Capacitance - MCFLY!

Question 1

what is a capacitor

Answer 1

an electric component in which charge is separated. consists of 2 metalic plates seperated from eachother by an insulator (dielectric).

Question 2

What does dielectric mean?

Answer 2

The material will polarise when in contact with an electric field.

Question 3

Define capacitance:

Answer 3

The charge stored per unit pd across it

Question 4

What is the relationship between charge and pd?

Answer 4

Charge on the capacitor is directly proportional to the pd.

Question 5

How to work out total capacitance in parallel?

Answer 5

Add them up. Together capacitance is greater than individual charges stored by the capacitors.

Question 6

Where does the energy in a capacitor come from in a capacitor and how Is it stored?

Answer 6

Energy stored comes from energy of battery or power supply. Work must be done in order to deposit negatively charged electrons onto the negative plate (like charges repel coulombs law) . Same way work is done to remove electrons from the positive plate since negative attracted to positive.

Question 7

How to calculate the work done on a capacitor?

Answer 7

Area under a pd - charge graph

Question 8

What is the time constant for a discharging capacitor?

Answer 8

The time taken for the pd to decrease to e^-1 (37%) of its initial value.

Question 9

How do capacitors work?

Answer 9

• When the capacitor is connected to the cell , electrons flow from the cell for a short time.
  
  • The brief current means electrons are removed from plate A and at the same time electrons are deposited onto plate B.
  • Plate A gets positive net charge and plate B gets negative net charge so there is no net overall charge
  • Current in the circuit falls to 0 when the pd across the plates is equal to the emf of the cell, capacitor is then fully charged
    The build of the electrons on one side means there is stored energy which can be used to do work

Question 10

Total capacitance In series?

Answer 10

Use kirchoffs second law. 1/c = 1/c1 + 1/c2 . Together their capacitance is less than their individual capacitances so the combination will store less charge for a given pd. All capacitors in series store the same charge even when they have different capacitances.

Question 11

Capacitor circuit practical:

Answer 11

• Ammeter, resistor, 2 capacitors and a resistor are all connected in series to a power supply
  
  • When switch is closed ammeter regsiters a current but quickly goes to 0 again which shows that electrons move only until capacitors are fully charged.
  • Pd across each capacitor can be measured using a voltmeter
  • Using c = q/v work out the charge stored by the capacitors which should be the same
    Any differences are because of uncertainties in the voltmeter readings and manufacturer values

Question 12

What is the equation for the energy stored in a capacitor?

Answer 12

W = 1/2QV
W = 1/2V^2C
W = 1/2 Q^2/C

Question 13

What graphs show how the capacitor discharges over time?

Answer 13

V-t, i-t, and q-t all show exponential decay graph

Question 14

What is the time constant of a capacitor resistor circuit?

Answer 14

Product of the capacitance and resistance.

Question 15

Time constant definition

Answer 15

Time constant t for a discharging capacitor is equal to the time taken for the pd charge and currentto decrease to about 37% of its initial value

Question 16

In parallel voltage is the same across each parallel branch of the circuit
Charge is shared as current is shared

Question 17

In series

Answer 17

Each capacitor will experience the same current so it will acquire the same charge
Pd is shared because of Kirchhoffs 2 law.

Question 18

Micro

Answer 18

10 ^ -6

Question 19

What is the constant property ratio of exponentials

Answer 19

In equal time intervals the exponential function will be decreasing by the same factor.

Question 20

How do smoothing capacitors work

Answer 20

domestic supply is ac which is a sine wave. without capacitor output voltage would be positive cycles only but with the capacitor the output voltage is smoothed out.