is commonly called V in textbooks, but in reality this voltage represents the voltage difference between the two terminals of the battery or power supply. When the electric field goes through the wires and components of an electric circuit, the free electrons present in conducting materials feel a force, F = qE. This force pushes all the free electrons in the same general direction. The problem is, those electrons move short distances before they run into atoms and molecules of the material, and bounce around as if they are in a pinball machine. With all those collisions, energy is lost by the electrons and transferred to the lattice of the material. The vibrational energy of the atoms and molecules is felt by us as heat, and those collisions make it tougher for current to flow – this is electrical resistance, measured in ohms (Ω).
Ohm’s law is the relationship between the voltage difference of the battery, V, and the resulting current. The bigger the voltage difference, the stronger the electric field and therefore the larger flow of electrons per unit time, which is electric current, I. Resistance is the constant of proportionality, and therefore Ohm’s law is V = IR.
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