Capacitance

• Capacitance is the ability of an object or surface to store an electrical charge.
• Capacitance is a measure of the capacity of the electrical storage capability of the object.
• Capacitors are typically arranged in a parallel plate configuration and are defined in terms of charge storage: Q = magnitude of charge stored on each plate, V = voltage applied to the plates.
• A capacitor stores energy.
• A capacitor stores equal amounts of opposite charge on its two plates.
• The electrical work done in adding a small amount of charge (dQ) to the capacitor is given by dW = dQ x V.
• The total work done is the sum of all such strips and is the area under the line.
• The voltage, current, and charge all decay exponentially during the capacitor discharge.
• We can note the voltage and current at time intervals and plot the data, which gives us the exponential graph.
• The shape of the graph is unaffected by the voltage.
• The half life of the decay is independent of the voltage.
• The current follows exactly the same pattern as I = V/R.
• The charge is represented by the voltage, as Q = CV.
• The graph is asymptotic, i.e. in theory the capacitor does not completely discharge.
• For voltage and current, the equation becomes: V= V0 e –t/RC, I = I0 e –t/RC.
• The product RC (capacitance × resistance) which we see in the formula is called the time constant.
• The units for the time constant are seconds.
• If we discharge the capacitor for RC seconds, we can easily find out the fraction of charge left.
• To increase the time taken for a discharge, we can increase the resistance or the capacitance.