4 Power in DC Circuits
Thursday, 12 July 2018
9:38 AM
When there is a current through a resistor, electrical energy is converted into heat. Heat is then radiated from the resistor. The rate that heat is dissipated is called power. Power is measured in units of joules per second (J/s), which defines the unit called the watt (W). The power dissipated by a resistor is given by the power law equation:
𝑃=𝑉𝐼
By applying Ohm's law to the power law equation, two more useful equations for power can be found. These are:
𝑃=𝑅𝐼2
and
𝑃=𝑉2𝑅
The three power equations given above are also known as Watt's law. In this experiment, you will determine power using the last equation. Notice that if you measure the voltage in volts (V) and the resistance in kilohms (kΩ), the power will have units of milliwatts (mW).
Metal-film composition resistors are available with standard power ratings ranging from 1/8 W to 2 W. For most typical low voltage applications (15 V or less and at least 1 kΩ of resistance), a 1/4 W resistor is satisfactory.
Power in DC Circuits
Measure the resistance of a 3.9 kΩ resistor.
R1 =
Construct the circuit shown in Figure 2-8 (a). Figure 2-8 (b) shows an example of the circuit constructed on a breadboard. R2 is a 10 kΩ potentiometer. Connect the centre (variable) terminal to one of the outside terminals. Use this and the remaining terminal as a variable resistor. Adjust the potentiometer for 0.5 kΩ. 
Caution!
Always remove power when measuring resistance and make certain you are measuring only the potentiometer's resistance. Have a lab tutor check your procedure if you are unsure.
(a) Schematic
(b) Breadboard wiring
Figure 2-8
Use Ohm's Law to compute the total current in the circuit. The total voltage is +12.0 V. The total resistance is R1+R2. Enter the total current in Table 2-7. The first entry has been completed as an example. (There are no measurements to put in the table yet, just the calculation for IT ).

Table 2-7
Variable
Resistance
Setting (R2)
𝑉1
(measured)
𝑉2
(measured)
Power in 𝑅2:
𝑃2
0.5 kΩ
2.727 mA
1.0 kΩ
2.0 kΩ
3.0 kΩ
4.0 kΩ
5.0 kΩ
7.5 kΩ
10.0 kΩ
Measure the voltage across R1 and the voltage across R2. Enter the measured voltages in Table 2-7. As a check, make sure the sum of V1 and V2 is equal to 12.0 V. Then compute the power in R2 using either of the following equations:
𝑃2=𝑅2𝐼𝑇2         or          𝑃2=𝑉22𝑅2
Enter the computed power, in milliwatts, in Table 2-7.

Turn the output of the power supply off and set R2 to the next value shown in Table 2-7. Turn the output of the power supply on and repeat steps 3 and 4. Continue in this manner for each of the resistance settings shown in Table 2-7.

Using the data in Table 2-7, graph the relationship of the power, P2, as a function of resistance R2 in Excel and place it in Plot 2-2. Since resistance is the independent variable, plot it along the horizontal axis and plot power along the vertical axis. An implied data point can be plotted at the origin because there can be no power dissipated in R2 without resistance. A smooth curve can then be drawn to the origin.
Plot 2-2
Insert your Excel graph here:




















