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Physics > Capacitors > Flashcards

Capacitors Flashcards

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1
Q

What is meant by the capacitance of an object?

A

The capacitance of an object is the amount of charge it is able to store per unit potential difference across it.

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2
Q

What is the unit of capacitance?

A

Farads (F)

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3
Q

What is a capacitor?

A

A capacitor is an electrical component which can store electrical charge.

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4
Q

Describe how a capacitor works.

A
  • Two parallel plates connected to a d.c. power source
  • One plate becomes negatively charged and the other positively charged
  • Plates separated by electrical insulator so no charge can pass between (potential difference builds up between them)
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5
Q

Describe the circuit needed to investigate V and Q using a capacitor.

A
  • Battery connected to a variable resistor, connected to an Ammeter, connected to a capacitor, connected to a switch, connected back to the battery.
  • Voltmeter across the battery
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6
Q

How could you investigate V and Q using a capacitor?

A
  • Charge capacitor using a constant current
  • Constantly adjust variable resistor to keep charging current constant for as long as possible
  • Record the p.d. at regular time intervals until it equals the battery p.d.
  • Plot a graph of fixed charging current against time taken to charge capacitor
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7
Q

What is the area under a charging current - time taken to charge capacitor graph?

A

The charge stored in the capacitor.

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8
Q

What do you get if you plot charge stored against p.d.? (using Q = )

A

A straight line through the origin - Q and V are directly proportional.

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9
Q

Give 3 examples of real world uses of a capacitor.

A
  • Camera flash - camera battery charges the capacitor over a few seconds, then the entire charge is dumped into the flash
  • ‘Ultracapacitors’ can be used in back-up power supplies
  • To smooth out variations in d.c. voltage supplies
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10
Q

How can you find the energy stored by a capacitor?

A

By using the graph of potential difference against charge for the capacitor (area under it).

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11
Q

What are the 3 main equations to do with energy stored by capacitors?

A 
E = 1/2 QV
E = 1/2 CV^2
E = 1/2 Q^2 / C
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12
Q

What is meant by permittivity?

A

Permittivity is a measure of how difficult it is to generate an electric field in a medium. The higher the permittivity of a material, the more charge is needed to generate an electric field of a given size.

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13
Q

What is relative permittivity? What is relative permittivity also know as?

A

The ratio of the permittivity of a material to the permittivity of free space. (permittivity of material / permittivity of free space).
Also know as the dielectric constant.

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14
Q

What is a polar molecule?

A

A molecule with a positive end and a negative end.

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15
Q

What happens to polar molecules between the plates of a capacitor when an electric field is generated between them?

A

The negative ends of the molecules are attracted to the positively charged plate and vice versa. This causes the molecules to rotate and align themselves anti-parallel to the electric field.

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16
Q

Describe how a capacitor is charged in terms of electrons.

A
  • Electrons flow from the negative terminal of the supply to the plate connected it (this plate becomes negatively charged)
  • At same time, electrons flow from other plate to positive terminal of supply, making that plate positive
  • These electrons are repelled by the negative charge on the negative plate and attracted to the positive terminal of the supply
  • Equal and opposite charge builds up on each plate
17
Q

What happens to the current when the capacitor is fully charged?

A

The current falls to zero.

18
Q

What happens if you charge a capacitor through a fixed resistor?

A

The resistance of the resistor will affect the time taken to charge the capacitor.

19
Q

Explain what happens to the p.d. and current when charging a capacitor through a fixed resistor.

A
  • P.d. across capacitor is zero at first, therefore no p.d. opposing the current, meaning an initial high current from battery
  • As capacitor charges, p.d. across it increases and p.d. across resistor decreases, and the current drops
  • Charge on capacitor is proportional to the potential difference across it
20
Q

Describe the I-t, V-t and Q-t graph when charging a capacitor.

A
  • I-t graph - Starts at initial current and is a curve which decreases with a less and less negative gradient
  • V-t graph - Starts at zero and is a curve which increases with a less and less positive gradient until capacitor is fully charged
  • Q-t graph - Same as V-t graph
21
Q

In the equation for charge of the capacitor at time t, what does each letter mean? (Q, Q0, t, R and C)

A

Q - charge of the capacitor at time t / C
Q0 - charge of the capacitor when fully charged / C
t - time since charging began / s
R - resistance of fixed resistor / Ohms
C - capacitance of capacitor / F

22
Q

What is the equation for voltage across a charging capacitor and the charging current?

A
  • Voltage equation is in the same from but with V instead of Q
  • Charging equation - I = I0e^(t/RC)
23
Q

How do you discharge a capacitor and why does it discharge when you do this?

A

Take the battery out of the circuit and reconnect the circuit. This discharges it because when a charged capacitor is connected across a resistor, the p.d. drives a current through the circuit which flows in the opposite direction from the charging current.

24
Q

Outline an experiment which investigates capacitors discharging.

A
  • First charge the capacitor
  • Then open the switch, remove the power source, and add a voltage sensor across the capacitor and a data logger across the ammeter and voltage sensor
  • Close switch and allow capacitor to discharge
  • When ammeter reading reaches zero, use computer to calculate charge on capacitor over time
  • Computer can then plot variety of graphs showing how current, p.d. and charge vary over time
  • Can also create log-linear plot of results to produce straight-line graphs
25
Q

Describe the I-t, V-t and Q-t graph when discharging a capacitor.

A
  • I-t graph - curve starting at initial current and gradually decreasing to zero
  • V-t graph - same as I-t graph but starts at V0 (p.d. across capacitor when fully charged)
  • Q-t graph - same as other 2 graphs but starts at Q0 (charge of capacitor when fully charged
26
Q

What is the equation for voltage across a discharging capacitor and the discharging current?

A

The same form as the charge equation.

27
Q

How do you change the decay of charge equation into a log equation?

A
  • Take logs of both sides
  • Split up the logs on the right hand side
  • Use the ln(e^A) = A rule to make the equation
28
Q

What do you get if you plot a graph of ln(Q) against t and what is the gradient equal to?

A

A straight line graph. The gradient is equal to -t/RC.

29
Q

What is RC?

A

The time constant.

30
Q

What two factors does charging and discharging times depend on?

A
  • The capacitance of the capacitor

- The resistance of the circuit

31
Q

What is the time constant?

A

When t=RC and it is the time taken for the charge on a discharging capacitor to fall to about 37% of the original charge.
Also time taken for the charge on a charging capacitor to rise to about 63% of Q0.

32
Q

How can you find the time constant using a graph?

A

By finding 37% (on a discharging graph) or 67% (on a charging graph) of Q0 and seeing which time it is equivalent to.

33
Q

What is the time to halve?

A

The time taken for the charge, current or potential difference of a discharging capacitor to decrease to half of the initial value.

34
Q

What is the equation for time to halve and what do the letters mean?

A

T1/2 = 0.69RC
R - resistance of fixed resistor
C - capacitance of the capacitor

Physics (8 decks)

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