Physics Lab Report
Lab Notes – Ohm’s Law – Physics 2426
Equipment:
·
· Voltmeter
· Ammeter
· 11 Ω and 44 Ω variable resistors
· Connecting wires
Theory:
If a potential difference ΔV is applied across some element in an electrical circuit, free charges will flow through the element and thus a current I is produced in the element. The current is proportional to the potential difference but is limited by a quantity known as the resistance R. The larger the resistance is, the more difficult it is for charges to flow through the element which lowers the current.
The right hand side of the above equation can be considered as the definition of resistance. That is to say, resistance is the ratio of the potential difference across an element to the current passing through it. For some electrical devices that ratio may change as the potential difference and current change, but nonetheless we will still consider that ratio to be the resistance of the device. The units of resistance are therefore Volt/Ampere which is given the name Ohm, named after German physicist Georg Simon Ohm. The capital Greek letter omega (Ω) was rather cleverly chosen to represent the unit of Ohm. On a somewhat unrelated but amusing note; the reciprocal of resistance is a measure of electrical conductance, the ease with which an electrical current flows. The unit of electrical conductance is therefore Ω -1 = ℧ which is called Mho.
For some electrical devices the ratio of the potential difference across it to the current passing through it is constant under the right circumstances. This means that the resistance is constant. Such devices obey what we call Ohm’s Law and are called ohmic devices.
In order to test if a device obeys Ohm’s Law it is necessary to vary the potential difference (the current will then also vary) and then determine if the ratio of potential difference to current is in fact constant. This will be accomplished by plotting voltage versus current. If the data lie in a straight line, then that is a good indication that the device obeys Ohm’s Law. What will the slope of a fit line through the data points represent?
C:\Users\wje7460\Desktop\Ohm.jpg Setup and Procedure
Figure 1: Circuit diagram for measuring current through and voltage across resistor.
1. Connect the ammeter (A) in series between the DC power supply and the 11 Ω wire variable resistor.
a. There are three connection points, use the two that are on the “side”.
2. Connect the leads of the DMM in parallel to the same terminals of the resistor. Make sure the DMM is set to measure voltage.
3. Make sure the dial of the power supply is set to zero before you turn it on.
4. With the power supply on, turn the dial and watch how the voltage on the multimeter and the needle on the ammeter change.
a. If you don’t see the needle deflect on the ammeter, you either have a bad one or you wired it up backward. Remedy that before continuing.
5. Starting with zero voltage and zero current (that counts as a data point right?) record the current and voltage in increments that seem convenient to you up to either a maximum of 2 A or 20 V. Do not exceed 2 A or 20 V! Only leave power supply on while collecting data!
a. You should collect at least 5 pairs of current and voltage in addition to (0A, 0V). More is typically better!
6. Remove the 11 Ω resistor and replace with the 44 Ω resistor and repeat the above steps.
7. Put your data into a clearly labeled Excel file and make a scatter plot of your two sets of data. Make it look like this:
Figure 2: Made up data with random noise.
Questions
1. Can you determine if the resistors are obeying Ohm’s Law or not? (Hint: The R2 value is pretty much saying what percentage of the data can be explained by the fit line.)
2. What is your best estimate of the actual resistance of each resistor?
a. “11 Ω” = Percent error:
b. “44 Ω” = Percent error:
3. The resistance of a wire of length L and cross section A with a resistivity ρ can be modeled with the following equation: . Look at the two resistors, the coiled up wires are made of the same material so ρ should be same for both of them. What can you see would explain why the resistances would be different?
4. The resistance of a metal is actually temperature dependent: where α is just a temperature coefficient of resistivity and R0 is the resistance at temperature T0. If you allowed the resistor to get warm because you didn’t turn the power off when you weren’t taking data then the resistance might not be constant throughout your data. Is there any indication that this may be the case with your data?
5. Why did I prohibit you from going above 20 V or 2A?