College Physics Lab
PH 144
Return to Lab main
Ohm's Law and Electrical Power

Purpose: To verify Ohm's law and determine the resistance of a particular resistor. To verify that the electrical energy delivered to a resistor is converted to heat energy, and that the total amount of energy is conserved.

Discussion:
    Ohm's law states that the voltage difference V between the two sides of a resistor is proportional to the current I flowing through the resistor, i. e. V = IR. The constant of proportionality R is the resistance, and its SI unit is the ohm (W).  Furthermore, the electrical power (energy per time) delivered to the resistor is P = I2R.  The electrical potential energy change as a result of the current flow is D(PEq) = -I2RDt, where Dt is the time during which the current flowed.
    In this experiment the resistor will be a coil of wire immersed in water. The electrical potential energy lost will be converted to heat energy in the resistor. The heat energy flowing out of the resistor will be detected as a rise in temperature in the water. The heat flowing into the water is positive and is given by Q = (mwcw + mccAl)DT, where mw is the mass of water, cw is the specific heat of water, mcis the mass of the aluminum calorimeter cup, cAl is the specific heat of aluminum, and DT is the temperature change.

Notes about safety: The equipment you will be using is quite safe as long as you observe a few safety precautions. Do not spill water on your hands or anywhere around the electrical equipment. Never touch a 'live' electrical circuit with both hands. Make sure that you turn down all the control knobs before switching the power supply on or off. Adjust the current and voltage in your circuit slowly.

Procedure:

1. Measure mc. (Check to see whether the cup is made of aluminum or copper. That will determine which value of specific heat you should use.) Fill the cup with about 130 - 150 g of water.  Place the cup in the calorimeter and insert the resistor in the water.  Don't run current through the resistor without water in the cup!

2. Connect the circuit as requested. Pay particular attention to the placement of the voltmeter and ammeter. Do not turn on the power supply until your circuit has been checked.

3. Take data of voltage V and current I. You should vary the current from 0 to 2 A.

4. Make a graph of V vs. I. Draw the best line through your points. The resistance R is the slope of this line, calculate it.

5. Adjust the current I to about 2.5 A. You are going to take data of the temperature T as a function of time t. You want the temperature to increase 1 Co every 45 to 90 s. This may require that you adjust the current somewhat. Once you have found an acceptable current setting, turn off the power, fill the cup with about 130 - 150 g of cool tap water, and determine mw. You want to start with cool water in order to minimize heat losses during the actual experiment. Turn the power back on, adjust your current to the acceptable setting, and start your timer at some whole number temperature (28°C for example). Stir water occasionally. Record the time at 1Co increments for ten or more increments.

6. Make a graph of T vs. t. Draw the best straight line through your points. A straight line indicates a constant electrical power. Determine the slope of your line. Compare the slope you determined with the slope you would predict if all of the electrical energy dissipated is absorbed by the water. Express the difference as a % difference.

7. Using the total time and temperature change, calculate |D(PEq)| and |Q| in J. Calculate the % difference.
% difference = difference/average X 100.