Physics Lab #16

 

Ohm’s Law and Resistivity Lab

Theory

Current (I) is a measure of the rate of flow of charge:

 

                    I = Δq/Δt       (units: C/s=amperes, amps or A)

 

Voltage (V) is the change in electric potential energy difference (ΔEPE) between two points (say the terminals of a battery) divided by the charge (q) that moves between the points:

 

                    V = ΔEPE/q   (units: J/C=Volts or V)

 

 

Resistance (R) is a measure of the tendency of something to resist the flow of charge.  This is sort of a circular (non) definition.  Resistance is better defined below in Ohm’s Law.  We also came up with an equation for resistance by way of analogy with heat flow that related the resistance to the length (L) and cross-sectional area (A) of the particular resistor, and the resistivity (ρ) of the material of which it is made:

 

                    R = ρL/A       (units: for R units are Ohms or Ω, for L units are m, for A use m² and for ρ use Ω-m)

 

Ohm’s Law relates the voltage (V) across and the current (I) through a resistance (R) via:

 

V=IR

 

 

 

Procedure

This lab involves lots of new equipment such as voltmeters (for measuring voltage), ammeters (for current) power supplies, resistors and nichrome wire, and some new techniques.  I will work along with you and show you how to use these to build a working circuit and how to determine the resistance of a resistor from the color coded bars on it as well as how to draw the circuit diagram for the circuits that you build.

 

1. Identify all the pieces of equipment on your lab bench.  (I will provide lots of help here).
2. Build a simple circuit with resistors and power sources provided and draw it.  Have the power supply set to 3V to begin with and have me check it out before you plug it in.
3. Add the voltmeter and the ammeter to measure the voltage and current across the resistor.  If the ammeter reading is too low increase the voltage until you can get a reasonable reading.
4. Use Ohm’s Law to calculate the resistance of the resistor and compare the result to the one written on the resistor.
5. Repeat steps 2 to 4 for other resistors.
6. Repeat steps 2 to 4 for the piece of nichrome wire.
7. Measure the length and radius of the wire.  Calculate the cross-sectional area from the radius and use the results to these and to step 6 to find the resistivity of nichrome. 
8. Compare 7 to the expected result (ρ = 1.0 x 10^–6 Ω-m).
9. Try confirming the nature of the relationship for resistance and resistivity by twisting two or three nichrome wires together to make a longer wire (hence doubling or tripling L – what should happen to R?) and by twisting two or three nichrome wires together side by side (hence doubling or tripling A – what should happen to R?).