Ohm's law

As you should remember from GCSE voltage, current and resistance are related via ohm's law:

V = IR

This is fundamental to many questions relating to voltage, current or resistance and therefore you must remember this equation for the exam. From the equation it is possible to deduce that a greater the resistance will provide a smaller current, assuming that the potential difference remains constant.

Definitions:
Resistance The ratio of potential difference across a resistor to the current flowing through it.
Ohm For many materials when at a constant temperature the current across the sample is proportional to the voltage across it.

Notice how with the definition of the ohm it states that 'when at a constant temperature'. This is very important as you'll see in the next section.


Ohmic and non-ohmic resistors
For an ohmic resistor (one which follows ohms law as expected) the following graph is expected:

Non-ohmic resistors are resistors, which don't obey ohms law. Metal conductors will obey ohms law as long as their temperature remains constant, however this is difficult to achieve in the lab and therefore they too behave as non-ohmic resistors.

As the current is increased the electrons collide with the atoms in the material with greater kinetic energy resulting in energy being transferred as heat. This causes the atoms to vibrate more, increasing the resistance. This trend is more apparent in a filament lamp which gets hotter as it is used.

Semi-conducting diodes too behave as non-ohmic resistors but in an entirely different way. They only allow current to flow in one direction when the voltage is above a certain value, if it is below that value no current will flow.

 

Environmental dependence
A number of other electrical components experience environmental dependence.

· NTC (negative temperature coefficient) thermistors are resistors that are depended on the temperature. An increase in temperature gives a lower resistance. It must also be noted that thermistors resistance may therefore decrease slightly as they're used, due to their temperature rising similar to that of a filament lamp or a conductor.

· LDRs (light dependent resistors) give a low resistance in the light and a high resistance in the dark. These resistors depend on the photoelectric effect which is discussed in more detail later in the module.