Dieletrics

Question 1

How can charge stored between plates be increased?

Answer 1

• inserting a dielectric between the plates
• dielectrics are electrically insulating materials that increase the ability of a parallel-plate capacitor to store charge when dielectric is placed between the plates of the capacitor
• polythene and waxed papers are examples

Question 2

What happens when a dielectric is placed between the oppositely charged parallel plates connected to a battery?

Answer 2

• becomes polarised
  
  • electrons are pulled slightly towards the positive plate
  • surface facing the +ve plate gains a -ve charge
  • vice versa
  • each molc has its own electrc field, which in the alignemnts opposes the applied magnetic field
• more charged stored on the plates because
  
  • the positive side of the dielectric attracts more electrons from the battery onto the -ve plates
  • the -ve side of the dielectric pushed electrons back to the battery from the +ve plates

Question 3

What is relative permittivity?

Answer 3

• ε_r
• _{aka dielectric constant}
• ratio of charge stored w/ the dielectric to the charge stored without the dielectric is defined as the relative permittivity of the substance
• ε_r = Q / Q₀
  
  • Q = charged stored by parallel-plates when space completely filled with dielectric
  • Q0 = empty
• also = C / C₀

Question 4

Water has a large relative permetivitty constant, what does this explain about waters oproperties?

Answer 4

explains why salts can dissolve in water

Question 5

What is the equation for capacitance when filled with dielectric?

Answer 5

• C = Aε₀ε_r / d
  
  • A is surface area of each plate
  • d is spacing

Question 6

What does this equation show how to make a large capacitance?

Answer 6

• increasing area
• decrease plate spacing
• filling the space between plates w/ dielectric with a larger relative permeativityy

Question 7

What happens in an alternating electric field?

Answer 7

• polar dipolses roate
• non-polar dipolses oscillate alternating ways and the field strength increases and decreases

Question 8

What is permittivity

Answer 8

measure of how difficult it is to generate an electric field in an equaitom

Question 9

Explain why there is an increase in the energy stored by the capacitor when the polythene sheet is pulled out from between the plates.

Answer 9

• Energy increases
• In the polar dielectric molecules align in the field with positive charged end toward the negative plate
• Work is done on the capacitor separating the positively charged surface of the dielectric from the negatively charged plate