Capactiors

Attachments:

• A Level Physics

1. Capacitor Basics
   1. Capacitors are devices that store electrical charge. They are made by having 2 metal plates separated by an air gap.
   2. The amount of charge a capacitor can store per volt is the capacitance (C) measured in Farads (F). It can be calculated using the potential difference across the capacitor when fully charged, and the charge stored (Q)
2. Charging and Discharging Capacitors
   1. When a power supply is connected to a capacitor, positive and negative charge build up on opposite plates. This creates a potential difference. This creates a uniform electric field between the plates.

      Annotations:

      • Charging of a capacitor
      1. The charging and discharge of a capacitor happens exponentially (starts off quick, then slows down.)
   2. If a capacitor is disconnected from a cell and then used connected to a resistor, it'll discharge. This is because the electrons will move over to the positive side of the capacitor and by discharging through a resistor.
   3. We can calculate the new voltage (V) after a given amount of time discharging (t) given that we know the initial voltage (Vo) and the time constant of the capacitor (RC)
      1. The equations for the charge and discharge of a capacitor all follow the same pattern to show the changes in charge, current and voltage
   4. The time constant is the time a capacitor will take to discharge down to 37%.
      1. If we know the capacitance of the capacitor (C), and the resistance of the resistor (R) that it is discharging through, we are able to calculate the time constant of the capacitor
         1. T=RC
3. Energy stored in a capacitor
   1. A charged capacitor is a store of electrical potential energy. When the capacitor discharges, electric potential energy is transferred into other forms.
   2. When calculating electric potential energy, our value is halved. This is because work is done when charging a capacitor, this leads to some energy being dissipated.
      1. This leads to the energy stored being the area under a Voltage (V) against Charge (Q) graph.