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I-V characteristics of circuit components

Pure metal conductors are conductors made only of metal atoms of a single chemical element such as copper.

The graph of $$I$$ vs $$V$$ for a pure metal conductor at a constant temperature shows the direct proportionality of $$I$$ and $$V$$ $$$\text{gradient}=\frac{\Delta I}{\Delta V}=\frac{1}{R}$$$

The relationship between the current and the voltage of a conductor is often referred to as its current-voltage (I-V) characteristic.

The graph goes through the origin at $$x = 0$$ and $$y= 0$$.

A pure metal conductor at constant temperature is an example of an ohmic conductor. This means that it obeys Ohm's law: its resistance is constant and the current and voltage are directly proportional.

$$I/V$$ characteristics of two typical pure metal conductors at constant temperature
$$I/V$$ characteristics of two typical pure metal conductors at constant temperature

At low voltages and currents, the gradient of the graph of $$I$$ vs $$V$$ for a filament bulb is constant. Under such conditions, it acts like a pure metal conductor.

At higher currents and voltages, the temperature of the filament increases. This means that the atoms inside the filament have more energy and vibrate more.

The more the atoms vibrate, the harder it is for electrons to pass through the filament: the increase in the current is reduced.

I-V characteristics of a filament bulb
I-V characteristics of a filament bulb

A filament bulb is an example of a non-ohmic conductor.

A filament bulb
A filament bulb
I-V characteristics of a semiconductor diode.
I-V characteristics of a semiconductor diode.

An ideal semiconductor diode allows current to flow in only one direction. It is an example of a non-ohmic conductor.

The flow of current in forward bias is very small until the potential difference across it approaches a certain value (called the diode voltage $$V_{d}$$ ). The current through it then begins to increase exponentially.

This means that the resistance decreases greatly as the voltage approaches the diode voltage.

Almost no current passes through in reverse bias, regardless of voltage. In other words, resistance is very high in the negative direction.

The resistance of a thermistor varies continuously with temperature.

Thermistors are used in temperature sensors in appliances such as air conditioners, refrigerators and water heaters.

The resistance of most thermistors decreases with increasing temperature (there are exceptions).

Thermistors for which an increase in temperature translates into a decrease in resistance operate through the following mechanism:

  • As the current and voltage across the thermistor increases, more electrical energy is converted into heat.

  • Unlike conductors, thermistors are made of a semiconductive material.

  • The reduction in resistance caused by the generation of free electrons is greater than the increase in resistance caused by the increased atomic vibrations when a semiconductor is heated. This lowers the overall resistance.

The graph of $$I$$ vs $$V$$ of a thermistor shows an increasing gradient as $$I$$ and $$V$$ increase (recall that $$\text{gradient}=\Delta I/ \Delta V=1/R$$).

I-V characteristics of a thermistor.
I-V characteristics of a thermistor.