Ch20_Ohms+Law+Lab

Purpose:

Find the relationship between current and pressure difference. Find the difference between Ohmic and non-Ohmic materials.

Hypothesis:

Pressure difference and current will have a direct relationship. As pressure difference increases, current increases, and vice versa. The rationale behind this hypothesis is that as more batteries are added to a circuit, the bulbs in the circuit get brighter. We can deduce that the bulbs get brighter because more charge is flowing through the filament, indicating a greater flow rate, or current.

Ohmic materials will be materials that follow Ohm's Law -- V = IR. Non-Ohmic materials will not follow Ohm's law.

Materials:

Variable power supply, batteries/holders, long bulb and socket, assorted resistors, multimeters, lead wires.

Set Up:



Procedure:

1. Set up the above circuit. 2. Insert a resistor where the symbol for resistor is in the diagram above. Use a long bulb and socket, round bulb and socket, a standard resistor, and another (different form the first) standard resistor. 3. Measure the current, using the ammeter, flowing through each resistor. Use 4 different voltages for each resistor. 4. Record the current and voltage for each resistor. 5. Graph voltage vs. current for each current and find the line of best fit.

Data:









Analysis:

We can think of Ohm's Law - V = RI as following the linear relationship of y = mx + b. Assuming this, y would be Voltage or pressure difference and the x would be Current. The slope would be Resistance. If a resistor follows Ohm's law, and the relationship between pressure difference and current is linear, then the resistor is said to be "Ohmic." If the relationship is non-linear then the material is said to be "non-Ohmic." As the graphs shows, the Ohmic materials (the resistors) have a linear relationship with a very strong R^2 value. The non-Ohmic materials (the bulbs) have a strong power relationship.

Sample Calculations:

Percent Error



Experimental Resistance



Discussion Questions:


 * 1) In terms of experimental data, how is resistance defined and what are its units?

Resistance is the slope of the graph of the linear relationship V=RI. It is defined as voltage or pressure difference divided by flow rate or current. R = V/I.
 * 1) Imagine that you had a third resistor that has a much smaller resistance than the ones used in the lab activity.
 * 2) Sketch a graph of pressure difference vs. flow rate that shows your 2 original resistors and this new resistor (sketch them on the same axes). Clearly label the lines.
 * 3) [[image:Photo_on_2011-11-06_at_15.30.jpg]]
 * 4) Explain why you drew it this way.
 * 5) Ohms law follows a linear relationship, V=RI where R is the slope. The resistor with the greatest resistor will have the steepest slope and the third resistance, with the smallest resistance, will have the flattest slope.
 * 6) How would the flow rate through this resistor change as the pressure difference decreases?
 * 7) The flow rate would also have to decrease because flow rate and pressure difference have a direct relationship.
 * 8) Assume that resistor A has 10 times the resistance of resistor B. What would a graph of resistance vs. current look like for these two resistors (sketch them on the same axes)? What about a graph of resistance vs. voltage? Justify your answers.

Resistance v. Current



Justification: Resistance does not have a relationship with current. Only voltage has a direct relationship with current. Resistance always remains constant.

Resistance v. Voltage



Justification: Resistance does not have a relationship with voltage. Only current has a direct relationship with voltage. Resistance always remains constant.

Examine the graph of electric pressure difference vs. flow rate on the right. Ohmic Resistance = slope.
 * 1) Is this resistor Ohmic or non-Ohmic?
 * 1) What is the resistance of the object from which this data was collected? (Show your work.)

Point 1 = (0,0) Point 2 = (1,5)

m = y2-y1/x2-x1 m = 5

Resistance = 5 Ohms

Conclusion:

My hypothesis was correct. It can be seen from the graphs that pressure difference and flow rate have a direct relationship even in non-Ohmic materials. In the graph of the bulbs, voltage increased as current increased, supporting my hypothesis. Same with the Ohmic materials. The only difference is that Ohmic materials have a direct-linear relationship, where non-Ohmic materials have a direct-nonlinear relationship. When dealing with Ohmic materials and their graphs, the slope of the graph is the resistance of the resistor.

There were very few sources of error in this lab, which is why our percent error was quite low. One source of error was the ammeter. The readings it displayed kept jumping around; they were not consistent, and so we had to "pick" one of the readings it gave as the numbers went up and down. Also, the pressure applied with the ammeter to the resistor affects the readings - greater pressure equals higher readings. This can definitely throw off results. In addition, the resistors themselves bore some of the error. Both our resistors had gold bars on the end indicating a tolerance of 5%. This means the resistor will have a resistance of, for example, 1000 ohms, plus or minus 5% of 1000. We were, however, well within the threshold, as our percent error never exceeded 5%. For example, one flow rate reading we got for the 1000 ohm plus/minus 5% resistor under a pressure difference of 7 volts was 985.9. If we calculate the percent error, |theoretical - experimental|/theoretical *100, we get:


 * 1000-985.9|/1000 * 100 = 1.4085%, well within the tolerance of the resistor.

One way I would change this lab is to use ammeters that provided more accurate and stable readings and to use resistors with extremely minimal tolerance, say, less than 1%. As a result, our data would be much more precise and accurate. A real life application of this lab would be making sure the electrician did his job right and installed the proper resistors, otherwise it could mess up the current running throughout the house. Also, it allows us to better understand how the circuits within our homes work and makes us smarter in general :).