Capacitors (Part 1) – A2 Physics Revision

Introduction

  • A potential difference is a difference in energy (electrical, in this case) between two places
  • If there’s a potential difference, electrons will try to flow to even it out
  • A capacitor is a component which stores electrical charge
  • Capacitors can be charged up by putting a potential difference across them
  • They can only discharge if a path between their two ends is made available
  • If you charge a capacitor from a battery and then remove the battery – breaking the circuit – the capacitor will retain the charge
  • You could then put a light bulb where the battery was – completing the circuit – and the current stored in the capacitor would flow through the bulb, lighting it up briefly

Structure

  • A capacitor is effectively a pair of metal plates, separated by an electrical insulator
  • The insulator (called the dielectric) could just be air, but the capacitor is more effective if a better insulator is used
  • To make small capacitors that can hold a lot of charge, large, flat plates and dielectrics are wrapped up tightly and put into small cylindrical cases with a wire at each end
  • Some capacitors are polarised and some are not
  • Polarised capacitors have a positive and a negative end
  • Non-polarised capacitors can be connected either way ’round without exploding being damaged

Charging Up

  • Normally, the free electrons in the capacitor are spread out evenly
  • When a potential difference is connected across the capacitor, the electrons are attracted to the positive charge
  • The electrons gather on the plate connected to the positive end of the battery
  • The plate connected to the negative end of the battery loses the majority of its electrons
  • As long as the battery is connected, the electrons are forced on to one plate
  • The potential difference of the battery determines how many electrons it is able to squeeze on to the plate
  • As the build up of electrons grows, it becomes harder and harder for the battery to force more electrons around, as they don’t appreciate being crammed together
  • This is why charging a capacitor starts off fast and gets slower towards the end
  • When there are so many electrons on one plate that the potential difference between the two plates is equal to the potential difference being used to charge the capacitor, the charging can go no further

Storing

  • When the battery is disconnected, the electrons try to spread out evenly again
  • They find themselves trapped on one plate, because the circuit has been broken
  • The other plate, with its lack of electrons, is now positively charged
  • The electrons are attracted to it, because their magnetic field can sense the positive charge through the dielectric, and pulls them towards it
  • Since the dielectric doesn’t conduct electricity, there’s no way for the electrons to actually reach the other side
  • They stay like this until somebody offers them an escape route by connecting the two ends of the capacitor
  • (They don’t quite stay like that forever, or we’d be using capacitors instead of batteries. Electrons can gradually leak from one side to another, so over a long period of time the capacitor will discharge on its own)

Discharging

  • When a path becomes available between the two plates, the electrons can spread out
  • This happens very quickly to begin with, as the potential difference between the two plates is high
  • The potential difference decreases as more electrons reach the other plate
  • This means that the electrons spread out less quickly as time goes on
  • The capacitor discharges in the opposite direction from which it charged (when charging, the electrons moved towards one plate, and when discharging, they move towards the other, back to where they were originally)
  • Putting a resistor on the discharging path limits the current, so the electrons can’t flow as fast, and the capacitor takes longer to discharge
  • The battery, if left connected, is not a suitable discharge route because its potential difference is still forcing the electrons on to the plate

Data, Formulae and Relationships Booklet

  • No equations to remember from this post, thankfully
  • There are some coming in parts two and three, though!
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About Matt

I like writing, filmmaking, programming and gaming, and prefer creating media to consuming it. On the topic of consumption, I'm also a big fan of eating.
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