Book Reference : Pages To understand what a capacitor is 2.To understand their applications 3.To understand how we define capacitance

A capacitor is a passive electronic device which stores charge. They consist of two or more conducting plates separated by an insulating material. +Q-Q Capacitor Symbol electrons When connected to a battery, the plate connected to the positive side of the battery loses electrons to the battery and the other plate gains electrons from the battery. We say that the capacitor has a charge Q when the positive plate is at +Q and the other plate is at -Q

We will see that capacitors effectively block direct current (DC) but allow alternating current (AC) to flow Smoothing circuits : Noise on power supply rails can be removed by conducting away to ground This  processor on a computer motherboard wants nice smooth stable power supply lines. The high speed clock (several GHz) cause unwanted noise on these power lines. A suitable capacitor connected between the power and ground will conduct away the AC noise and leave the DC supply unchanged

Store power : A large capacitor can be thought of as a small battery. It can store power for backup purposes, (for example in devices where you need to change batteries but not lose configuration). They can also suddenly dump a very large amount of energy in a short time (camera flash gun)

Tuned Circuits : In traditional radios and TVs the thing you twiddle to change station is a variable capacitor. This makes the tuned circuit sensitive to only the station you want Filter Circuits : Remove unwanted frequencies Timing and pulse producing circuits : on and off after a preset delay, on/off repeatedly

Definition : The capacitance of a capacitor is the charge stored per unit potential difference (voltage) C = Q/V The unit of capacitance is the Farad and is equal to 1 Coulomb per volt Note a Farad is a very large unit, in everyday life capacitors are found with values of pF, nF and  F (so take care with calculations)

Definition : Current : An electric current measured in Amperes is defined as the amount charge. (Coulombs) flowing per second (I = Q/t) A V Switch Variable Resistor Microammeter High  Voltmeter Close the switch, record the voltage at given times and use the variable resistor to keep the current constant (e.g. 15  A [or use data logger]

For a constant current of 15  A find the charge in the table below: Time /sPD / VQ/C Plot a graph of Charge Q against Potential Difference V What can we find from the graph?

Graph of Charge Q against Potential difference V The gradient is the value of capacitance (1015  F)

A capacitor is charged by means of a constant current of 0.5  A to a P.D. 5.0V in 55s. Calculate: The charge stored The value of the capacitor A 22  F Capacitor is charged by means of a constant current of 2.5  A to a PD of 12.0V. Calculate: The charge stored The time taken

A capacitor is charged by means of a constant current of 24  A to a P.D. 4.2V in 38s. The capacitor is then charged from 4.2V by means of a constant current of 14  A in 50s Calculate: Charge stored at a pd of 4.2v The value of the capacitor The extra charged stored at a current of 14  A The new PD after the extra charged was stored