graphs

GRAPHS

Graphs are one way of showing the relationship between two variables (things that can change in value).

The graph above shows how the brightness of the sun is related to the time of day.

From the start at the bottom left hand corner until just before 6 am brightness is zero. (It is dark).

Brightness increases as time passes being at maximum about 1 pm when the sun is highest in the sky.

Brightness then falls becoming dark at about 9 pm when the sun sets.

Now look at the following graph.

It shows the voltage of the batteries in my bicycle lights.

After four weeks it has fallen to about 3/4 of a volt and my lights are noticably dimmer.

The current through a resistor can be calculated if we know the value of the resistor and the current across it from I = V/R

However we can use a graph to do this as shown below.

We can see that if the voltage is 10 volts and the resistance is 2 ohms then the current is 5 amps.

For any voltage between 0 and 20 we can find the corresponding current.

This works only for a 2 ohm resistor.

For other values of resistor we would need a different sloping line.

The point where both values are zero is called the ORIGIN.

The last diagram demonstrates how we can show negative currents and voltages as well as positive ones using a graph.

It is divided into four QUADRANTS.

If we increase the positive voltage the current remains at zero until the voltage is just under a volt.

After this the current increases rapidly.

If we reverse the voltage and increase it from zero, then the current stays at zero until the reverse voltage reaches 6 volts.

The current then increases extremely rapidly in the reverse direction.

These are the characteristics of the diode displayed in graphical form.