S11052005_PH103-Experiment 2
PH103: Quantum and Electrical Physics
Laboratory Report #: 1
Title: Experiment 2- Review of Electrical
Circuit
Name: Ellery Ratu
ID Number: s11052005
Group 6
Lab Member: Lepeka Vaokakala Aslyne Napaa
ID: S1122282
S11052005_PH103-Experiment 2
A. Statement of Objective:
To build some simple DC circuits, and to measure currents, voltages and resistances.
B. Introduction:
The DC circuits produce small electric currents. So, the small electric currents are
detected by the multimeter. In a multimeter it has a galvanometer in it. This is a
sensitive instrument used for measuring and detecting small electric currents in a
circuit and it is function using principle of electromagnetic induction.
Thus, when an electric current flows through the coil, it generates a magnetic field
around itself. This magnetic field interacts with the external magnetic field created
by the permanent magnet. The interaction results in a torque being exerted on the
coil, causing it to rotate or deflect.
However, a galvanometer can be used as an ammeter to measure current in the
resistance and also as a voltmeter to measure the voltage in the resistance. When it
is used as an ammeter, a resistor called shunt is placed parallel with the
galvanometer in order to reduce the sensitivity of the galvanometer to obtain
accurate readings. And when it is used as a voltmeter a large series resistor is added
in order to restrict the flow of current goes through the circuit. So, a multimeter that
was used consist of a galvanometer, shunt and series resistors which can be used as
either an ammeter or voltmeter when it was switched in or out.
Moreover, when the voltage is applied across a circuit, it drives the current through
the circuit. The resistance in the circuit opposes the flow of the current. The
relationship between the voltage, current and resistance is called the ohm’s law.
The formula is V=IR.
Where, V is the voltage in volts (V), I is the current in amperes (A) and R is the
resistance in ohms (Ω).
Ohm’s law stated that the current in a circuit is directly proportional to the voltage
across it and is inversely proportional to the resistance.
Note: 1A =1000mA.
C. List of Equipment Used (Apparatus):
•
•
•
•
•
•
Multimeter,
0-100 mA milliameter,
Resistance box,
2 carbon resistors,
2.5 volt dry cell,
1 switch.
S11052005_PH103-Experiment 2
D. Procedure:
Part 1:

Measured the potential difference of the dry cell.
Part 2:
 A simple circuit was contracted. The positive terminal of the dry cell was
connected to the positive terminal of the milliameter. The negative terminal
of the milliameter was connected to the resistance box. And from the
resistance box it connected to the negative terminal of the dry cell to form a
complete circuit. The circuit was connected as shown below.
Part 3:
From the circuit that was constructed above, the voltage drops across the resistance
box and across the dry cell were measured.
Part 4:
The resistance of each of the carbon resistor were measured using a multimeter. Then
compared those measurements values with their stated resistance and tolerance using
the color chart.
Part 5:
A simple circuit was contracted by using two carbon resistors and the resistance was set
to 80Ω. The resistance box was connected parallel with resistor 1 (R1). The currents and
the voltages reading were taken.
Part 6:
This is a substitution method, so the circuit was set up to determine the value of R1. The
resistance box was used to determine the current same as the current across R1 then
the both have the same resistance.
S11052005_PH103-Experiment 2
E. Data: Experiment Results
Part 1:
Table 1. The table shows the voltage (V) value of the dry cell and its measured voltage (V)
value.
Component
Dry Cell Voltage Rating
Measured Voltage
Value
1.5 V
1.515V
Part 2:
Table 2. The table below shows the measured current (I) value in the circuit.
Component
Measured Current in the Circuit
Value
15.5mA
Part 3:
Table 3. The table below shows the voltage (V) drop across the Resistance Box and the Dry
cell.
Component
Voltage drop across Resistance Box
Voltage drop across Dry Cell
Value
1.362V
1.3V
Part 4:
Table 4.1. The table shows the resistance (Ω) value of the two resistors used.
