Expt.

# CKTS 1 L- 3
Power- Heat-Light

Objectives:

A. To 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.
B. To determine that the resistance of a lamp filament varies with temperature.

Equipment/Materials:
Power source
Milliammeter
VOM (Electronic/Analog)
Miniature Lamp
Resistors - 1K ohms & 1.5 K ohms

Procedure:
Objective A : To demonstrate that the electrical power is a function of voltage and current by calculating and
measuring the power dissipated in a resistance as the voltage is increased.
1.
a) Connect resistors R1 and R2 in parallel, then connect them through the milliammeter to the power source.
b) Set the VOM to +DCV, 9 V range and connect it across the parallel resistors.
I m= 0.015A, Ic= 0.015A Pm= .135 W watts Pc= .135 W

2.
a) Refer to Table 1 and use Ohm’s law to calculate the current through the resistors for each of the voltages
shown. Use 1K ohms as the load resistance.
b) Record your calculated current values in Table 1.
c) Calculate the power dissipated in the resistors for each voltage and enter values in Table 1.

TABLE 1:
VOLTAGE (E) CURRENT (IC) POWER (PC)
1.5 0.0025 A 3.75 x 10^-3 W
3 0.005 A 0.015 W
4.5 0.0075 A 0.03375 W
6 0.10 A 0.6 W
9 0.015 A 0.135 W
3.

a) Set the milliammeter to the 25 mAdc range and the volt meter to 10 volts range.
b.) Adjust the power source to 1.5Vdc as indicated by the VOM.
c.) Record the current reading from the milliammeter in Table 2.
d.) Adjust the power supply to each of the remaining voltages listed in Table 2 and record the current for each
voltage. After the 8-volt reading is recorded, reset the VOM to 50 Vdc range and the milliammeter to the 25 mAdc
range for the other voltages.
e.) Return the voltage to zero.
f.) Calculate and record the power for each of the voltages and currents listed in Table 2.

TABLE 2:
VOLTAGE (V) CURRENT (IM) POWER (PM)
1.5 2.511 mA 3.7665 mW
3 5.021 mA 15.063 mW
4.5 7.532 mA 33.894 mW
6 10.042 mA 60.252 mW
9 15.063 mA 135.567 mW

4.
a.) Compare the calculated values of Table 1 with the measured values of Table 2. What factors would account for
any differences in the two sets of values? The difference in the two sets of values is their tolerance, the computed
value is an ideal value in which it depends on the given, In the other hand the measured value is the exact or
precise value.
b.) Consider the current and power levels for 2 and 4 volts in Table 2. As the voltage doubles from 2 to 4 volts,
what does current do? Doubling the voltage will cause the current to increase or be doubled.

What does power do? It is the rate of doing work.

c.) Consider the current and power values recorded for 4 and 8 volts. Did current double? Yes, the current double.

Did power increase 4 times? The current is directly proportional to the voltage and inversely proportional to
the resistance. Any alteration in the voltage will result in the same alteration of the current. So, doubling or
tripling the voltage will cause the current to be doubled or tripled.
Objective B: To determine that the resistance of a lamp filament varies with temperature.

5.
a.) Set the electronic VOM to the Ω function and measure the cold resistance of lamp DS1.
R cold = 1 ohm this is the resistance when it is not connected.

b.) Connect lamp DS1 through the milliammeter to the power source.
c.) Set the VOM to +DCV, 2.5 volt range and connect it across the lamp.
d.) Set the milliammeter to the 250 mAdc range.
e.) Adjust the power source, in turn, to each of the voltages listed in Table 3. Caculate and record the
current for each voltage.
f.) Return the voltage to zero.
g.) Calculate and record the power dissipated by the filament for each set of voltage and current values
recorded in Table 3.

TABLE 3: Computed Current

Table 4: Measured current Adjust the power source, in turn, to each of the voltages listed in Table 4.
Measure and record the current for 1.5 voltage.
E,Vdc I, mAdc P,w R,ohms
1.5 1.5 A 2.25 W 1 ohms
6. Calculate and record the filament resistance of the lamp for each set of voltage and current values
recorded in Table 3.
b. Examine your recorded measurements in Table 3. As voltage and current increase, does the
resistance of the filament increase? the resistance of the filament will change as it heats up and cools
down.
c.) How much greater was the filament resistance at one volt than when cold? 1 ohm
d.) What was the resistance increase from?:
0.5 Vdc to 1Vdc? 1 ohm
1 Vdc to 1.5Vdc? 1 ohm
1.5 Vdc to 2 Vdc? 1 ohm
e.) Was the increase the same for each 0.5-volt increment? The increase is the same

QUIZ:

1. What is the relative amplitude of the current flow in the filament of a miniature lamp the instant power is
applied in comparison with the current at full brightness?

a. high
b. low
c. the current is the same at all times.
d. It depends on the type of lamp.

Answer: C

2. Current flow through a dc circuit is 2.5 amperes and the voltage across it is 120 volts. How much power
is dissipated by the circuit?

a. 5760 watts
b. 120 watts
c. 300 watts
d. 19.2 watts

Answer: C

3. How much power is dissipated by a dc circuit with a resistance of 80 ohms if the current through the
circuit is 1.5 amperes?

a. 120 watts
b. 180 watts
c. 80 watts
d. 270 watts

Answer: A
4. If 15 volts is applied to a dc circuit which has a resistance of 75 ohms, how much power will the circuit
dissipate?

a. 1125 watts
b. 0.2 watts
c. 5 watts
d. 3 watts

Answer: A

5. The normal current through a load resistor is 2 amperes and the normal voltage drop across it is 120
volts. What is the half power point of the load power?

a. 169.68 wattts
b. 240 watts
c. 120 watts
d. 84.84 watts

Answer: C

6. When current flows through a resistance:

a. heat is produced
b. power is dissipated
c. neither a nor b
d. both a and b.

Answer: D