 Picture 5.11. Table of results for RC discharge 
 Picture 5.12. Exponential decay of charge on a capacitor.
    The discharge curve (picture 5.12) is exponential. Let's see what this means.      

 Picture 5.13. Exponential changes.
   Exponential curves    We can always recognise an exponential curve because the yaxis changes by an equal proportion for equal divisions on the xaxis. For example, bacterial growth is often exponential – it doubles every 3 seconds. Radioactive decay is exponential – the activity of carbon14, for example, halves every 5700 years. Now we can see that our capacitor discharging through a resistor is exponential – it drops to 87.5% of its starting value every 10 ms.      Does it reach zero?    Although, in theory, exponential curves never reach the origin, in reality a capacitor will always discharge. Here are two views as to why:  The engineer: it will eventually get so small that we can’t measure it.
 The physicist: it will eventually get down to a small number of individual electrons on the negative plate. These will, randomly, leave the plate.
Even so, we can’t measure the time it takes to fully discharge. So we have to measure how long it takes to reach a certain proportion (rather like half lives for radioactive isotopes). We’ll see how on the next page.     
 