# capacitor and inductor

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#### bhuvaneshnick

##### Member
voltage and why voltage cannot change instantaneously in capacitor but current but why current cannot change instantaneously in inductor.Thank you in advacne

#### 4pyros

##### Well-Known Member
Because a cap stores voltage and a coil stores current.

#### cowboybob

##### Well-Known Member
Welcome to ETO, bhuvanesick!

Because a cap stores voltage and a coil stores current.
Both of which take a period of time, starting at zero, to reach their maximum (circuit allowable) value.

#### 4pyros

##### Well-Known Member
A cap will hold the voltage for a time like a small battery.
A coil will hold current for some time as the magnetic field collapses.

#### bhuvaneshnick

##### Member
expecting some detail and in depth explaination

#### 4pyros

##### Well-Known Member
expecting some detail and in depth explaination
Then google it or get a book, Its basic stuff but I am not a teacher.

thank you

#### Ratchit

##### Well-Known Member
voltage and why voltage cannot change instantaneously in capacitor but current but why current cannot change instantaneously in inductor.Thank you in advacne
If you could instantaneously change the charge distribution on the plates of the capacitor, i.e. add charge carriers to one plate and remove the charge carriers from the opposite plate, then the voltage would change instantaneously, too

Ratch

#### rumpfy

##### Active Member
bhu,
A capacitor is different to an inductor and your question seems not to recognise the differences between these devices.
A capacitor is a charge storage device. The charge in the capacitor (in the dielectric), and the voltage between the plates is given by the relationship; Q=C*V. Since the movement of charge is 'current', then Q = I*t.
When one changes the voltage across a capacitor, we are changing the charge. If we consider the rate of change of charge, there is a corresponding rate of change of voltage, which is related by the capacitance value. So for a step voltage change in the terminal voltage across a capacitor, there is a very high rate of change of voltage and consequently a high rate of change of charge. This high rate of change of charge, leads to a very large current flow.
With an inductor, the defining relationship is that; E = - L *di/dt. This is mathematically saying that the self induced voltage in an inductor is proportional to the rate of change of current in the inductor.
It is possible to force a sudden interruption to the flow of current in an inductor. In this case, the factor di/dt is very high and so the back emf across the inductor is also very high. However, when an inductor is connected across a voltage supply, the current will increase linearly with time and the rate of change of that current will be proportional to E/L.
So it is possible to have a high rate of change of terminal voltage with a capacitor and this requires a high current to either charge or discharge the capacitor.
Similarly, it is possible to have a high rate of change of current in an inductor, and this requires a high terminal voltage. Both of these situations require a supply with infinite current or voltage capacity, so often in the real world we dont see things exactly as they are supposed to be.
So your question was a bit not quite right, but you obviously have something in your mind. So maybe tell us what you had in your mind when you asked the question.
Hope this helps.

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