Capacitance COPY Flashcards
What is capacitance measured in?
Farads (F)
What is the most common type of capacitor?
The parallel plate capacitor
Define the capacitance of a capacitor
The charge stored per unit potential difference between it plates
What is the equation for capacitance?
C = Q/V
What does the graph of Q (on y) against V look like?
Straight line through the origin
What is the potential difference across a capacitor directly proportional to?
The charge stored on it
What does the gradient of a Q (on y) against V graph represent?
Capacitence
Describe how a capacitor is charged
- A p.d. is applyed across an uncharged capacitor
- There is a flow of e- in the circuit
- The plate connected to the negitive terminal of the battery has e- pushed onto it so becomed negatively charged
- On the other plate e- are drawn from it to the positive terminal of the battery. The plate is now positively charged.
How do you know when a capacitor is fully charged?
- The plates have opposite charges, so there is a p.d. across them
- When the p.d. across the capacitor is equal to that of the battery the charging stops
- There is no longer current flow and each plate is left with equal and opposite amounts of charge
What is a diaelectric?
An insulator
Define permittivity
Permittivity is a measure of the difficulty of forming an electric field within a particular medium
What does it mean if something has a high permittivity?
A large charge is required to produce an electric field
What is the permittivity of free space?
8.85 x 10-12 Fm-1
Why is an electric field easy to create in a vacuum?
The permittivity of free space is very low so it is relatively easy.
What is the relative permittivity of a material also known as?
The dielectric constant
What is the dielectric constant equal to? (er = e1 / e0)
The ratio of the permittivity of a material to the permittivity of free space.
What is the relative permittivity of a material equal to?
The ratio of the permittivity of a material to the permittivity of free space. ( er = e1 / e0)
What happens when a polar molecules are placed in an electric field?
- They will rotate and align with the electric field.
- The allignment is anti-parallel so that the + end of one molecule faces the - of the capacitor plate and vice versa
What happens to the value of er when polar molecules are put in an electric field?
- They have the effect of shielding the electric field
- er increases
Why does er equal 1 in a vacuum?
There are no polar molecules hence no opposition to the field and so er = 1
What does a capacitor consist of?
Two pates separated by a dielectric material
What does the capacitance of a capacitor depend on?
- The area of the plates
- The distance between the plates
- The relative permittivity of the dielectric
What is the parallel plate equation concerning capacitors?
What does the work done depend on?
The charge transferred and the potential difference
W = Q/\V
