Physics Lab

Physics 202

Physics 202 Lab: Ohm’s Law

Lab: Ohm’s Law

Please note that this assignment should be done COMPLETELY on your own (no lab partner collaboration). The only person you should ask a question of is your instructor.

Apparatus: Microsoft Excel, Phet Circuit Construction Kit: DC, Phet Ohm’s Law

In this lab we will investigate Ohm’s Law. Ohm’s Law is the mathematical relationship between Voltage (V), Current (I), and Resistance (Ω).

Where V is voltage, I is current, and R is resistance.

Part 1 https://phet.colorado.edu/en/simulation/ohms-law Please follow the link to the simulation

Investigate the relationship between voltage (V) and current (I) by sliding the Voltage slider. Make sure that you only slide one slider at a time, so leave the resistance slider alone for now.

1) What happens to the current as voltage increases?

2) What do you think the value of current would be if voltage was 0?

3) The current is ( directly | inversely ) proportional to the voltage. Please circle your answer

4) Draw a graph of the relationship between voltage and current.

Please use this site “https://www.desmos.com/calculator” to help guide your graph prediction. Try typing in a few of the following equations- 3x , 3x^2 , 3x+2 , 3/x , 3x^3 , 3+x , 3^x

Hint: You can copy and paste each equation into the website

Voltage

Current

Reset the voltage slide back to the middle position. Now investigate the relationship between the resistance (R) and the current (I) by sliding the resistance slider. Now you want to leave the voltage slider alone.

1) What happens to the current as resistance increases?

2) The current is ( directly | inversely ) proportional to the resistance. Please circle your answer

3) Draw a graph of the relationship between resistance and current.

Resistance

Current

Notice that the value for current is in mA (milliamps). Milliamps are used to display a value when it is very small. There are 1000 mA in 1 A. For example, if you have 9 mA you will have 0.009A

Write down a few of your measured values in mA and convert these values to A.

Current (mA)	Current (A)

Part 2 https://phet.colorado.edu/en/simulation/circuit-construction-kit-dc

Procedure:

1 Click on the url above to open the Phet Circuit Construction Kit: DC. Then click on the Play symbol on the Colorful Picture of the Monitor , then click on LAB. To construct your circuit, you will click and drag the circuit elements (wires, battery, current meter, resistors) onto the drawing screen to start drawing what is illustrated below. Once on the screen you can drag them about by first clicking near the center of each piece and then holding down the mouse while you move the circuit element. You can rotate these circuit elements by clicking on one of the circles that are on each end. You can make connections between pieces by bringing the circle on one element onto the circle of another. You can tell that you have made a connection, when the circle between the two is black, and you can no longer see a red circle. If you need to break a connection, click on one of the circles, and then click on the scissors.

2 Set up the circuit shown in the figure below. First set up only that part of the circuit connected by straight wires, the Battery, ammeter, and resistor. Now connect the voltmeter to the resistor, by putting the voltmeter probes on both sides of the resistor as indicated in the illustration. Click on the “Show Current”, “Labels” and “Values boxes in the upper right hand corner of the screen Click on the plus sign next to the Resistivity and Battery Resistance and make sure that they are at zero. If your circuit is drawn correctly, you will see electrons moving through the circuit and the ammeter will have a value other than zero.

3 When you click on an element of the circuit, it will be surrounded by yellow box. This allows you adjust the voltage or resistance. Once the circuit element is highlighted with the yellow box, you can also click on Delete and eliminate it. Click on the resistor and set it to 50 ohms. Click on the Battery and set it to 12 Volts. Record the value of the current in amps in the data table for the 50-ohm resistor. Now fill in the rest of table for the 50-ohm resistor, by clicking the battery and changing to every value on the table.

4. Now click on the resistor and change it to 100 ohms, and then fill in all of the current values in the 100-ohm table.

Give all currents as positive values. Record your currents in the table as AMPS

Resistance - 50 Ohms Resistance - 100 Ohms

Current (amps)	Voltage	Current (amps)	Voltage
	0		0
	2.0		2.0
	4.0		4.0
	6.0		6.0
	8.0		8.0
	10.0		10.0
	12.0		12.0

4. Now, click the box on the right hand side of the simulation for advanced settings.

5. Check the box for ‘Add Real Light Bulbs’

6. Recreate the circuit above, replacing the resistor with a REAL light bulb (make sure it is the light bulb marked as real)

7. If we replace the resistor with a light bulb, we get the following data (adjust the voltage of your battery accordingly then record Current).:

Current (Amps)	Voltage (Volts)
0	0
	2.0
	4.0
	6.0
	8.0
	10.0
	12.0

Analysis

1. Resistors

a. Plot the data from the 50-ohm resistor on Excel. Put the current in column A (x-axis) and the voltage in column B (y-axis). On a new sheet, plot the data from 100-ohm resistor. Insert these graphs below. Label the axes properly (see axes below).

b. Find the slope of each line of each line using the trendine feature of Excel.

*Slope of Line 1 (50ohm resistor)_________ Slope of Line 2 (100 ohm resistor)______

*c. What does the slope of the line represent?

*d. According to your graph, how could you describe the relationship between Voltage and Current?

(directly proportional, Inversely proportional, etc.)

V (volts)

I (amps)

2. Light Bulb

a. Plot the data from the light bulb on excel. Once again, Plot Current on the X axis and Voltage on the Y axis. Insert this graph below. Label the axes properly (see image above).

b. If these dots form a straight line, then find its slope. If they don’t form a straight line, then draw a curve that come closest to connecting the dots.

Slope of the graph (if it’s a straight line) ________________

If it’s a curve, is its slope increasing or decreasing.

What is happening to the resistance of the light bulb as the voltage is increased?

(Hint: Look at the answer to “1c” of the analysis to find out what the slope represents)

.

The resistance is increasing The resistance is decreasing The resistance is staying the same

Final Questions

1) What is Ohm’s Law in equation form? Please circle all that apply, and show your calculations.

2) What would happen if we were to increase the voltage, would you expect there to be more or less current?

3) Using your graph for the 50 Ohm resistor, what would the current be from an applied voltage of 150 volts?

4) For the 50-ohm resistor, does the resistance change when more voltage is applied?

5) For the 100-ohm resistor, does the resistance change when more voltage is applied?

6) If there are 1000 mA in 1 A, how could we display a value of .007 A in mA?

7) If there are 1000 Ω (Ohms) in 1 kΩ (kilo-Ohm). How many kΩs do we have if the resistance is 1500 Ω?

8) Looking at the curve for the light bulb, what happens to the resistance as we apply more voltage? Is this the same as the resistors?

9) Describe the shape of the graph and what information is given by this shape.

10) What do you expect the current in the light bulb to be if the voltage was increased to 15 V? There is a range of answers for this question.

Please include the 3 excel graphs in your submission.

Tidewater Community College Virginia Beach, VA

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