Resistor
R2=
R1=
Measured Resistance
149.1 Ω
237.9 Ω
Table 4.2. The table shows resistance of the two resistors using the Resistor Color Code
Chart.
Decode Value using Resistor Color Code Chart
Color Bands
Chart
Value
R2= Brown, Green, Brown, Gold 150 Ohms
R1= Red, Yellow, Brown, Gold
240 Ohms
Tolerance
± 5%
± 5%
S11052005_PH103-Experiment 2
Part 5:
Table 5. The table below shows the voltage (V) and the current(C) across R1 and R2 in a
circuit that has a dry cell (1.5V) when the Resistance Box was set to 80Ω.
Component
R1
R2
Rb
Voltage(V)
0.2744V
1.192 V
0.2728V
Current(mA)
1.82mA
5mA
3.2mA
Part 6: Substitution method.
Table 6: The table shows the resistance value and current across the R1 in the circuit and the
expected value in the Resistance Box in a substitution method.
Component
R1
Resistance Box
Resistance(Ω)
237.9
238
Current(mA)
6.22
6.22
F. Analysis of the Results
Part 1.
The measured voltage of a dry cell is 1.515V which is very close to its stated
voltage1.5V.
Part 2.
By using the resistance box (80Ω), the milliammeter and the dry cell (1.5V) the
current across the circuit measured was 15.5mA.
Ohm’s law calculation:
V = IR,
= (15.5x10-3A) (80Ω)
= 1.24V
So, the voltage of 1.24V is the experimental value is close to the theoretical voltage
value of the dry cell which is 1.5V. This shows that circuit did obey the ohm’s law.
Part 3.
The voltage drop across the resistance box was 1.362V and the voltage drop across
the dry cell was 1.3V.
The result shows that the voltage drop across the resistance box was slightly
different from the voltage drop across the dry cell.
S11052005_PH103-Experiment 2
This is due to the properties of electrical components and the principle of electrical
circuits such that the voltage across the resistance box depends on the current that
flows through it.
While the voltage across the dry cell is its open-circuit voltage and can be affected
by the internal resistance and load conditions.
When the voltmeter was added to the circuit the current does not change.
The diagram below showed how the two voltages were measured.
Ohm’s law calculation:
V=IR
So, I =V÷R
= 1.3V ÷ 80Ω
= 0.01625 A or 16.25mA.
Therefore, 16.25mA is close to the measured current value which is 15.5mA. This
shows that the Ohm’s law was obeyed.
Part 4.
The measured resistance value of resistor1 (R1) and resistor2 (R2) are 237.9 (Ω) and
149.1 (Ω) respectively.
The decode value for resistor1 (R1) was 150 (Ω) with a tolerance value of ±5%, so the
measured resistance value was still on the right range.
Likewise, for resistor2 (R2) the decode resistance value was 240 with a tolerance of
±5%, thus, the measured resistance value was still in the right range.
This shows that the measurement in this part of the experiment is 100% accurate.
Part 5.
The measured current across the resistor (R1) is 1.82mA, current across resistor (R2)
is 5mA and across the resistance box is 3.2mA. The voltage across each component
are 0.2744V, 1.192V and 0.2728 respectively.

The sum of the current across through the parallel resistors:
R1 + Rb = 1.82mA + 3.2mA = 5.02mA

The current flow through the dry cell is equal to R2 which is 5mA.
Thus, the sum of the currents through the parallel is equal to the current through
the dry cell. This indicates that the ohm’s law was obeyed.
S11052005_PH103-Experiment 2
Also, the VAC = VAB + VBC
= 1.192V + 0.2744V
= 1.4664V
The measured voltage value of 1.4664V is very close to 1.5V of the dry cell.
Part 6.
In the Substitution Method, R1 =237.9 Ω and the current in R1 = 6.22mA.
Thus the expected value of Rb, should be the same as R1 because they both have the
same current = 6.22mA.
Therefore, Rb = 238Ω
7. The carbon resistor of 0.5watt and dry cell of 1.5V. To find current power ÷
voltage. So, I = P ÷ V
= 0.5watt ÷ 1.5V
= 0.333A or 333.3mA.
