Ohms Law lab Report
Ohm's law states that the current through a conductor between two points is directly proportional to the voltage across the two points, and inversely proportional to the resistance between them.
The mathematical equation that describes this relationship is
V = IR (1)
where V is the voltage (or potential) measured in volts (v), R is the resistance measured in Ohms (Ω), and I is the current measured in amperes. If the resistance is constant over a large range of values of current and voltage, the resistor is referred to as an ohmic device. In this experiment, you will examine the relationship between current and voltage in both ohmic (ceramic resistors) and non-ohmic (ie: a light bulb) devices.
This lab also examines the effect of placing resistors in series and parallel with each other. Placing resistors in series should have the effect of increasing the total resistance, as predicted by the following formula:
RTotal = R1 + R2 + ... (2)
Placing resistors in parallel should have the effect of decreasing the total resistance, as predicted by the following formula:
1/RTotal = 1/R1 + 1/R2 + ... (3)
- To examine the effect on total resistance of placing resistors in series and parallel
- To examine the differences between ohmic and non-ohmic devices
- 4-Battery pack or variable power supply
- Alligator wires
- Various Resistors
- Multimeters for voltage and current
Activity 1 – Measuring resistance with a multimeter
On your table are two different ceramic resistors, in blue containers labeled A and B. Make sure that they are not connected to any circuit, especially to a power supply. Dial your volt/ohm multimeter to the resistance range, then place the probes across the terminals of the eachresistor. Record the resistance of each. Include your uncertainty.
Activity 2 – Graphically determining resistance
The purpose of this activity is to graphically determine the resistance of two ohmic resistors. If they are ohmic, then the resistance should remain constant over a wide range of voltages and currents.
Draw the circuit diagram using the symbols below.
Figure 1: Measuring Voltage and Current in a Simple Circuit (the block is a resistor)
The above circuit will allow you to simultaneously measure the current through, and the voltage across, a resistor connected to the battery pack. In Figure 1, the resistor is represented by the rectangular block. The "A" represents the ammeter, and the "V" represents the voltmeter. The symbol with the long and short line represents the battery pack.
Notice that the ammeter is in series with the other components of the electric circuit. The voltmeter, however, will be placed in parallel using the black and red probes. Your ammeter reads in units of mA. The current in your circuit should never exceed 200 mA, or 0.2 A. Your volt meter reads in units of volts. If your voltmeter has a range setting, set it to the 20 V range.
Using Resistor "A, " construct your circuit, but do not connect to the battery pack yet. Before you can connect to the batteries, you must get affirmation from your laboratory instructor.
Note that the 4-battery pack has notches cut out where the batteries meet. There is a Fahnestock clip that fits into the notch and between batteries. By inserting the clip between the first and second battery (going from negative to positive), and then connecting the rest of your circuit to that clip, you are effectively bypassing the other three batteries.
By moving your clip from one notch to the next, you are now including the next battery in the circuit. This second battery is now in series with the first battery, which results in an increase in the circuit voltage. As the voltage across the resistor changes, the current through the resistor will also change.