Download TEP Ohm`s law with Cobra4 Mobile

Ohm’s law with Cobra4 Mobile-Link
TEP
Related Topics
Ohm’s law, Resistivity, Contact resistance, Conductivity, Power and Work
Principle
The relation between voltage and current is measured for different resistors. The resistance is the derivative of the voltage with respect to current and is measured in dependance on current. In case of an
incandescent lamp the temperature rise with higher current leads to a considerable higher resistance.
Equipment
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1
1
1
1
1
1
1
1
2
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Cobra4 Mobile-Link incl. Zubehör
Cobra4 Sensor-Unit Energy
Digital Function Generator, USB, incl. Cobra4 Software
Connection box
Resistor 100 Ω, 1 W
Resistor 220 Ω, 1 W
Resistor 330 Ω, 1 W
Lamp socket E10
Filament lamps, 12 V/0.1 A, E10, 10 pcs.
Connecting cord, l = 500 mm, red
Connecting cord, l = 500 mm, blue
12620-10
12656-00
13654-99
06030-23
39104-63
39104-64
39104-13
17049-00
07505-03
07631-01
07631-04
Additionally required
1 PC with USB-interface, Windows XP or higher
Fig. 1:
Experimental set-up.
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Ohm’s law with Cobra4 Mobile-Link
TEP
Set up and procedure
The experimental set-up is shown in Fig. 1.
Connect the Cobra4 Mobile-Link to the USB-interface of the computer and plug the Cobra4 Sensor-Unit
Energy onto the Mobile-Link. Connect the Digital Function Generator to the Sensor-Unit and the USBinterface of the computer. Load “Ohm’s law” experiment. (“Experiment”  “Open experiment”). All presettings that are necessary for data acquisition are now carried out.
Click on
in the icon strip to start measurement. A sample measurement is given in Fig. 2.
Fig. 2:
Current curve of an incandescent bulb.
2. Resistance measurement
Now put a resistor into the place where the bulb had been. Record a curve again. You may put several
curves into one diagram with “Measurement“  “Assume channel...“. The curves may look like this (see
Figs. 3 and 4):
Fig. 3:
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Current-voltage diagram of commercial resistors
Fig. 4:
The power dissipated as heat in the resistors
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Ohm’s law with Cobra4 Mobile-Link
TEP
Theory and evaluation
1. Incandescent bulb
With the “Regression“ tool of the “measure“ software it is possible to determine the resistance of the bulb
at different applied voltages. E.g. around zero Volt the slope
of a slowly measured curve is 90 mA/V, thus is the resistance of the bulb 11.1 Ohm at room temperature. A check
with an ohmmeter yields 11.2 Ohm. Around 9 Volt is the
slope about 5.6 mA/V and that yields a resistance of 179
Ohm, sixteen times the resistance at room temperature.
The curve is not symmetrical because of the heat capacity of
the bulb. When increasing the voltage, a part of the dissipated energy is necessary to warm the bulb and so the temperature is not the same as with the same voltage when decreasing the voltage. If the curve is recorded very slow, the
state of thermal equilibrium is better approximated and the
curve gets more symmetrical (Fig. 6). You can do so by setting the “Pause time“ in the function generator’s parameters
to some seconds (Fig. 5). The rise of resistance with temperature in metals is due to electron scattering at phonons,
the quanta of crystal lattice vibrations. The rise can only be
considered to be linear for small temperature changes.
Fig. 5: Function Generator settings.
Fig. 6:
Curves recorded with different speed.
2. Resistors
The resistors for industrial use are usually metal film resistors or carbon resistors. The thermal coefficient
of them is usually 0.025%/K or 0.0125%/K. The allowed power dissipation specified for them is such that
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TEP
Ohm’s law with Cobra4 Mobile-Link
they don’t heat up much. Here no deviation from linearity can be seen – the resistance stays the same
over all the range of the meaurement and they follow Ohm’s law,
with resistance R, current I and voltage U.
The slopes read out with the “Regression“ function are for the
330 Ohm resistor: 3.04 mA/V
220 Ohm resistor: 4.65 mA/V
100 Ohm resistor: 10.14 mA/V
and this yields 329 Ohm, 215 Ohm and 98.6 Ohm, respectively. This corresponds well with the manufacturer’s data and the 5% tolerance specification for them.
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