I-V Characteristics

In a nutshell

The term I-V characteristics refers to graphs that show the relationship between current and potential difference. The LDR is a resistor whose resistance depends on the intensity of the light hitting it. The thermistor is a component whose resistance depends on temperature. The diode only allows current to flow in one direction.

I-V characteristics

The relationship between current and potential difference can be very helpful to understand how a component works. This can found by investigating the I-V characteristics of a component or more simply put, by plotting a graph of current against potential difference. 

For ohmic conductors (components that follow Ohm's law) the I-V graph looks like the following at constant temperature:

The negative current and voltage occur when the component is connected in reverse polarity (flipping positive and negative).

To find this graph for a component you can use a standard test circuit. Place the component into the circuit and take readings on the ammeter and voltmeter at different current intervals. Repeat the same procedure but in reverse polarity and plot the graph with your measurements.

An I-V graph can also be used to find the resistance of a component as the gradient is $$\dfrac{1}{R}$$.​

Filament lamp

A filament lamp is a component that emits light when current passes through it. It has a symbol:

The I-V graph of the filament lamp looks like the following:

This graph shows no straight line meaning that the filament lamp is a non ohmic conductor (does not follow Ohm's law). This is because as current passes through the lamp, the temperature increases meaning that resistance increases and hence produces a drop in current. The higher the voltage the faster the temperature increases which leads to the graph leveling off.

LDR

LDR stands for light-dependent resistor. This is a type of resistor whose resistance depends on the intensity of the light hitting it. LDRs are typically made by semiconducting materials in which more charge carriers are released with an increasing light intensity hence, the darker it is the higher the resistance and the brighter it is the lower the resistance. 

Below are the symbol for the LDR along with a graph showing the relationship between resistance and light intensity:

The I-V graph for an LDR looks like the following:

As you can see the LDR is an ohmic conductor whose gradient depends on light intensity; the greater the light intensity, the lower the resistance and the greater the gradient since for an I-V graph the gradient is $$\dfrac{1}{R}$$.​

Thermistor

When increasing the temperature of a component their resistance also increases. This is not the case for components made of semiconductors with a negative temperature coefficient. Such components are called NTC thermistors and as the temperature increases their resistance decreases due to more charge carriers being released. The change in resistance is very dramatic which makes them ideal to monitor temperature.

Below is the symbol for the thermistor along with a graph showing the relationship between resistance and temperature:

The I-V graph for a thermistor is as follows:

Since a thermistor produces no straight lines it is a non ohmic conductor. The reason for this is that when current passes through it it heats it up releasing more charge carriers, decreasing the resistance and therefore making the graph steeper.

Note: At this level you can always assume a thermistor is an NTC thermistor.

Diode

Diodes are components that allow current to flow only in one direction. They are made from semiconductors and once a specific voltage called the threshold voltage is reached (usually $$0.6\ V$$​) and they are connected in the right direction, they become highly conductive. When in the direction that lets current flow they are said to be in forward bias, when in the opposite direction they are said to be in reverse bias. LEDs (light-emitting diodes) are a type of diode that lights up when current passes through it. 

The symbol for the diode is:

The property of the diode illustrated more clearly:

The I-V graph for the diode is as follows:

When in forward bias, the current is low until it reaches the threshold voltage after which the current increases exponentially. When in reverse bias, is always low meaning that the resistance is always very high.