Year 2 Chapter 6: Capacitance

Question 1

What is capacitance?

Answer 1

The charge stored by a capacitor per unit potential difference

Question 2

What is the equation for capacitance?

Answer 2

C= Q/V
C: capacitance/ F
Q: charge/ C
V: potential difference/ V

Question 3

What is a capacitor?

Answer 3

An electrical component that stores charge. A parallel-plate capacitor is made of two parallel conducting plates with an insulator between them
(dielectric).

Question 4

What happens when a capacitor is connected to power?

Answer 4

Opposite charges build up on the two parallel conducting plates forming a uniform electric field between them

Question 5

What is a dielectric?

Answer 5

An insulating material placed between the two plates of a capacitor in order to increase the amount of charge it can store

Question 6

What is permittivity for dielectrics?

Answer 6

A measure of the ability to store an electric field in the material

Question 7

How is relative permittivity of a dielectric found?

Answer 7

Relative permittivity, εr, also called the dielectric constant is the ratio of the permittivity of the dielectric to the permittivity of free space

Question 8

What is the equation for relative permittivity?

Answer 8

εr = ε / ε0
εr: relative permittivity/ dielectric constant
ε: permittivity of dielectric
ε0: permittivity of free space

Question 9

What is the equation for capacitance using relative permittivity?

Answer 9

C = A ε0εr / d

Question 10

What are dielectrics made of and how does it behave?

Answer 10

Made of polar molecules which are normally randomly arranged but with an E field, they align themselves with the field, with + facing negative plate and - facing positive plate

Question 11

How does the behaviour of dielectric affect the capacitor?

Answer 11

When the molecules align, they have their own electric field, the strength of which depends on the dielectric permittivity. These fields oppose the capacitor’s E field, decreasing its strength, so V required to charge capacitor decreases meaning C increases

Question 12

How do capacitors get charged?

Answer 12

Current flows and negative charge builds up on the plate connected to the negative terminal. Electrons on the opposite plate are repelled by this negative charge building causing them to move to the positive terminal and an equal and opposite charge is formed on each plate, creating a pd. As the charge increases, pd increases but electron flow decreases as replusion increases meaning current reduces until zero.