capacitors

• A capacitor consists of two conductors separated by an insulator called dielectric.
• The SI unit of capacitance is farad (F).
• Capacitance is the ability to store charge.
• Permittivity - ε0 - The ability of a material to conduct electricity, how much a force is affected by the dielectric material
• Energy stored in a charged capacitor can be calculated using U = 1/2CV^2.
• Energy stored = Charge squared / 2 * Capacitance
• Charge stored = Capacitance x Voltage
• Charging time constant = RC
• Relative permittivity - the ratio of permittivity to the permittivity of free space
• Capacitance = charge / voltage
• Capacitance depends on the distance between plates (d), area of overlap (A) and relative permittivity (εr)
• Capacitance - the charge stored per unit applied voltage
• why is the charge storing ability of a capacitor increased by a dielectric?


  the molecules in a dielectric align with the electric field when a charge is applied across it. The negative ends of the molecules on the surface reduce the negative plates potential, and the positive ends on the surface reduce the positive plates’ potential. More charge is needed on the plates to make the potential difference equal the applied voltage - so capacitance is increased.
• How is a capacitor made?


  A positive metal plate is parallel to an equally negative metal plate seperated by a dielectric (insulating material)
• when there is no charge applied across a dielectric the polar molecules are aligned in random directions as there is no electric field between the plates.
• when there is a charge applied across a dielectric there is an electric field present between the metal plates, so the polar molecules of the dielectric align with the electric field lines.
• When a charge is applied across a dielectric, the polar molecules become aligned with the direction of the electric field. This reduces the strength of the electric field between the plates because the dipoles act like small charged particles that oppose the original charges.
• Work is done to store charge on a capacitor. As the negative charge on the plate increases the potential difference across the capacitor also increases so more work must be done to store more charge as this is increasingly difficult
• To charge a capacitor there is a flow of current from the battery and electrons build up on one plate forming positive and negative plates. As the negative charge builds up it gets increasingly difficult for more electrons to get onto the negative plate due to electrostatic repulsion. When no more electrons can get on the charging stops and the potential difference across the capacitor equals the EMF of the circuit.
• Energy stored = 1/2 x Q x V
• Capacitance is measured in farads (F). A farad is defined as the amount of capacitance when a potential difference of 1 volt causes a charge of 1 coulomb to appear on its plates.
• What is meant by a dielectric constant of 6.0

  The permittivity of the dielectric constant is 6 times the permittivity of free space
• What is meant by a capacitance of 370F


  370C of charge can be stored per 1 volt
• If the plates are connected to a source of emf then the capacitor will hold a certain amount of charge q and have a potential difference v across it. If the plates are disconnected from the source of emf then the charge remains constant but the potential difference decreases until it reaches zero.
• Capacitors have many uses such as smoothing out fluctuations in mains electricity supply, timing circuits, tuning radio sets, and storing energy from solar panels.