IB Physics Lab Report 5
Lukas Bosko 000771-008
Introduction
Resistance is one of the most important characteristics in any electric circuit. Resistors are widely used in any
electronic device. There are many different types of resistors, but the most important property is their
measured resistance.
Problem statement
What is the resistance of an unknown resistor?
Hypothesis
Value of resistance changes value of other variables in the circuit. According to Ohm’s law, voltage is directly
proportional to the current (U=R.I). Changing terminal voltage changes also voltage and current in circuit.
Measuring these we can obtain data from which value of constant resistance of unknown resistor can be
calculated. Final value can be as well obtained from graph, where R is slope of the graph of linear trendline. In
conclusion will be compared measured value to official value of producer.
Independent variables:
Resistance of rheostat
Dependent variables:
Current
Voltage
Constant:
Resistance of resistor
Materials:
An unknown resistor, rheostat, a source of a voltage, ammether, voltmeter, connecting wires
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IB Physics Lab Report 5
Lukas Bosko 000771-008
Procedures
1. Prepare electric circuit designed on the scheme.
2. Swich on the variable power supply and try to obtain measurable values on both ammeter and
voltmeter.
3. Change value of terminal voltage.
4. Measure value of current from ammeter and voltage from voltmeter.
5. Repeat steps 3 and 4. For every measurement write down measured values.
6. Calculate resistance from Ohm’s law.
7. Create a graph from measured values.
8. Using technology make trendline and find the resistance of resistor as an slope of it.
9. Compare obtained value with value from producer.
10. Evaluate the experiment and its errors.
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IB Physics Lab Report 5
Lukas Bosko 000771-008
Data processing
1
2
3
4
5
6
7
8
9
10
Current
A
0.24
0.28
0.34
0.40
0.52
0.22
0.20
0.16
0.60
0.46
Voltage
V
3.40
4.00
4.60
5.40
7.00
3.00
2.60
2.20
7.60
6.00
average
Resistance
Ω
14.17
14.29
13.52
13.50
13.46
13.64
13.00
13.75
12.67
13.04
13.50
Error
Ω
0.67
0.79
0.02
0.00
0.04
0.14
0.50
0.25
0.83
0.46
V
I
sample calculation
R=
R=
3.40 V
= 14.166 Ω ≅ 14.17 Ω
0.24 A
Average resistance RA = 13.50 Ω
Average error RE = 0.37 Ω
Calculated resistance
RC = (13.50 ± 0.37) Ω
Relative error RE / RA = 2.7%
Current vs. Voltage
9.00
y = 13.273x
8.00
7.00
Voltage [V]
6.00
5.00
4.00
3.00
2.00
1.00
0.00
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
Current [A]
From linear function V = R. I was made trendline (for y = 0 equals x = 0) with equation y = 13.273 x. Value of
resistance of the unknown resistor is thus from graph RG = 13.273 Ω.
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IB Physics Lab Report 5
Lukas Bosko 000771-008
Conclusion and evaluation
From measured data were obtained two different values of resistance. Firstly, resistance was just simply
calculated from formula V = R.I for every measured value. Error was also calculated and thus was obtained
value RC = (13.50 ± 0.37) Ω.
Graph of volt-ampere characteristics shown, that voltage is directly proportional to the current in the circuit
(Ohm’s law). Constant in graph V=R.I is this time resistance of our resistor, which was calculated by Microsoft
Excel from trendline(zero voltage -> zero current and therefore for trend line y(0)=0)graph as RG = 13.273 Ω.
Third value (written down on the resistor by producer and shown to us after the experiments) is 12 Ω. This
can be assumed as the most accurate one.
The lab report showed validity of Ohm’s law and provided measurement for resistance of resistor. Final
result obtained is not in range of third value, which is caused by our experimental method and our equipment.
Measurement was done precisely, which supports small value of relative error. Because only one device was
used, it can not be stated whether it was measuring properly. Devices were old and probably inaccurate. For
two given values of voltage could be obtained more than one value of different current in different time.
Resistance of resistor is not the only resistance in the circuit, but in our case was considered as the greatest
comparing it to the internal resistance of the source and resistance of the wires.
Even that our experimental data were not such close to expected result, we were able to explain the result
and identify problems of our experiment. The task was thus fulfulled.
Limitations and weaknesses in this lab were:
-
Only one method was used even though results were not accurate.
-
Method of measuring voltage and current – ammeter and voltmeter were too old and we could not
state whether they are working properly
-
Resistance of a wires as well as internal resistance of a source (which actually caused inaccuracy of the
final result) were neglected
-
Human error in reading the data from ammeter and voltmeter
Suggestions to improve this lab:
-
Try to use 2 resistors, from which only one will be unknown
-
Use digital ammeter and voltmeter with greater preciseness
-
Measure internal resistance of source and try to estimate value of resistance of wires
-
Use more different and even harder schemes – resistance of resistor is still the same – and obtain
various other results
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