CAPACITORS

Question 1

What are capacitors made from

Answer 1

Two conducting plates separated by a gap or an insulating material.

Question 2

Why is there a PD across capacitor

Answer 2

The +ve and -ve plates build up charge charge but this cannot transfer due to the gap and therefore a uniform magentic feild is created +ve to -ve which cuases a P.D

Question 3

Capacitor equation

Answer 3

Q=CV. Charge = capacitance (farda) x voltage

Question 4

Capacitance in series circuit

Answer 4

The series the total is 1/c = 1/c1+1/c2 +…. This is opposite to reistance

Question 5

In parallel circuit for capacitance

Answer 5

You say Ctot = C1+C2+C3+….

Question 6

Capacitor graph for charge and P.D

Answer 6

The gradient of the graph gives the total capacitance, if there is more then one line, the total off all gradients are the capacitance

Question 7

For capacitor graph current against time

Answer 7

Area = It there fore the charge, so area under graph gives total charge stored by capacitor

Question 8

How to investigate capacitors in series and parrallel

Answer 8

• set up circuit with power supply, variable resistor and ammeter
• adjust the veraible resistor to keep current constant
• data logger contected to voltmeter can see voltage change over time
• open switch and let discharge
• redo with different type of circuit
• for analysis do a graph for current against time and charge against p.d

Question 9

Work done and capacitor and energy

Answer 9

Energy stored on a capacitor is equal to the work done to deposit charge on the plate

Question 10

Graph energy on capacitors

Answer 10

P.d across capacitor is equal to proportional to the charge stored on it, so graph straight line through origin and therefore E=1/2 x QV

Question 11

All equation for energy stored by capacitor

Answer 11

E=0.5(v)^2 x C. And E=0.5QV. And E = 1(Q)^2 / 2C

Question 12

Uses of capacitors

Answer 12

Camera flash, can discharge very quickly
Back up power supply, can store charge to mitigate small power interruptions
Smoothing out P.D, smooth out peaks and troughs

Question 13

What happens when charging a capacitor

Answer 13

When switch is closed charge starts to deposits on the negative terminal this then repels electrons off positive plate which causes an equal but opposite charge on positive plate, initially the current is high but once the capacitor is fully charged the current is 0

Question 14

Discharging capacitors how to

Answer 14

Set up charged capacitor with a data logger which can see voltage and current over time and set up a resistor of a known resistance for the capacitor to discharge through +_Q will get closer to 0 over time as current decreases

Question 15

Exponential decrease when discharging capacitor.

Answer 15

Q= Q0e^-t/CR. Q’s can be substituted to be I or V if looking for initial or end of either of the three.

When doing the maths divide the Q/Q0 before doing ant log removal of e

Question 16

When charging a capacitor maths.

Answer 16

Q= Q0(1-e^-t/cr). Q can be replaced by V or I to get charging levels for the capacitor.

Question 17

What affects time taken to charge or discharge a capacitor?

Answer 17

• the capacitance of the capacitor as affects amount of charge that can be transferred at a given voltage
• the resistance of the circuit as current is affected.

Question 18

What is time constant

Answer 18

Time constant = CR this is then time taken from the charage, P.D or current to fall to 37% of its initial value.

Also time taken from the charage, P.D or current to rise by 63% of maximum value when charaging

Question 19

How dose the discharge resistor affect time to discharge a capacitor?

Answer 19

The larger the resistor the in series with capacitor the longer it takes to discharge, the time for full charge of discharge is about 5CR