Capacitors Flashcards
What does a capacitor do?
Stores electrical energy
Define capacitance
The amount of charge a capacitor is able to store per unit of potential difference across it, Capacitance is measured in Farads (F)
Define one Farad (F)
1 Coulomb per Volt
Describe the structure of a capacitor
It consists of two conducting plates that are separated by an insulating material called a dielectric, the plates store opposite chares when a voltage is applied across them
On a graph of charge against voltage what does the gradient show?
Capacitance
What is represented by the area under a graph of charge against voltage?
The energy stored by the capacitor
What is produced between capacitor plates when a voltage is applied?
A uniform electric field
Describe the structure of a dielectric
A dielectric is an insulating material placed between capacitor plates, it is made up of polar molecules that have a positive and negative end
Describe how the polar molecules of a dielectric behave when the capacitor is uncharged
The polar molecules are arranged randomly
Describe how the polar molecules of a dielectric behave when the capacitor is charged
The uniform electric field between the plates causes the polar molecules to align with the plates facing towards their opposite charges (due to the opposites attracting)
What is meant by the term ‘permittivity’?
It is a measure of how well a material can store electrical energy in an electric field, it describes how much the electric field is reduced compared to a vacuum when the material is present
What is relative permittivity?
The ratio of a material’s permittivity to the permittivity of freespace, it shows how much better a material is at reducing the electric field compared to a vacuum
What is another name for relative permittivity?
The dielectric constant
What is the relative permittivity of air?
1
What does it mean if one material has a higher relative permittivity than another?
The material with the higher relative permittivity can store more electrical charge when the same amount of voltage is applied to both materials
Draw the 3 charging graphs
Look in booklet for answers
Draw the 3 discharging graphs
Look in booklet for answers
Describe what happens as a capacitor is charged up
When a voltage is applied across the two plates (and the circuit is complete), electrons are from one plate are attracted towards the positive terminal of the cell, leaving this plate positively charged as it has lost electrons, the electrons then flow around the circuit and build up on the other plate causing it to become negatively charged, as it gains electrons
Describe what happens as a capacitor is discharged
When a charged capacitor is connected across a circuit with no battery the electrons flow fron the negatively charged plate back to the positively charged plate, so the negative plate looses electrons and becomes more positive and the positive plate gains electrons and becomes more negative, this neutralises the positive plate and removes excess electrons from the negative plate, which gradually reduces the p.d. across the plates until it is fully discharged
What is the time constant?
It is a measure of how quickly a capacitor charges or discharges, it is the time taken for charge, current or voltage to REACH 63% of its final value during charging or to FALL by 37% of its initial value during discharging
