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## Class 12 Physics (India)

### Course: Class 12 Physics (India)>Unit 2

Lesson 9: Capacitance & parallel plate capacitors

# Capacitance

Introduction to the capacitance of a two-plate capacitor. . Created by Sal Khan.

## Want to join the conversation?

• Why do we need a capacitor? •   Or, in the case of musical equipment, they store charge that the circuit requires to operate other components. Once the ON switch is hit, they release the charge stored to fire up parts of the circuit. Like a quick burst of power (i.e. camera flash). Also, they're used in Equalizers, crossovers and tone control as they're important to create filters. Especially important in vacuum tube amplifiers.
• Why do the field lines at bulge out when they get to the edges? •  just imagine that there were a positive charge kept near the edge of the positively charged plate then the charge would be pushed outward because the horizontal components due to the charges on the left are not cancelled.but when is more near to the negatively charged plate there would be a net attractive force on it and it would curve inwards hence giving the field lines that kind of shape.
the field lines at the edges of a capacitor plate have the same shape as of the field lines due to an electric dipole.you might want to see earlier videos to get that
• Whats the difference between a capacitor and a battery? •  Batteries use an ongoing chemical reaction to generate a potential difference between two terminals. Capacitors can store charge like a battery, but have no internal chemistry to maintain a potential; however they can discharge very rapidly whereas batteries have a limit on how much current they can output. Once the charge in a capacitor has been used up, it cannot be replenished until the capacitor is charged by an external current. Capacitors have high power density, but low energy density. Batteries have low power density, but high energy density.

If someone can invent a device which has both high power and energy density, they will be very rich.
• why a capacitor does not allows dc current and why it allows ac •   You can imagine a capacitor like a "balloon", and the DC as a man blowing on the balloon.
The balloon at the start allows the man to keep blowing into it but at some point the balloon can no longer stretch and no more flow of air can get in.

For AC you can imagine it as man who inhales and exhales in the balloon (Don't try this at home XD) The balloon expands as the man exhales and contracts while the man inhales the air he blew in. You can see that air is constantly flowing in and out of the balloon.

The analogy is an oversimplification but it gives you a rough idea of how a capacitor works for electricity in AC and DC. DC stands for Direct Current and so the electric current only flows in directly in one direction (man constantly blowing), while AC stands for Alternating Current where it alternates electricity from one direction to the opposite direction back and forth (man inhaling and exhaling). The flow of the air in my analogy is the flow of electricity and so there's an initial flow during DC when the capacitor is empty/discharged but soon doesn't allow a flow when its fully charged whereas the capacitor constantly charges and discharges during AC allowing a constant flow of electricity.
• At he goes from work= charge*Electric Field*distance to work=Electric Field*change in joules. How did he arrive at the second definition? Would that not give you (N/C)*J, which is not the units that work is in. • Since the charge is 1 in his example, he drops the "charge" from the work equation. It then becomes E*d, with units J. While it is easy to confuse d as distance, when placed next to the unit J, with the notation for an infintesimal part of the unit, this is not what was intended in the circumstance here! In other words, it is d=distance, not dJ="change in Joules."
• why we always talk about INFINITE charged plates?why infinite?? • How to know when is the Capacitor fully charged? Is there a Formula? • Does current flow or not when capacitors are connected in a circuit? • Raghav's answer is partially right.
DC current can flow through a capacitor when it is not fully charged. While he is correct that no current flows across the capacitor gap, a current will still flow through the circuit because as electrons accumulate on one side of the plate, they will repel the electrons on the other side of the plate through electrostatic repulsion causing a DC current to flow. Once the capacitor is fully charged, then no more electrons can accumulate and the DC current will be zero.
• In , where did he get Joules from? It was 1C(E)d =E(d)J ? I that later he divides that J/1C = V, but where did the J come from? • Work has to be in Joules, right? Work is force*distance, and force is qE and distance is d, so the units of qE*d must somehow be joules. Sal knows that he used SI units all along, so he already knows that the formula for work is going to give him Joules, so he writes Joules.

To see that indeed it does come out to be joules, note that the units of E can be expressed EITHER as N/C, or V/m (those are the same thing). If we take E in V/m, then you have (just using units )
work = C*(V/m)*m, which simplifies to C*V, and a volt is a joule per coulomb, so you end up with joules.

If you want to use the other route , you have C*N/C*m, which is n*m, which is, again, a joule. 