G. Discussion:
The results obtained from the experiment shows that in part 1, the measured
voltage of the dry cell was 1.515V and it is slightly closed to the voltage of the dry
cell which is 1.5V. The slightly difference here was due to some component in the
measuring device.
In part 2, the setup of the simple circuit which has a resistance of 80 ohms
has 15.5mA current passed through. Thus, when using ohm’s law the voltage was
1.24V. This is less than the voltage of the dry cell measured which is 1.515V. The
reason for this decrease is that when a circuit experience load, in this case the
resistance of 80 ohms the voltage drop across it can be substantial and this cause the
lower of the voltage measured where compared with dry cell voltage. So, ohm’s law
was proved to be true.
Part 3 of the experiment shows that the voltage measured across the
resistance was 1.362V and the voltage across the dry cell was 1.3V. The difference
here tells that due to the properties of electrical components and the principle of
electrical circuits such that the voltage across the resistance box depends on the
flow of current through it and the voltage across the dry cell is its open-circuit
voltage. Therefore, since the voltage drop across the resistance box is determined by
the product of current and resistance, this the reason of it having slightly higher
value than the dry cell voltage. And the adding of the voltmeter does not change.
This is due to the designed of the voltmeter to measure voltage, and it is typically
connected in parallel with the component for which its voltage is to be measured.
In part 4, the two resistors R1 and R2 resistance measured were 149.1Ω and
237.9Ω respectively. When compared to their decode reading from the color chat,
the two resistors were in the right range. For instance, decode showed that R1 is 150
with a tolerance of + or – 5%. So the right value range should be between 157.5Ω to
142.5Ω which 149.1 was within the range. Likewise, for R2 decode showed 240 with
S11052005_PH103-Experiment 2
a tolerance of + or – 5%. So the right value range should be between 252Ω to 228Ω
and 237.4 was within that range.
In part 5, the circuit has R1 and resistance box are parallel. The current across
R1 was 1.82mA and it has a voltage of 0.2744V. R2 current was 5mA and voltage of
1.192V. Resistance box current was 3.4mA and voltage of 1.192V. By calculating the
sum of the resistor and the resistance box that were connected parallel, it is the
same as the current across R2. Also VAC = VAB + VBC. Thus the ohm’s law was obeyed.
In part 6, the substitution method used to measure the value of the resistor
by using the same current for both the resistor and the resistance box. And when
they got the same current value it showed that the resistor and the resistance box
both have the same resistance. Therefore, the expected value of the resistance box
should exactly 238Ω. This is because they both have the same current across them
and which is 6.22mA.
QUESTIONS:
1. From the question 240V indicates that the voltage at which the bulb is designed to
optimally operates and 30W is the power that the bulb consume when 240V
supplied to it.
Calculating the flow of current through the filament when the bulb was connected to
240 volts.
Ohm’s law: V =I x R
First: calculate the current (I)
Use power (P) = I x V
I = P ÷V = 30W ÷ 240V = 0.125A
I = 0.125 A or 125mA
So, the current that flows through the filament of the bulb when it was connected
to 240 V is 0.125 amperes (A).
Next, calculate the resistance (R) of the filament:
Now the current was calculated; use ohm’s law.
R = V ÷ I = 240V/0.125A = 1920 Ω
R = 1920Ω is the filaments resistance.
2. To reduce the current through the filament to 0.1A, connect a resistor of 480 Ω in
series with the bulb.
S11052005_PH103-Experiment 2
H. Conclusion:
To conclude, by constructing some simple DC circuit, it is possible to measure the
current that flows through the resistors and the resistance box in the circuit. Also
they allowed voltages across the resistors, resistance box and the dry cell to be
measured and the measurement of resistance using the multimeter. However, all of
the calculations and the unknown values were obtain by using ohm’s law. Therefore,
it is clear that the relationship between the current, voltage and resistance was
define well by ohm’s law. That is current is directly proportional to voltage and
inversely proportional to the resistance.