capacitance save my exams

Question 1

what is a capacitor?

Answer 1

• an electrical device that stores charge

Question 2

what is the purpose of the dielectric?

Answer 2

• ensures charge does not flow freely between the plates

Question 3

what is capacitance

Answer 3

• charge stored per unit potential difference between the plates

Question 4

give the equation for capacitance

Answer 4

C = Q / V

C = capacitance ( F )
Q = charges on the plates ( C )
V = potential difference ( V )

Question 5

what is an objects capacitance?

Answer 5

• the ratio of charge stored by the capacitor to the potential difference between the plates

Question 6

what are dielectrics made up of?

Answer 6

• many polar molecules

( molecules with partial charges )

Question 7

what happens when there is a charge across the capacitor : polar molcules

Answer 7

• one plate becomes positively charged, the other negatively charged
• an electric field is generated
• negative ends of polar molecules align with positive plate, vice versa
• all molecules rotate and align themselves parallel to the electric field

Question 8

why does capacitance increase with dipole alignment?

Answer 8

• there is an original electric field (E•)
• when a dielectric is added dipole alignment occurs, creating opposite charges on the dielectric
• these aligned dipoles create their own E field as : E = E• - Ede
• as V = Ed, E goes down so V goes down
• C = Q/V so if V goes down, capacitance increases

Question 9

what is permittivity ?

Answer 9

• a measure of how easy it is to generate an electric field in a certain material

Question 10

what is relative permittivity?

Answer 10

• ratio of permittivity of a material to the permittivity of free space

Question 11

what is the equation to calculate capacitance of parallel plate capacitors

Answer 11

C = Aeoer / d

C = capacitance ( F )
A = cross sectional area ( m^2 )
eo = permittivity of free space
er = relative permittivity
d = seperation of the plates ( m )

Question 12

explain how capacitor charging works

Answer 12

electrons flow and build up on one side of the plate, creating a negatively charged plate

Question 13

what can we deduce about the potential difference across a capacitor?

Answer 13

• potential differences across the capacitor increases as the amount of charge increases

Question 14

what are the three equations for energy stored by a capacitor

Answer 14

E = 1/2QV

E = 1/2CV^2

E = Q^2/2C