Capacitors

the capacitor is an electrical device which has the capacity to hold and store an electrical charge. it is made of metallised plates layered with an insulating substance, rolled up tightly, and fitted inside a metal cylinder. A terminal at one end of the capacitor is connected to the moving contact, and the metal cylinder, acting as the other terminal of the capacitor, is connected to earth. The capacitor holds an electrical charge and releases its stored energy when connected in a circuit. The capacitor is connected across contacts of the contact breaker, and provides an alternative path for the current and accepts and stores and electrical charge from the primary coil until conditions are suitable for releasing it back into the circuit. This stored energy would otherwise cause destructive arcing across the contacts. The current thus ceases to flow, and the capacitor immediately discharges itself through the primary coil in the opposite direction to the flow of induced current. This reverses the polarity of the coil and increases the rate at which the field collapses. this in turn increases the voltage induced in the secondary winding. A capacitor takes time to charge, they charge faster when first connected to a power source and start to slow down as their charge builds up. The size of a capacitors capacitance is measured in Farads (F).

To calculate how fast a capacitor should charge you use the formula
                                           Resistance X Capacitance X 5 = Time required to charge
                                                       (R x C x 5= T)

In one of our practical lessons we had to build a circuit with
                                  1 x resistor
                                  1 x capacitor
                                  1 x bridging wire
                                  1 x voltage source (battery or power supply)

After building the circuit we had to time the capacitor to see how long high it would charge over the course of 3minutes (180secs). Recording the charge every 10seconds on a table (which can be found in my 'capacitor circuits workbook') We then had to graph the results. The results showed that during the first 90seconds the charging was going quite fast, shooting up from a starting voltage of 3.20v to 10.8v, the second half (90seconds) the charging slowed down because the electrical store was getting taken up. Kind of like how when you blow up a balloon and if you blow it up too much it gets harder to blow up, harder to store air. This made the second 90seconds only rise 0.8v as opposed to the 7v in the first 90 seconds. This graph can also be found in my capacitor circuits workbook.