Characteristic Curves
Current-voltage graphs are used to show how the current through a component varies with the voltage you put across it. It is a physicist's way to see at a glace how a component behaves when a particular range of potential differences are applied across it.
These graphs are called Characteristic Curves.
To find the characteristic for a component you vary the potential difference across it (so p.d. or voltage is the independent variable and it plotted on the x-axis) and note down the current that flows for each voltage (the dependent variable - plotted on the y axis).
This characteristic is described by Ohm's Law - The ratio of the potential difference across the ends of a current carrying conductor to the current flowing though it is constant providing the temperature is constant. (Click here for an interactive experiment)
For details on an experiment to determine the characteristic curve for a filament lamp: click here
You should know how to sketch these curves and be able to interpret them, explaining how they show that:
- The current through an ohmic conductor (e.g. a wire) (at constant temperature) is proportional to the voltage across the resistor. This is known as Ohm's Law.
- The resistance of a filament lamp increases as the temperature of the filament increases.
- The current through a diode effectively only flows in one direction only. It's resistance is very low as long as it has a potential difference of more than 0.6 volts across it. The diode has a very high resistance in the reverse direction therefore only a tiny current flows.Click here for more detail.
- The resistance of a light dependent resistor decreases as the light intensity increases.
- The resistance of a thermistor decreases as the temperature increases. (There are some thermistors which behave in the opposite way to this but all of your questions will be set on this version).
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