Capacitance

Question 1

how do electrons flow around a circuit with a capacitor, and a what point do they stop flowing?

Answer 1

negative electrons build up on side of capacitor attached to negative terminal and electrons flow off plate attached to positive terminal to positive terminal

until p.d. across capacitor = E.M.F. of cell (this means capacitor fully charged)

Question 2

define capacitance

Answer 2

charge stored per unit potential difference

Question 3

how can a dielectric increase the capacitance of a capacitor?

Answer 3

polarisation of dielectric created internal electric field which partially cancels electric field created by charge on plates - therefore to return pd across capacitor to original value, additional charge must be stored onto plates

this means more charge can be stored per unit potential difference therefore increasing capacitance of capacitor

Question 4

define relative permittivity of a dielectric (dielectric constant)

Answer 4

factor by which the electric field between two charges is decreased by presence of dielectric, relative to a vacuum

Question 5

what is the relative permittivity or air?

Answer 5

one

Question 6

how do polar molecules in dielectric rotate?

Answer 6

rotate to align with field, negative part of dielectric molecule faces positive plate and vice versa

Question 7

equation for work done by the battery in charging the capacitor

Answer 7

W = 1/2QV

Question 8

how is energy stored on a capacitor related to energy supplied by the battery?

Answer 8

energy stored on capacitor = 1/2QV which is half energy supplied by battery = QV because half energy supplied by battery transferred is resistance of circuit

Question 9

define time constant

Answer 9

time for charging current to fall to 1/e of its original value

or time for charge/pd to reach 1-1/e of its maximum value