What does a Capacitor do

Short answer

• A capacitor stores electrical charge, and with that charge it stores electrical energy in an electric field, then releases that energy when it is needed. It does this with two conducting plates separated by an insulator. When a voltage is applied, one plate collects a positive charge and the other collects an equal negative charge, while the capacitor as a whole stays electrically neutral. That one job, store and release, explains every capacitor use. A camera flash capacitor stores energy slowly and releases it in a sudden burst of light. A power supply capacitor smooths out the ups and downs of voltage so a device gets steady power. A timing capacitor charges at a predictable rate, so a circuit can measure out a delay. A filter capacitor lets some signals through and holds others back. A capacitor is not a battery, and it does not create energy. A battery stores energy in chemical reactions and can deliver it steadily for a long time. A capacitor stores energy in an electric field between its plates, and it is usually much better at delivering a quick burst than at supplying steady power for hours.

The one job: store charge, store energy

OpenStax, in University Physics Volume 2, defines a capacitor in a single sentence that is worth learning word for word: a capacitor is a device used to store electrical charge and electrical energy.

First, a capacitor stores charge. Charge is the electrical property carried by particles such as electrons and protons, measured in coulombs. When a capacitor is charged, charge is separated: positive charge sits on one plate and negative charge sits on the other plate. The capacitor itself gains no net charge, because the two plates carry equal and opposite amounts.

Second, a capacitor stores energy. The separated charges create an electric field in the space between the plates, and the energy sits in that field. OpenStax puts it plainly: a charged capacitor stores energy in the electrical field between its plates, and when a charged capacitor is disconnected from a battery, its energy remains in the field in the space between its plates.

This is why the water tank comparison is popular and also why it needs a warning label. Like a tank, a capacitor fills up and empties. Unlike a tank, a capacitor does not hold a substance. It holds a separation of charge, and the energy is in the field that separation creates.

Inside a capacitor: two conductors and an insulator

Almost every capacitor follows the same pattern. It has two electrical conductors, usually called plates, placed close together but not touching. The space between them is filled with an insulating material called a dielectric, or in some designs it is simply a vacuum. The plates are connected to the rest of the circuit by terminals, the wires or legs that carry current to and from the plates.

The plates do not have to be flat. In a parallel plate capacitor they are flat sheets facing each other. In the rolled capacitors found in many electronic devices, two long conducting sheets with a dielectric sheet between them are rolled up into a compact cylinder, so that a large plate area fits into a small case.

The dielectric is not just empty filler. It is an insulator, so it stops charge from flowing directly from one plate to the other, which is what forces the charge to stay separated. It also increases how much charge the capacitor can hold at a given voltage, as explained in the section on dielectrics below. A capacitor blocks direct current once it is fully charged, because the insulator between the plates stops a steady current from crossing. That is the property that makes capacitors useful as filters and as coupling parts between stages of a circuit, where a steady voltage must be blocked while a changing signal is allowed through.

Charging: how a capacitor fills up

Charging a capacitor means connecting it to a voltage source, such as a battery, so that charge moves onto the plates. OpenStax describes the process in concrete terms. When battery terminals are connected to an initially uncharged capacitor, the battery potential moves a small amount of charge from the positive plate to the negative plate. The capacitor remains neutral overall, but with charges of plus Q and minus Q residing on opposite plates.

The battery does not pump new charge into the capacitor from outside. The circuit already contains movable charge in its wires. The battery pushes charge around the loop: electrons are drawn away from one plate, leaving it positively charged, and pushed onto the other plate, making it negatively charged. The total charge of the capacitor is still zero. What has been stored is separation, not extra charge.

As charging goes on, the voltage across the capacitor rises. At the start, with no charge on the plates, the voltage is zero and current flows freely. As charge builds up, the growing voltage across the capacitor pushes back against the battery, so the current falls. When the capacitor voltage equals the battery voltage, current stops, and the capacitor is fully charged. Real circuits always have resistance, so charging takes time, as set by the RC time constant covered later in this article.

Discharging: how a capacitor gives energy back

Discharging is charging in reverse. If a charged capacitor is connected to a circuit without a battery, such as a lamp or a resistor, the separated charges now have a path to reunite. Charge flows from the positive plate around the circuit to the negative plate, current flows through the load, and the load does something with the energy: a flash lamp flashes, a motor turns, a speaker sounds.

