Capacitors

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• Created by: Sagaana
• Created on: 11-11-18 13:46
• Capacitors
• What is a capacitor?
• It is an component made up of two conducting plates separated by a gap or a dielectric.
• When  connected to a power source positive and negative charge builds up on the opposite plate creating a uniform electric Field
• Capacitance= charge per unit potential difference
• unit= Farad (this is a huge unit)
• Dielectrics
• Permittivity= how difficult it is to generate electrical field
• Relative Permittivity= the ratio of Pmaterial and Pvacuumn
• aka Dielectric Constant
• when no charge applied
• Polar Molecule that have a positive and negative end are in random directions
• When charge is applied
• polar molecules are aligned that negative ends are towards the positive plate and vice versa
• Polar Molecules have their own Electric Field and this opposes the applied electric field
• Larger permittivity the larger the opposing field
• so capacitance increases
• Capacitors Stores energy
• as charge builds up on the plates it stores electrical energy
• This can be seen in the area underneath the graph of charge against p.d
• Charging Capacitors
• Connect switch to uncharged capacitor to the power supply
• This should charge up the capacitor. Ammeter is 0 this means the capacitor is fully charged
• Current flows  ans electrons flow to the plate connected to the negative terminal so a negative charge builds up
• Build up of negative charge causes electrons to repel and attract to the positive plate.
• Equal and opposite charge builds up on each plate creating a potential difference
• As charge builds electrostatic repulsion makes it harder for more electrons to be deposit causing the ammeter to fall to 0.
• Discharging Capacitors
• Remove the power supply to discharge the capacitor
• when both the p.d and current is 0 the capacitor is fully discharged
• Time Constant
• time taken to charge depends on two things: capacitance and resistance
• Time taken to charge the charge/p.d/current of a discharging capacitor to fall to 37% and also the time take for capacitor to rise to 63%
• larger resistance the  longer it takes to charge and discharge.
• You can find the time constant using the log linear graph
• Half life =0.69RC