# physics capacitors

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 Author: ghoran ID: 292491 Filename: physics capacitors Updated: 2015-01-07 12:33:26 Tags: phsyics Folders: Description: revision Show Answers:

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1. capacitor
device that stores electrical charge and therefor electrical energy
2. capacitor consists of
two conductors placed near each other but not touching
3. parallel plate capacitors are separated by
an electrically charged insulating material called a dielectric
4. one capacitor plate is
positively charged the other is negatively charged
5. electric potential of positive plate exceeds that of the negative plate by an amount
V
6. when the magnitude of the charge on each plate is doubled the electric potential is
• doubled
• Q is proportional to V
• Q=CV where c is capacitance and is a constant
7. units of capacitance
• CV-1
• F
8. 1uF =
1*10-6F
9. 1nF
• 1*10-9F
10. 1pF
1*10-12F
11. capacitance
the charge stored per unit potential difference across a capacitor
12. initially the flow of charge , current , to an uncharged capacitor is
high as time passes the rate reduces as the charge from the cell must overcome the build of of charge on the plate
13. on charging electrons flow from
negative terminal of cell to one end of plate (which becomes negative) and then from the other end of the plate (which becomes positive) to positive terminal of cell
14. it isn't possible to store more and more charge by applying an ever increasing pd across the capacitor
eventually the pd becomes too large for the insulating material between the plates . electrical breakdown then occurs and conduction takes place across the gap
15. various dielectrics are used to
increase the capacitance , the material used and its thickness determine the max working pd of the capacitor
16. for a Q VS pd graph the gradient is
C
17. .... must be done to charge up a capacitor therefor
• work
• electrical potential energy is stored as a result
18. the amount of energy stored depends on
charge stored by the capacitor and the pd across it
19. when a capacitor is being charged
charge flows on to the capacitor
20. the battery has to
transfer energy to do work against the repulsive force from the charge already on the capacitor
21. work done in moving charge through a pd is
W=QV
22. ... across a capacitor isn't constant
pd
23. average pd is
0.5pd (pd increases from 0 to max pd when capacitor is fully charged)
24. work done in charging a capacitor is therefor
0.5QV
25. energy stored by the capacitor is therefor
0.5QV
26. Q=CV so energy stored also =
0.5CV2 or 0.5Q2/V
27. draw a Q vs V graph
28. draw a V vs Q graph
what does the area under the graph represent
• 1/C
• work done in charging a capacitor , energy stored by a capacitor 0.5QV
29. capacitor discharge circuit
30. charging through a fixed resistor
when an uncharged capacitor is placed in a circuit to be charged the initial current I0 depends on the
• emf of the battery and the resistance of the circuit
• I0 = V0/R
31. as charge accumulates on the capacitor the
pd across it increases .
32. the pd opposes that of the battery and it becomes
increasingly difficult to push charge on to the capacitor and the charging current drops
33. eventually the pd across the capacitor is
equal but opposite to that of the battery and the charging current drops to zero
34. as the capacitor charges the pd across the resistor
gets smaller because the pd across the capacitor gets bigger and so the current drops
35. Q ∞ V sp Q vs t graph is the same as V vs T graph
draw these graphs for charging through a fixed resistor
36. the rate at which the current drops depends on
resistance and on the capacitance of the circuit
37. the larger the resistance
the lower the current the capacitor therefor takes longer to charge
38. a large value of capacitance
also increases the charging time since it takes more charge to fill the capacitor
39. the product RC is known as
the time constant for the circuit
40. draw an I vs T graph for charging through a fixed resistor
41. time constant is the time
it takes for current to drop to 1/e of its original value
42. draw a V vs T graph for charging across a fixed resistor showing curves for pd across capacitor and pd across resistor

• where :
• A = across capacitor and
• B = across resistor
• Vc + Vr = EMF
43. exponential decay curves
have the property that they always take the same time to decrease by a given fraction
44. to discharge a capacitor
take out the battery and reconnect the circuit
45. when a charged capacitor is connected across a resistor the pd
drives a current through the circuit
46. the current flows in the
opposite direction from the charging current
47. the capacitor is fully discharged when
the pd across the plates and the current in the circuit are both zero
48. draw the three  vs T graph for discharging through a fixed resistor
49. the I vs T graph is the same as the one for charging
the current starts off relatively high and gradually decreases to zero . this is because  initially when a charged capacitor is connected across a resistor the pd drives a current through the circuit but the pd across the resistor decreases as the charge on the capacitor decreases
50. Q =
Q0e-t/RC
51. I =
I0e-t/RC
52. V =
V0e-t/RC
53. time constant is the
• time taken for charge/voltage/current on a discharging capacitor to fall to 0.37Q0/V0/I0
• its's also the time taken for voltage/charge on a charging capacitor to rise to 0.63V0/Q0
• it's also the time taken for current on a charging capacitor to fall to 0.37I0
54. Q =
It
55. capacitors discharge energy quickly so they can be used in
applications where a lot of energy is required rapidly , a taser is an example . Filters also use capacitors to smooth electrical signals
56. if we have two capacitors in parallel the voltage of each capacitor has to be the same because
the capacitors are in parallel
57. if we have two capacitors in parallel the final charge in each capcitor is
• capacitor A
• ------------         x total charge
• total capacitance

• and the same goes for B
• total charge is Q=CV for capacitor that is being charged
• voltage can be worked out in the same way but you will find the voltage is the same for both capacitors as discussed in previous question
58. if we have two capacitors in parallel the final energy stored in each capacitors is
E = 0.5CV2 where v is the final pd across the capacitor
59. why is there a loss of energy stored
because battery has to transfer energy to do work against the repulsive force from the charge already on the capacitor so energy is wasted and hence less is stored
60. battery has more energy because
pd in battery is constant so E=QV

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