As discharge goes on, the voltage across the capacitor falls, because voltage is tied to the amount of charge on the plates. As voltage falls, current falls too. The discharge therefore starts strong and fades away, rather than delivering steady power until the last moment and then stopping suddenly. That fading shape is why a capacitor is good for bursts and poor for long, steady supply.

OpenStax uses the camera flash as its classic example. When you use a flash camera, it takes a few seconds to charge the capacitor that powers the flash, and the light flash discharges the capacitor in a tiny fraction of a second. The battery charges the capacitor slowly through a relatively high resistance, and the flash lamp discharges it quickly through a very low resistance. A safety note belongs here, stated plainly and without how to instructions. Some capacitors, especially large ones in power equipment and older display devices, can hold a dangerous charge after the power is switched off. Do not open such equipment and do not handle its internal parts. Servicing and repair of that equipment belong to qualified technicians.

Capacitance: how much charge per volt

If one capacitor holds more charge than another at the same voltage, it has a larger capacitance. Capacitance, symbol C, is defined as the ratio of the charge Q stored on one plate to the voltage V across the plates: C = Q / V. So capacitance answers the question: for each volt applied, how many coulombs of charge separation does this device produce? Rearranged, the same relation gives the charge at a given voltage: Q = C x V. Double the voltage on the same capacitor and you double the stored charge. Double the capacitance at the same voltage and you also double the stored charge.

The SI unit of capacitance is the farad, symbol F, named after Michael Faraday. One farad equals one coulomb per volt, written 1 F = 1 C / V. OpenStax stresses how large a farad is: a 1.0 farad capacitor stores 1.0 coulomb of charge when the potential difference across its plates is only 1.0 volt, and one coulomb is a very large amount of charge. Everyday capacitors are therefore labeled in much smaller units. Typical values range from picofarads to millifarads, including microfarads. One picofarad is 10 to the power of minus 12 farads, one microfarad is 10 to the power of minus 6 farads, and one millifarad is 10 to the power of minus 3 farads. A capacitor marked 100 microfarads has a capacitance of 0.0001 farads, and at 12 volts it stores 0.0012 coulombs of charge on each plate.

Capacitance is a property of the device, not of how much charge happens to be on it at a given moment. OpenStax states this explicitly: capacitance is independent of Q or V, because if the charge changes, the potential changes correspondingly so that Q / V remains constant. Change the voltage and the charge follows; the ratio stays the same unless the device itself is physically changed, for example by changing the plate area, the plate separation, or the dielectric.

What sets capacitance: area, separation, and dielectric

For the simplest shape, the parallel plate capacitor, capacitance can be calculated from geometry alone. If each plate has area A and the plates are separated by distance d, and the space between them is a vacuum, the capacitance is C = epsilon0 x A / d, where epsilon0 is the permittivity of free space, equal to 8.85 x 10 to the power of minus 12 farads per meter. Larger plates hold more charge at a given voltage, so capacitance grows with area A. Plates closer together attract each other’s opposite charges more strongly, so capacitance grows as separation d shrinks. Halve the separation and you double the capacitance, for the same plate area.

OpenStax works an example that shows how hard it is to reach a large capacitance with flat plates in air. An empty parallel plate capacitor with plates of 1.00 square meter each, separated by 1.00 millimeter, has a capacitance of 8.85 nanofarads. At 3,000 volts it stores 26.6 microcoulombs of charge.

The dielectric multiplies capacitance by a factor called the dielectric constant, symbol kappa, which is greater than 1 for real materials. Insert a dielectric that completely fills the gap, and the capacitance becomes kappa times the empty capacitance. OpenStax gives a worked example: an empty 20.0 picofarad capacitor charged to 40.0 volts holds 0.8 nanocoulombs. Insert Teflon with a dielectric constant of 2.1, with the battery disconnected so the charge stays at 0.8 nanocoulombs, and the capacitance rises to 42.0 picofarads while the voltage falls to 19.0 volts. The dielectric does not add charge. It lets the same charge sit at a lower voltage, which by C = Q / V means a larger capacitance. The field polarizes the dielectric, and the induced charges on its surfaces partly cancel the field of the plates, so the same plate charge produces a smaller voltage. Every dielectric also has a voltage limit, and exceeding a capacitor voltage rating can damage the part.

Energy stored: why voltage matters more than charge

The energy stored in a capacitor is electrostatic potential energy, held in the electric field between the plates. OpenStax gives three equivalent forms of the same relation: U = 1/2 x C x V squared = 1/2 x Q x V = Q squared / (2 x C), where U is the stored energy in joules, C is capacitance in farads, Q is charge in coulombs, and V is voltage in volts. The first form is the one most often quoted, and it carries an important warning: stored energy grows with the square of voltage. Double the voltage and the stored energy quadruples, for the same capacitor. Double the capacitance at the same voltage and the energy only doubles.

