How does the load resistance define the current
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Definition of electrical resistance
What is a resistance anyway? In everyday parlance we know the resistance under the wrongly used generic term "consumer". But a resistor consumes neither electricity nor energy, energy is converted in it. In technology in particular, resistors are indispensable because they are needed in every electrical circuit.
In the section The electric current in a simple circuit it was already mentioned that the electric current depends on the applied voltage and the resistance. In the following, we want to get to know exactly how these quantities relate to one another.
With the following interactive screen experiment, we investigate the change in current strength with increasing voltage on a resistor. Battery cells, which are connected in series, serve as the voltage source. As we got to know in the section Connecting two batteries in series, double, triple, ... n-fold voltages are obtained by connecting two, three, ... n batteries of the same type in series. The value pairs of voltage and current are entered in the graph at the same time as the experiment. You can run the experiment step by step by pressing the slider.
The resistor is a coiled iron wire.
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One can see in the graph that the value pairs are not on a straight line, as one might expect. Rather, you can see that with increasing voltage, the current strength only increases more slowly than with low voltages. This is because the iron wire is heated by the applied voltage and current.
We know from the section The Thermal Effect of Electric Current that a higher current strength causes a greater temperature increase in the wire. So, by gradually increasing the voltage, the current strength and thus the number of electrons that flow through a conductor cross-section per second also increase. Thus, the wire is heated to a greater extent. As a result, the atoms inside the wire move faster, i.e. their oscillation increases around the equilibrium position. The movement of the electrons flowing through the wire is increasingly inhibited due to the interaction with the positive atoms. As a result, there is not such a large increase in current.
In order to be able to make quantitative statements about the relationship between voltage and current strength for an electrical conductor, the definition of resistance is introduced:
- The electrical resistance
- The respective resistance of a conductor is calculated from the quotient of the applied voltage and the current strength in the conductor. The unit of electrical resistance is the ohm, by Georg Simon Ohm, where applies
With this definition, the above situation can be expressed as follows: With increasing voltage and smaller increase in current strength, the resistance increases because of. This is exactly the case with the above wire, you increase the voltage and the current strength increases more slowly.
You might ask yourself: weren't the wires that we always used before also resistors? Strictly speaking, yes, which is why when investigating voltage and current strength in the electrical circuit, ideally conductive wires that are free of resistance were always spoken of. However, in most experiments and also in everyday life, copper lines are used; these have a much lower resistance compared to the iron wire above. Even when compared to the resistance of a connected consumer, its resistance can then be neglected in most cases.
This shows that the resistance must be material-dependent, which we will talk about later.
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