25.6:
Energy Stored in a Capacitor: Problem Solving
Capacitors act as temporary batteries storing electric potential energy that could be released over a short time.
In small engines, a capacitor-discharge ignition system is used that consists of a charging unit to charge a capacitor completely. Upon receiving a trigger, the capacitor rapidly discharges the stored energy to an ignition coil that, in turn, fires the spark plug to start the engine.
The amount of energy stored by a capacitor-discharge ignition system depends on the value of the applied voltage and the capacitance of the capacitor.
Consider a fully charged capacitor with a charge of 375 micro coulomb and capacitance of 1.5 micro farad. Here the charge and capacitance are known quantities, and the voltage and energy are unknown quantities.
Using the known quantities, the voltage supplied to the system could be calculated to be 250 volt.
Upon substitution, the voltage and capacitance values give the value of the electric potential energy saved in the capacitor to be 0.047 joules.
25.6:
Energy Stored in a Capacitor: Problem Solving
In 1749, Benjamin Franklin coined the word battery for a series of capacitors connected to store energy. Capacitors store electric potential energy that can be released over a short time. This property means capacitors have a wide range of applications.
Capacitor-discharge ignition is a type of ignition system commonly found in small engines where the energy released from a capacitor ignites an induction coil that, in turn, fires the spark plug.
To calculate the energy stored in a capacitor of a typical CDI system, at least two of the following parameters should be known:
- The voltage applied across the capacitor
- The charge stored in the capacitor
- The capacitance of the capacitor
Since these three parameters are related by the equation given below, the unknown parameter can be calculated if any two parameters are known:
For any given case, the potential energy stored can be calculated using the values of capacitance, voltage, and charge:
Suggested Reading
- OpenStax. (2019). University Physics Vol. 2. [Web version]. Retrieved from Pp. 361–363 https://openstax.org/books/university-physics-volume-2/pages/1-introduction