If you're interested in electronics, you've probably heard of capacitors. They're small but mighty components that store energy and help electrical circuits work properly. The amount of energy a capacitor can store is measured in Farad. It's kind of like a battery, but with a different job to do. You can find capacitors in lots of everyday devices like pacemakers, phones, and computers. They work together with other parts of the circuit to make sure the right electrical impulses get through. Think of them like a filter that lets some things pass and blocks others. It's pretty cool how they help make all our gadgets work! And if you're into electronics, you'll definitely want to learn more about capacitor discharge.

How does a capacitor discharge?

Capacitors have two conductive plates separated by an insulator material. When the capacitor is charging, the following two steps below occur in the order in which they are listed:

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When a capacitor discharges, the extra electrons on the negatively charged plate start to move towards the positively charged plate. This creates a flow of electrons in the circuit, which acts as a voltage source for a short period of time. Once the potential difference between the plates reaches zero, the flow of electrons stops. At that point, both plates become neutral, and the charge held by the capacitor is given back to the circuit.

But how does a capacitor behave when it discharges in different types of circuits? It actually depends on whether it's in an AC or a DC circuit. In an AC circuit, the capacitor discharges and charges repeatedly as the current alternates between positive and negative. This can be useful for things like filtering out unwanted frequencies. But in a DC circuit, the capacitor discharges only once and then remains uncharged until it's recharged. This can be useful in things like smoothing out voltage fluctuations. Understanding how capacitors behave in different circuits is important for designing and building electronic devices.

The capacitor’s discharging behaviour in DC circuits

In DC circuits, the capacitor charges and discharges only once. To understand the concept better, take a look at the circuit below.

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Right after we move the switch to position 3, electron flow from the capacitor starts. Since it is in the opposite direction to the electron flow that was happening when the capacitor was charging, the ammeter’s indicator for a short time turns in the opposite direction before going back to zero. This load flow ends when the charge of the two plates of the capacitor is at the same level, which indicates that the capacitor has discharged.

Since the capacitor in the circuit in Figure 2 is short-circuited, the time period while the electron flow is present is very short. To increase this time period and use the capacitor as a source for a longer time, resistors need to be connected to the circuit since they resist current flow.

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In the figure above, Vc is the voltage value of the capacitor, V is the voltage value of the capacitor when it is fully charged, and t is time.

As you can see, in DC circuits, we speak of the temporary state when the capacitor is discharging and the voltage level goes down to zero. When the capacitor is fully discharged, we speak of the steady state. This is the main difference between how capacitors behave in DC and AC circuits.

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The figure shows that the current (Ic) flowing through the capacitor is decreasing from a negative value to zero. This is because the capacitor is discharging, meaning that the electrons are flowing in the opposite direction to the direction they were flowing while the capacitor was charging. Once the capacitor is fully discharged, the current will remain at zero until the switch is moved to position 1, which will cause the capacitor to start charging again.

The capacitor’s discharging behaviour in AC circuits

Whereas a capacitator in a DC circuit discharges only once, in an AC circuit, it charges and discharges continuously. The current flow is also different compared to a DC circuit, where it flows in one direction until the capacitor is discharged and then stops. In an AC circuit, by contrast, current flows in both directions continuously.

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In this figure, V(t) is the voltage depending on time, i(t) is the current depending on time, Vm is the peak value of the voltage of the capacitor, Im is the peak value of the alternative current going through the capacitor, and θ is the phase difference between the voltage and the current of the capacitor.

To understand the concept better, we will look at it in different parts of a period. Normally, there are four parts where the capacitor behaves differently: 0-π / 2, π / 2-π, π -3π / 2, and 3π / 2-2π. Let’s say the phase angle is a. In the π/2<a<π and the 3π/2<a<2π periods, the capacitor is discharging while in the other two periods, it is charging.

The π/2 <a <π period

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In this figure, Vt is the AC voltage source, which depends on time, while Vmax ⋅ sin(wt) is the function defining its sinusoidal behaviour.

Because the capacitor’s voltage is at its peak at the a=3π/2 point, the load will be at its maximum as well. And because the capacitor is completely charged, there will be no current flowing through it at this precise moment. As a result, the current value is i = 0.

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In the circuit shown, the capacitor is initially fully charged to 10 volts. When the switch is closed at time t=0, the capacitor will start to discharge through the resistor. The time it takes for the capacitor to fully discharge can be calculated using the:

t = RCln(V0/Vt)

where R is the resistance of the resistor, C is the capacitance of the capacitor, V0 is the initial voltage across the capacitor (10V in this case), and Vt is the voltage at which we consider the capacitor to be fully discharged (0V in this case).

Let's assume that the resistance of the resistor is 1000 ohms and the capacitance of the capacitor is 1 microfarad. Then, the time it takes for the capacitor to fully discharge can be calculated as:

t = (1000 ohms) x (1 microfarad) x ln(10V/0V) = 6.93 milliseconds

Therefore, it will take approximately 6.93 milliseconds for the capacitor to fully discharge in this circuit. It's important to note that the discharge time will depend on the values of the resistance and capacitance in the circuit, as well as the initial voltage across the capacitor.

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The time it takes for the capacitor to discharge is 5T, where T is the time constant that can be calculated as: Entering the known values, we get: And, as already said, the discharge time equals 5T. This gives us:

Capacitor Discharge - Key takeaways

There is a difference in how capacitors operate in DC and AC circuits since the voltage levels are steady in DC and constantly changing in AC. Capacitator discharge happens when the electric field of the source surrounding the capacitor disappears, causing the start of the electron flow from the conductive plates to the circuit. The time it takes for a capacitor to discharge is 5T, where T is the time constant. There is a need for a resistor in the circuit in order to calculate the time it takes for a apacitor to discharge, as it will discharge very quickly when there is no resistance in the circuit. In DC circuits, there are two states when a capacitor is discharging. The first is the temporary state, which is while the capacitor is discharging. The second is the steady state, which is when the capacitor is fully discharged.

Capacitor Discharge

How long does it take a capacitor to discharge?

The time it takes for a capacitor to discharge is 5T, where T is the time constant.

What causes a capacitor to discharge?

When the capacitor is fully charged and the electrical field from the source surrounding the capacitor goes down to zero, it causes an electron flow from the conductive plates of a capacitor to the circuit, which then causes the capacitor to discharge.

What is a capacitor discharge?

A capacitor discharge is a situation that occurs when the electrical field from the voltage source around the capacitor goes down to zero, leading to an electron flow, which causes the potential difference between the two conductive plates to reach zero. This is possible when the charges of the two conductive plates are the same.

How do you discharge a capacitor?

You can discharge a capacitor by simply connecting it to a circuit without a source, or you can short-circuit the poles of the capacitor using a conducting material. 

When do capacitors discharge?

Capacitors discharge when another path in the circuit that allows the charges to flow to each other is created. This causes the charges to flow out of the capacitor, and the capacitor becomes discharged after some time.

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