An electric circuit is a system of a conducting path together with a source that creates potential difference in order to maintain the flow of charge. Two of the important characteristics of any circuit are the amount of charge that is flowing and the potential difference that is maintained by the power source.
The amount of charge flowing in a circuit is called the current. In SI units, current is measured in Amperes and is denoted by . the device that is used to measure the amount of current is called an ammeter.
In order for an electric current to flow in a circuit a potential difference needs to be maintained at one point in the circuit. The potential difference is the work done on a coulomb of charge when it passes between the potential difference. The potential difference is sometimes called the voltage. The SI unit for the potential difference is volt and is denoted by . The device that measures the potential difference in a circuit is called a voltmeter.
From the definition of current and potential difference, we see that these two values are related to each other. In order to show the relationship between voltage and current in a circuit we can draw current-voltage characteristic curves or simply I-V characteristics.
Resistor I-V characteristics
A resister satisfies Ohm’s law: , therefore we see that I and V are proportional to each other and the slope of the linear relationship is .
Filament lamp I-V characteristics
A filament lamp does not follow Ohm's Law. As the temperature of a filament lamp increases its resistance also increases. From this it follows that current through a filament lamp is not proportional to the voltage across it. The I-V characteristics of a filament lamp approaches to specific values as the voltage goes to large negative and positive values.
LED I-V characteristics
The ideal diode equation explains the relation between the current through a diode and the voltage across it. This equation is: . Graphing this equation would give the following I-V characteristic:
1.2 Aims and predictions
The aim of the experiment is to familiarize students with digital multi-meter to measure current, resistance and voltage and to give students the possibility of studying I-V characteristics of resistor, light filament lamp and light emitting diode (LED). Lastly, students are required to calculate the resistance of the resistor from the graphical data.
2.0 Method:
2.1 Apparatus
The apparatus consists of an adjustable power supply, ammeter, resistor and a voltmeter. Before the experiment it is necessary to assemble the apparatus and get the permission of the instructor to do the experiment.
2.2 Diagram
The circuit diagram for the experimental set-up consisting of a voltmeter, resister and an ammeter together with a power source is as follows:
For the experiment with filament lamp the resister is replaced with the filament lap and in the same way in the experiment for with the LED lamp the resister is replaced with the and LED.
2.3 Procedure
For all three experiments (resistor, filament lamp and LED) we shall measure the voltage and the current using the DMMs. Several measurements of the current and voltage (at least 8 measurements) was performed for different values of current and voltage.
3.0 Results
3.1 Tables
Resistor Measurements
Voltage V ()
Current mA
8.00
118.7
7.00
104.0
6.00
89.3
5.00
74.4
4.00
59.6
3.00
44.8
2.00
30.0
1.00
15.00
0.00
0.0
-1.00
-15.0
-2.00
-30.1
-3.00
-44.7
-4.00
-59.6
-5.00
-74.7
-6.00
-89.5
-7.00
-104.3
-8.00
-118.8
Filament Lamp Measurement
Voltage V ()
Current mA
8.00
143.4
7.00
133.4
6.00
122.7
5.00
111.1
4.00
98.3
3.00
84.1
2.00
67.8
1.00
47.6
0.00
0
-1.00
-47.9
-2.00
-67.9
-3.00
-84.2
-4.00
-98.2
-5.00
-111.4
-6.00
-122.8
-7.00
-133.5
-8.00
-143.6
LED Measurements:
Voltage V ()
Current mA
-8.00
0.0
-7.00
0.0
-6.00
0.0
-5.00
0.0
-4.00
0.0
-3.00
0.0
-2.00
0.0
-1.00
0.0
0.00
0.0
1.73
0.1
1.80
0.5
1.84
1.1
1.88
2.0
1.90
2.5
1.95
4.5
2.00
6.1
2.05
8.8
2.10
11.3
2.20
15.7
2.30
20.5
2.50
29.6
3.2 Graphs (with minor gridlines, error bars etc.)
I-V Characteristics of Resistor
I-V Characteristic of Filament Lamp
I-V Characteristic Of LED
4.0 Calculations (with error analysis)
Remembering that a resistor follows ohm’s law: we see that the slope of the I-V characteristic for the resistor is the inverse of the value of R that we need to calculate. From our data we then conclude the following:
Including the possible error in the measurement we have
5.0 Discussion
We graphed the I-V characteristics of resistor, LED and filament lamp. In all three cases the I-V curves that are based on the experimental data exhibit the same type of relationship that is predicted from the theoretical considerations. From the understand of the relationship between the current and the voltage in a resistor we calculated the resistance in the resistor.
The degree of uncertainty for the ammeter is taken to be this is because the device and our reading cannot be off by more than one unit of the measurement of the device which is mA. In the same way, the degree of measurement of the voltmeter is taken to be .
6.0 Conclusion
The experimental data agrees with the theoretical predictions within the degree of uncertainty in all three cases. The resistance of the resistor coincides with the resistance written on the resistor within the measured degree of uncertainty.