A worked example makes the numbers concrete. Take a 100 microfarad capacitor charged to 12 volts. Using U = 1/2 x C x V squared: one half times 0.0001 farads times 144 gives 0.0072 joules, or 7.2 millijoules. Now take the same capacitor charged to 120 volts instead: one half times 0.0001 times 14,400 gives 0.72 joules, one hundred times more energy, because the voltage went up tenfold and energy follows voltage squared.

OpenStax gives a medical example at the other end of the scale. A heart defibrillator delivers 400 joules of energy by discharging a capacitor initially at 10,000 volts, and solving U = 1/2 x C x V squared for C gives a capacitance of 8.00 microfarads. A modest capacitance at a very high voltage stores a large amount of energy, and it can deliver that energy in a short burst.

Series and parallel: combining capacitors

Capacitors are often used in groups. In a parallel combination, all the capacitors share the same voltage, and their charges add, so the equivalent capacitance is the sum: C equivalent = C1 + C2 + C3 and so on. In a series combination, all the capacitors carry the same magnitude of charge, and their voltages add, so the equivalent capacitance follows the reciprocal rule: 1 / C equivalent = 1 / C1 + 1 / C2 + 1 / C3 and so on.

These rules, summarized in the OpenStax chapter summary, have two consequences that beginners often get backwards. Adding capacitors in parallel always increases the total capacitance beyond the largest single capacitor. Adding capacitors in series always decreases the total capacitance below the smallest single capacitor. Series connection trades capacitance for voltage handling: the total voltage divides across the capacitors, so each one sees only part of it.

Worked examples help. Two capacitors of 2 microfarads and 4 microfarads in parallel give 6 microfarads. The same two in series give 1 / C equivalent = 1/2 + 1/4 = 3/4, so C equivalent = 4/3 microfarads, about 1.33 microfarads, which is indeed smaller than the smaller capacitor.

The RC time constant: why charging takes time

A capacitor rarely sits alone. It is usually paired with a resistor, forming an RC circuit, and the resistor sets how fast the capacitor can charge or discharge. OpenStax defines the RC time constant, symbol tau, as tau = R x C, where R is resistance in ohms and C is capacitance in farads, and tau comes out in seconds. After one time constant of charging, the capacitor voltage reaches about 63.2 percent of its final value. After one time constant of discharging, about 36.8 percent of the initial charge remains. In practice, after about five time constants a capacitor is treated as essentially fully charged or discharged, at about 99.3 percent of the way there.

A worked example shows the scale. A resistor of 10,000 ohms with a capacitor of 100 microfarads gives tau = 10,000 x 0.0001 = 1 second. Charging from zero, the capacitor reaches 63.2 percent of the supply voltage after 1 second, about 86.5 percent after 2 seconds, and about 99.3 percent after 5 seconds. Swap the resistor for one of 1,000 ohms and the time constant falls to 0.1 seconds, so the same capacitor charges ten times faster. This is also why the flash camera example works the way it does: the charging path has high resistance, so charging takes seconds, while the discharge path through the flash lamp has very low resistance, so discharge takes a tiny fraction of a second.

The five jobs a capacitor does in circuits

So far this article has covered one job, store and release, in several forms. In real circuits that job appears as five distinct roles.

First, energy bursts. A capacitor collects energy slowly and delivers it quickly. Camera flashes and defibrillators are the textbook examples, both given by OpenStax.

Second, smoothing power supplies. The power that comes from a wall outlet alternates, and after it is converted toward direct current it still carries ripples, regular ups and downs in voltage. A capacitor across the supply output charges when the voltage is high and discharges when it dips, filling in the valleys.

Third, filtering signals. Combined with a resistor, a capacitor forms a filter that can pass some frequencies and block others. OpenStax notes that capacitors are used for filtering static from radio reception, and that RC filters can be used to filter out unwanted frequencies. In audio circuits, a capacitor can block the steady direct current component of a signal while letting the changing audio signal through.

Fourth, timing and control. Because an RC circuit charges at a predictable rate set by tau, it can measure out a delay. OpenStax lists timers for intermittent windshield wipers, pacemakers, and strobe lights among RC circuit applications, and notes that increasing the resistance increases the time between wiper sweeps. The same idea drives the timed pulses of a 555 timer chip, a part whose timing is controlled by an RC circuit.

