A capacitor is a component capable of storing energy in the form of an electric field.
Whereas a resistor dissipates energy, a capacitor (or condenser) can temporarily store it.
This ability to store energy and release it later is the basis of power supplies, audio filters, and computer memories.
Think of a water pipe:
- A resistor would be a constriction in the pipe.
- A capacitor would be an elastic tank or a bucket inserted into the pipe. Initially, water flows quickly to fill it, but once full, the flow stops.
What is a Capacitor
Physically, a capacitor is tremendously simple: it consists of two conductive plates separated by an insulating material (called the dielectric).
When connecting a battery to the plates:
Positive charges accumulate on one plate.
Negative charges accumulate on the other.
Because the insulator is in between, the charges cannot touch each other (no current flows through the insulator).
But they attract each other through the insulator, creating an Electric Field.
The energy is stored in the electric field between the plates.
Capacitance (C) and Farad (F)
Capacitance is measured in farads. A 1-farad capacitor has a charge of 1 coulomb on each plate when there is 1 volt between them.
The Farad is a gigantic unit. A 1 Farad capacitor would be the size of a large soda can. In real electronics, we will always work with submultiples:
- Microfarads (µF): 10⁻⁶ F (Power supplies).
- Nanofarads (nF): 10⁻⁹ F (Filters).
- Picofarads (pF): 10⁻¹² F (High frequency).
Behavior in DC: The Transient
Here comes the crucial part. What happens when we connect a capacitor to a direct current (DC) battery?
If there is resistance in the charging path, the evolution is not instantaneous, and we can divide it into three stages:
The Initial Instant (
The capacitor starts discharged, and its voltage cannot change instantaneously.
- The initial current is maximum and is limited by the entire circuit resistance, including that of the source, conductors, and the capacitor itself.
- At the initial instant, an ideal discharged capacitor can be approximated as a short circuit.
The Charging
As the plates fill with charge, they begin to repel new electrons trying to arrive (“Hey, there’s no more room in here!”).
- The voltage across the capacitor rises.
- The current gradually decreases.
- This charging curve is exponential.
The Steady State (
The capacitor’s voltage approaches that of the source. It’s not that “no more electrons fit”: the voltage difference that caused the charging current disappears.
- In the ideal model, the current tends to zero.
- In the DC steady state, the ideal capacitor behaves as an open circuit (a real one retains a small leakage current).
Key idea in DC: A charged capacitor blocks the flow of direct current.
The Time Constant (
How long does it take to fill the tank? It depends on two things:
- The size of the tank (C).
- The resistance of the filling pipe (R).
We define the time constant Tau (𝜏) as:
- In 1𝜏, the capacitor charges to 63%.
- In
, it reaches approximately 99.3% of its final value and is usually considered fully charged for practical purposes.
This is important for designing timers. If you want a light to turn off 5 seconds after pressing a button, you will play with the values of R and C to achieve that delay.
Types of Capacitors
Just like resistors, there are several types based on their construction:
- Ceramic (discs): Small, cheap, non-polarized. Low values (pF, nF).
- Polyester / Plastic: Square, precise. Medium values.
- Electrolytic: Aluminum cylinders. They contain a liquid (electrolyte) inside.
- They achieve enormous capacitances (µF).
- THEY HAVE POLARITY! They have a marked positive and negative terminal.
Danger of Explosion: If you connect an electrolytic capacitor backwards (positive to negative), the electrolyte boils, gas is generated, and the capacitor explodes like a firecracker, releasing highly toxic smoke and metallic paper confetti. Be very careful.
