Introduction to the Capacitor in Electrical Engineering
We can think of capacitors are short term ‘charge-stores’. In other words, capacitors just store charge inside them. A capacitor consists of two metal plates separated by a layer of insulating material called a dielectric.
There are 2 types of capacitor:
- Electrolytic capacitors. These hold much more charge and must be connected with the correct polarity, otherwise they can explode.
- Non-electrolytic. These hold less charge and can be connected either way round in a circuit.
How Does a Capacitor Work?
When two conducting plates are connected to a battery, electrons move towards one plate. The positive plate loses electrons , eventually leaving both plates with equal and opposite charge, +Q and -Q. When a capacitor is charged, we say that the capacitor has charge Q.
Define Capacitance
Capacitance, C = “the charge, Q, required to cause potential difference, V, in a conductor. It is measured in Farads”.
“1 Farad is the capacitance of a conductor, which has potential difference of 1 volt when it carries a charge of 1 coulomb”.
Charging a Capacitor Using D.C.
At any time, t, after the switch is closed, the charge, Q, on the capacitor can be calculated using Q=It where I = the current (Amps).
The variable resistor can be altered to keep the current constant.
Plot a graph of capacitance against voltage and since:
The gradient of the graph will equal the capacitance of that capacitor.
Charge Stored in a Capacitor
Electrical potential energy is stored when a capacitor is charged.
The area under this graph is equal to the energy stored = (½bh)
We can combine previous equations to give the following:
Charging and Discharging a Capacitor
When discharging, the current decreases with the potential difference, p.d. This decrease is exponential as can be seen below. Q0, V0 and I0 are all the initial charge, voltage and current through the capacitor at the initial discharge.
Decay Curve
Let us now examine the process of discharging the capacitor. Charge, Q, falls to 1/e of its initial value in a time equal to the time constant, RC.
When the initial charge is Q0,
- After RC seconds = 0.37 x Q0
- After 2RC seconds = 0.37 x 2 x Q0
- After nRC seconds = 0.37n x Q0.
The time taken to halve, T½, is always the same:
Example of Capacitors in Series and Parallel
Interested in electrical and electronic engineering? Find out more about all the electrical engineering courses we have available by clicking here.
Diploma in Electrical Technology
Diploma in Renewable Energy (Electrical)
Diploma in Electrical and Electronic Engineering
Alternatively, you can view all our online engineering courses here.
Recent Posts
How can we analyse a single-degree-of-freedom lumped parameter systems with damping?
How can we analyse a single-degree-of-freedom lumped parameter system with damping? In our previous article on undamped vibration, we looked at solving vibration problems with a single degree of freedom. In this article, we’re going to look at systems with damping and how to analyse them. We’ll focus on the mathematical analysis of a single-degree-of-freedom […]
How can we analyse undamped vibration problems.
How can we analyse undamped vibration problems. In our previous article on harmonic motion, we looked at how we can analyse problems involving simple harmonic motion, now we’re going to look at solving vibration problems with a single degree of freedom. Vibration Analysis In vibration analysis, there are two approaches we are interested in. One […]
How do we analyse problems involving harmonic motion?
How do we analyse problems involving harmonic motion? In our previous article on mechanical resonance, we looked at harmonic motion and how it affects structures. Now we’re going to jump into how we can solve issues that involve simple harmonic motion. What is simple harmonic motion? Many systems consist of components that vibrate in the […]