Fifth, tuning and sensing. In a radio tuner, a variable capacitor changes the tuning of a circuit so it responds strongly to one station frequency and weakly to others. In sensing, a change in capacitance signals a change in the world: OpenStax describes an electronic stud finder whose plates act as a capacitor, and when it moves over a wooden stud the capacitance changes because wood has a different dielectric constant than a gypsum wall, triggering a signal.

Some electric motors also use a capacitor to help them start and run smoothly. Very large capacitors, sometimes called supercapacitors, store far more charge than a conventional capacitor of similar size, though still usually holding less total energy than a battery of similar size. Both points are stated here in general terms; detailed ratings vary widely by application.

Capacitor versus battery: the honest comparison

The capacitor versus battery question comes up in almost every discussion of this topic, and the honest answer is that neither device wins in general. Each wins at what it is built for.

A battery stores energy chemically. That gives it high energy density: a small battery can run a device for hours or days. But chemical reactions take time, so a battery delivers power relatively slowly, and repeated fast charging and discharging wears it out. A capacitor stores energy in an electric field. That gives it high power density: it can deliver its stored energy very quickly and recharge quickly, and it can repeat that cycle many more times than a typical battery. But its energy density is low: the same size capacitor holds far less total energy than a battery.

In short: energy per size, battery high, capacitor low. Power per size, capacitor high, battery moderate. Charge speed, capacitor fast, battery slow. Cycle life, capacitor generally very long, battery limited. Steady voltage during discharge, battery good, capacitor poor, because capacitor voltage falls steadily as it discharges. The pattern is consistent: batteries are for endurance, capacitors are for speed, which is why devices often use both.

Types of capacitor, in brief

Capacitors come in several families, each trading size, capacitance, voltage rating, stability, and cost differently. Ceramic capacitors are small, inexpensive, and widely used in high frequency circuits and for filtering noise. They are usually not polarized, meaning they can be connected either way around, and they come in small capacitance values. Film capacitors use a thin plastic film as the dielectric. They are stable and accurate and handle higher voltages well, though they are larger than ceramic parts of similar capacitance. Electrolytic capacitors pack a large capacitance into a small volume by using a very thin oxide layer as the dielectric and an electrolyte as one electrode. They are polarized: they must be connected with the correct polarity, and connecting them backwards can damage them. They are common in power supply smoothing, where large capacitance matters more than precision. Supercapacitors, also called ultracapacitors, use porous electrodes with an enormous effective surface area to reach capacitances of farads rather than microfarads. They charge fast and deliver bursts, but they typically work at low voltages and still store less energy than a battery of comparable size.

Common mistakes, and how to avoid them

Several mistakes recur in student answers and in online explanations.

Mistake one: saying a capacitor stores electrons. A capacitor stores charge separation, not a stockpile of electrons. Electrons move from one plate to the other through the external circuit during charging, but the plates end up with equal and opposite charges and the capacitor as a whole stays neutral.

Mistake two: saying current flows through a capacitor. In the ideal model, no current crosses the dielectric. What looks like current through a capacitor during charging is charge accumulating on one plate and leaving the other through the external circuit, at the same rate on both sides. Real capacitors leak a tiny current through the dielectric, but that leakage is a defect to be minimized, not the mechanism of operation.

Mistake three: treating capacitance as if it changes with voltage or charge. Capacitance is set by geometry and dielectric. Applying more voltage stores more charge, but the ratio Q / V stays the same.

Mistake four: calling a capacitor a small battery. The two devices store energy by different mechanisms and deliver it on different timescales. A battery holds its voltage fairly steady until it is nearly empty. A capacitor voltage falls steadily from the start of discharge.

Mistake five: assuming a bigger capacitor is always better. Larger capacitance means slower charging for the same resistance, larger physical size, and often higher leakage and cost. In a timing circuit, a bigger capacitor changes the timing. In a filter, it changes which frequencies pass.

Frequently Asked Questions

What does a capacitor do in simple terms? It stores electrical energy for a short time and releases it when needed. Two plates separated by an insulator hold opposite charges, and the energy sits in the electric field between them until the capacitor discharges.

Is a capacitor a battery? No. A battery stores energy in chemical reactions and delivers steady power for a long time. A capacitor stores energy in an electric field and delivers it quickly, usually in a burst.

How does a capacitor charge? Connecting it to a voltage source moves charge around the circuit so that one plate becomes positively charged and the other equally negatively charged. Current is largest at the start and falls as the capacitor voltage rises to match the source voltage, at which point charging stops.

