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Physics > Capacitance COPY > Flashcards

Capacitance COPY Flashcards

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1
Q

What is capacitance measured in?

A

Farads (F)

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2
Q

What is the most common type of capacitor?

A

The parallel plate capacitor

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3
Q

Define the capacitance of a capacitor

A

The charge stored per unit potential difference between it plates

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4
Q

What is the equation for capacitance?

A

C = Q/V

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5
Q

What does the graph of Q (on y) against V look like?

A

Straight line through the origin

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6
Q

What is the potential difference across a capacitor directly proportional to?

A

The charge stored on it

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7
Q

What does the gradient of a Q (on y) against V graph represent?

A

Capacitence

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8
Q

Describe how a capacitor is charged

A
  • A p.d. is applyed across an uncharged capacitor
  • There is a flow of e- in the circuit
  • The plate connected to the negitive terminal of the battery has e- pushed onto it so becomed negatively charged
  • On the other plate e- are drawn from it to the positive terminal of the battery. The plate is now positively charged.
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9
Q

How do you know when a capacitor is fully charged?

A
  • The plates have opposite charges, so there is a p.d. across them
  • When the p.d. across the capacitor is equal to that of the battery the charging stops
  • There is no longer current flow and each plate is left with equal and opposite amounts of charge
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10
Q

What is a diaelectric?

A

An insulator

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11
Q

Define permittivity

A

Permittivity is a measure of the difficulty of forming an electric field within a particular medium

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12
Q

What does it mean if something has a high permittivity?

A

A large charge is required to produce an electric field

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13
Q

What is the permittivity of free space?

A

8.85 x 10-12 Fm-1

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14
Q

Why is an electric field easy to create in a vacuum?

A

The permittivity of free space is very low so it is relatively easy.

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15
Q

What is the relative permittivity of a material also known as?

A

The dielectric constant

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16
Q

What is the dielectric constant equal to? (er = e1 / e0)

A

The ratio of the permittivity of a material to the permittivity of free space.

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17
Q

What is the relative permittivity of a material equal to?

A

The ratio of the permittivity of a material to the permittivity of free space. ( er = e1 / e0)

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18
Q

What happens when a polar molecules are placed in an electric field?

A
  • They will rotate and align with the electric field.
  • The allignment is anti-parallel so that the + end of one molecule faces the - of the capacitor plate and vice versa
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19
Q

What happens to the value of er when polar molecules are put in an electric field?

A
  • They have the effect of shielding the electric field
  • er increases
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20
Q

Why does er equal 1 in a vacuum?

A

There are no polar molecules hence no opposition to the field and so er = 1

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21
Q

What does a capacitor consist of?

A

Two pates separated by a dielectric material

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22
Q

What does the capacitance of a capacitor depend on?

A
  • The area of the plates
  • The distance between the plates
  • The relative permittivity of the dielectric
23
Q

What is the parallel plate equation concerning capacitors?

A
24
Q

What does the work done depend on?

A

The charge transferred and the potential difference

W = Q/\V

25
Q

How do you calculate the energy stored on a capacitor from a V (on y) against Q graph?

A

The energy stored on a capacitor with a charge Q and a voltage V is given by the area underneath the graph

26
Q

What does the energy of a capacitor equal in terms of Q and V?

A

E = 0.5QV

27
Q

How much energy supplied by the power source is gained by the capacitor?

A

Exactly half

The rest is lost as heat

28
Q

What does the energy of a capacitor equal in terms of C and V?

A
29
Q

Derive E = 0.5CV^2

A
30
Q

What does the energy of a capacitor equal in terms of Q and C?

A

E = Q^2 / 2C

31
Q

Derive E = Q^2 / 2C

A
32
Q

How does a capacitor discharge?

A
  • The charge decreases exponentially with time as e- on the negitive plate flow around to the positive plate
33
Q

What is the equation for the charge on a discharging capacitor?

A
34
Q

What is the equation for voltage on a discharging capacitor?

A
35
Q

What is the equation for current on a discharging capacitor?

A
36
Q

What is the graph of voltage against time for a discharging circuit?

A
37
Q

What is the graph of charge against time for a discharging circuit?

A
38
Q

What is the graph of current against time for a discharging circuit?

A
39
Q

What is I (0) equal to for a discharging circuit?

A
40
Q

What is charge equal to for a charging capacitor?

A
41
Q

What the the current equal to for a charging capacitor?

A
42
Q

What the the charge equal to for a charging capacitor?

A
43
Q

What does the graph look like for current (on y) against time for a charging capacitor?

A
44
Q

What does the graph look like for charge (on y) against time for a charging capacitor?

A
45
Q

What is the voltage equal to for a charging capacitor?

A
46
Q

How does the charging capacitor behave when there is a resistor in the circuit?

A

The p.d across the resistor decreases as the capacitor increases until the p.d. = the V supplied.

47
Q

What is RC (not the definition)?

A

The time constant for the discharging circuit. It is measured in seconds.

48
Q

Define the time constant

A

The time constant is the time for the charge / current / voltage to decrease to 1/e of the original value or approximately 37% of the original value in a discharging circuit.

49
Q

What happens to the value of the voltage if t = RC?

A
50
Q

Derive a formula that tells us the time it takes for the voltage on a discharging circuit to half.

A
51
Q
A
52
Q
A
53
Q

Sketch and label a graph of ln V against t.

A

Physics (15 decks)

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