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MODULE 2 : POWER, HEAT, LIGHT
At the end of this experiment, the student should be able to:
TLO 1 : Demonstrate that electrical power is a function of voltage and current by
calculating and measuring the power dissipated in a resistance as the
voltage is increased.
TLO 2 : Determine that the resistance of a lamp filament varies with temperature.
INTRODUCTION:
Power is the rate of doing work per unit of time. It is a measure of work done within
a specific length of time. In electricity, the unit of power measurement is the watt (W),
named for James Watt who perfected the steam engine. One watt of power is the work
done in one second by one volt of electrical pressure in moving one coulomb of charge.
Since one coulomb per second is one ampere, power in watts is the product of volts
times amperes.
When current flows through a resistance, heat is produced. This release of energy
(work) in the form of heat indicates that power is being generated by the power source
and dissipated (given off) by the resistance of the circuit. Because power is dissipated by
the circuit resistance, power is often calculated in terms of resistance. Two equations for
calculating power when resistance is known are derived from the basic equation
(๐ = ๐ธ ร ๐ผ) as follows:
(1) Substitute the Ohmโs Law equivalent for current (๐ธ โ๐
) for ๐ผ in the basic
equation. This results in
๐ = ๐ธ ร
2
(2) Substitute the Ohmโs Law equivalent for voltage (๐ผ ร ๐
) for ๐ธ in the basic
equation. This results in
๐ = (๐ผ ร ๐
) ร ๐ผ ๐กโ๐ข๐ ๐ = ๐ผ
2
ร ๐
The second equation reflects the power loss resulting from current flow through the
resistance of a circuit. Such power losses are frequently referred to as ๐ผ
2
๐
losses.
ENGAGE
- How does voltage and current affect the power output of a light bulb or electric
appliance?
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- What is the importance of determining power ratings of the electrical appliances
and devices we use every day?
EXPLORE
- Refer to this module and read in advance to have a better understanding of the
activities to be performed.
- Read about Ohmโs Law and Power using the references stated at the end of this
module or references taken online.
EXPLAIN & ELABORATE
EQUIPMENT/ MATERIALS NEEDED:
Power Source - 0 โ 30 Vdc, 25 mA
Milliammeter - 0 โ 0.1/ 1 Adc
Electronic VOM
Practical Electronics Trainer
R1 - 1 kฮฉ, 1W
R2 - 1.5 kฮฉ, 1W
R3 - 3.3 kฮฉ
R4 - 10 kฮฉ, 1W
R5, R6 - 100 ฮฉ, I W
DS1 - Miniature Lamp
Universal Experiment Board K
PROCEDURES:
Note: In the absence of an actual laboratory set-up, you may answer the module based
on the concepts learned from the lecture subjects. As a verification, perform a simulation
with your simulator of choice. For every circuit, attach a screenshot of the set-up in your
report.
Examples of simulators: LTspice (Free), Electronic Workbench, Circuit Construction Kit: DC
at phet.colorado.edu
TLO 1 : Demonstrate that electrical power is a function of voltage and current by
calculating and measuring the power dissipated in a resistance as the voltage is
increased.
- Connect resistors R1 and R2 in a parallel, then connect them through the
milliammeter to the power source as shown in Fig. 2 โ 1.
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- Set the milliammeter to the 100mAdc range.
- Adjust the power source to 2Vdc as indicated by the VOM.
- Record the current reading from the milliammeter in Table 2 โ 2.
- Adjust the power supply to each of the remaining voltages listed in Table 2 โ 2 and
record the current for each voltage. After the 4 โ volt reading is recorded, reset
the VOM to the 15Vdc range and the milliammeter to the 1Adc range for the
other voltages.
- Return the voltage to zero.
- Now calculate and record the power for each of the voltages and currents listed
in Table 3 โ 3.
VOLTAGE
E
CURRENT
I
POWER
W
Table 2 โ 2
- Compare the calculated values of Table 2 โ 1 with the measured values of Table
2 โ 2. What factors would account for any differences in the two sets of values?
_________________________________________________
_________________________________________________
_________________________________________________
Resistor tolerances, accuracy of the meters, and measurement error
(reading the meter at an angle) can cause your measured values to differ from
your calculated values. However, this difference should be no greater than 5 to
10 percent.
TLO 2 : Determine that the resistance of a lamp filament varies with temperature.
- Set the electronic VOM to the LPฮฉ function and measure the cold resistance of
lamp DS
Rcold = _______________ ฮฉ
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You should measure approximately six ohms.
- Connect lamp DS1 through the milliammeter to the power source as shown in Fig.
A
V
0 โ 5Vdc
0 โ 1Adc
DS
Fig. 2 โ 2
- Set the Electronic VOM to +DCV, 5V range and connect it across the lamp, as
shown.
- Set the milliammeter to the 1Adc range.
- Adjust the power source, in turn, to each of the voltages listed in Table 2 โ 3.
Measure and record the current flow for each voltage.
- Return the voltage to zero.
- Calculate and record the power dissipated by the filament for each set voltage
and current values recorded in Table 2 โ 3.
- Calculate and record the filament resistance of the lamp for each voltage and
current values recorded in Table 2 โ 3.
- Examine your recorded measurements in Table 2 โ 3. As the voltage and current
increase, does the resistance of the filament increase? ____
E
Vdc
I
mAdc
P
W
R ฮฉ 1 2 3 4