How does a capacitor discharge? Connecting a charged capacitor to a load gives the separated charges a path to reunite through that load. Current flows through the load, delivering energy to it, while the capacitor voltage falls. Discharge starts strong and fades, following an exponential curve set by the RC time constant.

What is capacitance? Capacitance is the charge stored per unit voltage, C = Q / V, measured in farads. A larger capacitance stores more charge at the same voltage. It depends on the geometry of the plates and on the dielectric between them.

Why does a capacitor block direct current? Once fully charged, the insulator between the plates stops a steady current from crossing, so no steady direct current flows. A changing signal still causes current to flow in and out of the terminals as the plates charge and discharge.

What is the RC time constant? The RC time constant, tau = R x C, is the timescale for charging and discharging. After one time constant of charging, a capacitor reaches about 63.2 percent of its final voltage. After about five time constants it is essentially fully charged or discharged.

What happens if you connect capacitors in series or parallel? In parallel, capacitances add, so the total is larger than any single capacitor. In series, reciprocals add, so the total is smaller than the smallest capacitor, though the total voltage divides across the series capacitors.

Do capacitors lose their charge over time? Yes, slowly. Real capacitors leak a small current through the dielectric, so a charged capacitor left disconnected gradually loses charge. Large capacitors in equipment can also retain a dangerous charge after switch off, so such equipment should only be serviced by qualified technicians.

Sources & references

• OpenStax, University Physics Volume 2, Section 8.1, Capacitors and Capacitance. A capacitor as a device used to store electrical charge and electrical energy; two conductors separated by a distance with a dielectric between them; battery charging moving charge from the positive plate to the negative plate with the capacitor remaining neutral overall; capacitance defined as C = Q / V; the farad as one coulomb per volt; typical values from picofarads to millifarads; parallel plate capacitance C = epsilon0 x A / d with epsilon0 = 8.85 x 10 to the power of minus 12 farads per meter; the worked example of a 1.00 square meter, 1.00 millimeter capacitor giving 8.85 nanofarads and 26.6 microcoulombs at 3,000 volts.
• OpenStax, University Physics Volume 2, Chapter 8 Summary. A capacitor as a device that stores electrical charge and electrical energy; capacitance as charge stored per unit potential difference; the farad as 1 F = 1 C / V; the series rule that the reciprocal of the equivalent capacitance is the sum of the reciprocals, and the parallel rule that the equivalent capacitance is the sum of the individual capacitances.
• OpenStax, University Physics Volume 2, Section 8.3, Energy Stored in a Capacitor. Energy stored in the electrical field between the plates and remaining there when disconnected from a battery; the relations U = 1/2 x C x V squared, U = 1/2 x Q x V, and U = Q squared / (2 x C); energy in joules for charge in coulombs, voltage in volts, and capacitance in farads; defibrillator energy stored in a capacitor; the worked example of a defibrillator delivering 400 joules at 10,000 volts with a capacitance of 8.00 microfarads.
• OpenStax, University Physics Volume 2, Section 8.4, Capacitor with a Dielectric. The dielectric constant kappa as the factor by which capacitance increases when a dielectric completely fills the gap; the relation C = kappa x C0; the worked example of a 20.0 picofarad capacitor at 40.0 volts holding 0.8 nanocoulombs, rising to 42.0 picofarads at 19.0 volts with Teflon of dielectric constant 2.1; the electronic stud finder example in which a change in dielectric constant changes capacitance and triggers a signal.
• OpenStax, University Physics Volume 2, Section 10.5, RC Circuits. An RC circuit as a circuit containing resistance and capacitance; the flash camera example in which charging takes a few seconds and discharge takes a tiny fraction of a second, with charging resistance significantly greater than discharging resistance; the charging relation q(t) = Q x (1 minus e to the power of minus t / tau); the time constant tau = R x C; charge reaching 0.632 of maximum after one time constant; discharge following q(t) = Q x e to the power of minus t / tau; RC applications including timers for intermittent windshield wipers, pacemakers, and strobe lights, RC filters for unwanted frequencies, the 555 timer, and power supplies turning alternating voltage into direct voltage.
• OpenStax, College Physics, Section 21.6, DC Circuits Containing Resistors and Capacitors. The time constant tau = R x C, illustrated by the defibrillator example with an 8.00 microfarad capacitor and 1,000 ohm path giving 8.00 milliseconds, with voltage falling to 0.368 of its initial value after one time constant.