Electrical principles - Properties
Capacitance
Capacitance is the ability of a device to store an electrical charge.
The unit of capacitance is the farad.
Quantity |
Symbol | Unit |
Abbreviation |
Meaning |
|---|---|---|---|---|
capacitance |
C |
farad |
F |
ability to store a charge |
The capacitor
A capacitor ia a device capable of storing an electrical charge. This is done by separating conductive plates with an insulating material. The insulator material is called a dielectric.
Internal view of a capacitor
Factors affecting the capacitance value of a capacitor:
- area of the plates (bigger plates, more capacitance)
- distance between the plates (bigger distance, lower capacitance)
- material used as dielectric (higher insulative qualities, more capacitance).
Types of capacitors
Various capacitor types are used for specific purposes.
Electrolytic capacitor
Electrolytic capacitor
These capacitors have aluminium or tantalum and the dielectric rolled together then enclosed. They are polarity sensitive and if connected incorrectly will explode.
Ceramic capacitor
Ceramic capacitor
A ceramic disc is silver plated on both sides. They are often used where a capacitor of high values is required. Ceramic capacitors are not polarity sensitive.
Variable capacitor
A variable capacitor is made up so that one plate can be moved in or out to change the capacitance.
Stacked plate capacitor
Stacked plate capacitors have the plates and dielectric stacked alternately. Each alternate plate is connected together to form one plate. These are often used where it is important not to have any self-inductance.
Dangers of capacitors
Capacitors will hold their charge until they are discharged. If a capacitor is not discharged before it is touched, it can cause electric shock.
Polarity sensitive capacitors can explode if they are not connected correctly.
Connection of capacitors
Series
Capacitors, when connected in a series, work the opposite to resistors and the overall value of the capacitance will reduce.
Here is the formula to calculate total capacitance in a series circuit.
Formula for total capacitance in a series circuit
Parallel
When capacitors are connected in parallel, the result is the sum of all the capacitors.
Here is the formula to calculate total capacitance in a parallel circuit.
Formula for total capacitance in a parallel circuit
Time constants
When voltage is applied to a capacitor it does not charge instantly. The capacitor is charged up over five time periods. These periods are called time constants. In the first time constant, a capacitor will always achieve 63.2% of total charge. The actual time that it will take can range from milliseconds to hours.
Select the switch to see the capacitor charge.
Charging of capacitor
The time it takes to charge up, depends on the size of the capacitor and the resistance in the circuit.
Now select the switch to see the capacitor discharge.
Discharging of capacitor
Calculations
The time constant depends on the resistance and capacitance (RC) in a circuit. The time constant can be calculated using the equation:
t = RC
Where:
t = time in seconds
R = resistance in ohms
C = capacitance in farads.
After five time constants a capacitor will be fully charged, therefore the charging time of a capacitor equals five times the time constant.
