# How do I know farad capacity of a capacitor printed its specification in Joule unit and volt unit?

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##### New Member
How to measure the farad unit capacity of a capacitor printed its specification in Joule unit and volt unit? Why is this kind of capacitor specified such that way?

#### JimB

##### Super Moderator
What capacitor is this?

Do you have a picture of the capacitor?
Do you have the datasheet?
Can you post them here?

JimB

#### JLNY

##### Active Member
Erm, I'm not entirely sure I understand the question, but capacitors are typically defined in terms of capacitance in farads because capacitors are often used in electronics for AC filtering and decoupling applications where the capacitance value is more useful for calculating reactance and frequency-domain characteristics than an energy value in joules. It would be highly unusual for a capacitor to have its value defined in terms of the number of joules it can store. This is also the case because capacitors are rarely charged up to their maximum value, so the energy will be less than whatever was printed. (Actually, it is considered good engineering practice to leave a reasonable margin of safety between the max voltage rating of the capacitor you plan to use and the maximum voltage you expect to see in your system).

The value of most smaller capacitors is typically marked in picofarads (1*10^-12 farads). For smaller capacitors, the value is usually printed as a three-number code such as "104", where the first two numbers are the first two digits, and the last number is the number of "zeroes" after it.

so for the value of "104" from above that would be a one, a zero, and 4 more zeroes to make 100,000 picofarads, or 100 nanofarads, or 0.1 microfarad (typically marked with the Greek letter μ, or mu, so 0.1μF) The maximum voltage rating may or may not be marked on the capacitor. The value "473" would be 47,000 picofarads or 47 nanofarads. The value "101" would be 100 picofarads.

for larger capacitor sizes, such as electrolytic "can" capacitors, the capacitance in μF and voltage rating are usually marked in plain text. For very small, surface mount "chip" capacitors, the value may not be marked at all. There are some rare cases of capacitors with colored bands like resistors, but the colors are used to represent numbers in a similar notation as above.

Once you know the value of the capacitor, one has to know what voltage it will be charged to. From there, the energy stored in the capacitor is 0.5*C*V^2, where C is the capacitance in farads and V is the voltage. So a small capacitor with a value of "104," or 0.1μF, charged to 12 volts, would have a stored energy of 0.5*10^-7*12^2=7.2*10^-6 joules, or 7.2μjoules. A large electrolytic capacitor with a value of 470μF charged to 240V would have a stored energy of 0.5*470*10^-6*240^2=13.536 joules.

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#### JLNY

##### Active Member
I suppose, conversely, if one knew the energy in joules at a given voltage, it could be calculated by reversing the formula above, so C=2*J/V^2, where C is the capacitance in farads, J is the energy in joules, and V is the voltage.

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##### New Member
I just figured out Energy in Joule = Integral ( Q dV )
as Q = CV
so E = 1/2 C V² Joule

simply get C from known Joule,
Thanks so much… God bless you all.. ameen

#### Ratchit

##### Well-Known Member
How to measure the farad unit capacity of a capacitor printed its specification in Joule unit and volt unit?
A farad is not a capacity, it is a attribute. You would not say a resistor has a capacity of one ohm, would you? Capacity implies a upper limit of a quantity. Is there any limit to how many farads a capacitor can have? Capacitance is defined by how much charge will separate per volt applied across its plates. It can be measured by what frequency it resonates in a defined circuit, how fast it reaches a voltage when a constant current is applied, its response to a step voltage, or many other ways.

Why is this kind of capacitor specified such that way?
All capacitors are specified that way. Capacitance is a basic attribute of a capacitor.

Ratch

P.S. When used as a unit of energy, joule is not capitalized.

#### JLNY

##### Active Member
P.S. When used as a unit of energy, joule is not capitalized.
Ah, oops. Duly noted. I have amended my ramblings above to avoid amateurish capitalization. #### large_ghostman

##### Well-Known Member
A farad is not a capacity, it is a attribute. You would not say a resistor has a capacity of one ohm, would you? Capacity implies a upper limit of a quantity. Is there any limit to how many farads a capacitor can have? Capacitance is defined by how much charge will separate per volt applied across its plates. It can be measured by what frequency it resonates in a defined circuit, how fast it reaches a voltage when a constant current is applied, its response to a step voltage, or many other ways.

All capacitors are specified that way. Capacitance is a basic attribute of a capacitor.

Ratch

P.S. When used as a unit of energy, joule is not capitalized.
TBH i wouldnt say a resistor has an attribute of 1 ohm either.

#### Ratchit

##### Well-Known Member
TBH i wouldnt say a resistor has an attribute of 1 ohm either.
Well, then it is up to you to explain why that is not so. Don't just leave your statement hanging. While you are at it, give the meaning of the acronym of TBH, too. It seems logical to assume that the resistor's size, composition, type, resistance, or wattage are all attributes of a resistor. Can you provide a succinct counter argument?

Ratch

#### large_ghostman

##### Well-Known Member 