Ch 10: Electricity and Magnetism

Question 1

In considering the capacitance (or charge stored per unit voltage), what would the effect on capacitance be if you decreased the distance between the plates?

Answer 1

Decreasing the distance (which is in the denominator of the equation) would INCREASE the capacitance.

(From Princeton Review, Practice Exam 1)

Question 2

Capacitors in parallel all have the same _______ (just like resistors in parallel), but the equivalent capacitance is the ____ of the _______ _________, like resistors in a series.

Answer 2

voltage
SUM of the individual capacitances

p. 341

Question 3

If the cell membrane is modeled as a pair of parallel plate capacitors, and the Na+/K+ ATPase pump is the source of the voltage, what is happening in the following graph?

Answer 3

The Na+/K+ pump is inactive, therefore the charge (Q) can no longer be maintained.

Question 4

What is capacitance?

Rearrange this equation to give the formula for the charge on a capacitor.

Answer 4

The ratio of charge to voltage for a capacitor:
C = Q / V

Q = CV

p. 327, p. 485

Question 5

Why is the NET charge on parallel-plate capacitors zero?

Answer 5

Bc one plate carries a positive charge, while the other carries a negative charge.

p. 326

Question 6

While charging a circuit with a battery and a parallel-plate capacitor, charge will stop flowing when the potential difference between the plates matches the….

Answer 6

…voltage of the battery.

p. 327

Question 7

Explain the units of capacitance.

Answer 7

Since capacitance is charge per unit of voltage, the units are coulomb/volt, which has its own name: the farad (F).

p. 327

Question 8

Capacitance is determined only by the sizes of the plates and how far apart they are (also whether there’s anything between the plates).

Give the formula for capacitance in terms of area and distance.

Answer 8

Question 9

What is the value of ε₀ (the permittivity of free space constant)?

Answer 9

8.85 x 10^-12 F/m

Note, this number is equal to 1/(4πk₀), where k₀ is Coulomb’s constant.

p. 328

Question 10

What is resistivity?

Answer 10

The intrinsic resistance of a material.

It is different from resistance because resistance depends on how we shape the material.

p. 302

Question 11

What formula relates resistance to resistivity?

Answer 11

• R =* ρ (L/A)
• Note*: Don’t confuse ρ (rho) with density. Here, it is the notation for resistivity.
  p. 302-303

Question 12

Resistivity is the inverse of __________.

Answer 12

conductivity

[From AAMC Practice Exam 3, #39]