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Physics > Current Electricity > Flashcards

Current Electricity Flashcards

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1
Q

charge on an electron

A

-1.6 * 10^-19 C

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2
Q

1 coulomb of charge has a deficit of

A

6.25 * 10^18 electrons

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3
Q

define current

A

rate of flow of charge across a cross section of a conductor

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4
Q

define ampere

A

1 ampere is the current that flows in a circuit when a charge of 1 coulomb passes through it in 1 second

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5
Q

direction of current in a circuit

A

opposite to direction of motion of electrons

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6
Q

define potential

A

the potential at a point is defined as the amount of work done per unit charge in bringing a positive test charge from infinity to that point

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7
Q

formula for potential

A

work done on getting charge from infinity to the point / charge
= W/Q

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8
Q

define potential difference

A

the p.d between two points is equal to the work done in moving a unit positive charge from one point on the other.

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9
Q

where is +ve of voltmeter connected

A

at higher potential point

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10
Q

define resistance

A

the obstruction offered to the flow of current by the conductor is called resistance

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11
Q

what is needed in a conductor for the continuous flow of current

A

a constant potential difference has to be maintained across its ends

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12
Q

statement of ohm’s law

A

the current flowing in a conductor is directly proportional to the potential difference applied across its ends, provided that the physical conditions and temp of the conductor remains constant

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13
Q

resistance of a conductor is equal to

A

potential difference applied across its ends for the flow of unit current through it

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14
Q

define conductance

A

the reciprocal of resistance is called conductance (G)
unit is ohm^-1, or siemen (S)

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15
Q

slope of I/V graph

A

reciprocal of resistance of conductor

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16
Q

limitation of ohm’s law

A

applied only when temperature remains constant

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17
Q

slope of V/I graph

A

resistance of conductor

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18
Q

what are ohmic resistors

A

conductors which obey Ohm’s law are linear resistances

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19
Q

examples of ohmic resistors

A

all metallic conductors, nichrome, copper sulphate soln. with copper electrodes, dil. sulphuric acid

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20
Q

examples of non-ohmic resistors

A

LED, solar cell, junction diode, transistor, filament

(graph plotted for these is a curve V/I)

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21
Q

factors affecting resistance

A
  1. material
  2. length
  3. thickness
  4. temperature
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22
Q

relation of resistance with radius of wire

A

inversely proportional to square of radius

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23
Q

define specific resistance

A

resistance of a wire of that material of unit length and unit area of cross section

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24
Q

unit of specific resistance

A

ohm metre

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25
Q

factors affecting specific resistance

A
  1. material/substance (very low for metals, low of semiconductors, high for insulators)
  2. temperature (decreases with increase in temperature of semiconductors)
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26
Q

for what materials will specific resistance never change even on changing temp?

A

constantan, manganin

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27
Q

specific resistance and resistance changes

A

specific resistance- remains constant with change in length or area
resistance- does not remain constant

28
Q

define conductivity

A

reciprocal of specific resistance is known as conductivity

29
Q

material of resistance wires

A

manganin, constantan (effect of change in temp is negligible)

30
Q

material of fuse wire

A

an alloy of lead and tin (because melting point is low, and resistance is more than connecting wire)

31
Q

material of filament of bulb

A

tungsten (because it has high melting point, and glows)

32
Q

material of heating element in appliances

A

nichrome wire (specific R is high, and resistance increases with increase in temperature)

33
Q

define superconductor

A

a substance of zero resistance/ infinite conductance at a very low temperature
current persists even when there is potential difference across the ends
(ex: mercury, lead, niobium)

34
Q

what is electric cell?

A

it maintains a constant potential difference between the two conductors in the electrolyte by a chemical reaction to obtain a continuous flow of charge between them

35
Q

define electromotive force emf

A

when no current is drawn from the cell (when the cell is in an open circuit) the potential difference produced by the chemical reaction between two terminals of the cell is called emf

the energy spent per unit charge in taking a positive charge around the complete circuit of the cell

36
Q

how can emf be measured

A

by connecting a voltmeter of high resistance across the cell

37
Q

factors affecting emf

A
  1. material of electrodes
  2. electrolyte used in cell
38
Q

factors NOT affecting emf

A
  1. shape of electrodes
  2. distance between electrodes
  3. amount of electrolyte
39
Q

define terminal voltage V

A

work done per unit charge in transferring a positive test charge around the external circuit connected across the terminals of the cell (when cell is in closed circuit)

40
Q

define voltage drop v

A

the work done per unit charge in carrying the positive test charge through the electrolyte
this voltage cannot be used

41
Q

relationship bw emf and terminal voltage

A

emf= terminal voltage - voltage drop

42
Q

terminal V depends on what

A

amount of current drawn from the cell

43
Q

define internal resistance of a cell

A

the resistance offered by the electrolyte, inside a cell, to the flow of current. voltage drop occurs due to this.

44
Q

relationship between emf, terminal V, internal r

A

emf of cell= Current(external resistance+internal resistance)

45
Q

factors affecting internal resistance of a cell

A
  1. surface area of electrode (inverse)
  2. distance between electrodes (direct)
  3. nature and concentration of electrolyte (more electrolyte, more Ir)
  4. temp of electrolyte (inverse)
46
Q

which combination to increase/decrease resistance

A

increase- connect in series
decrease- connect in parallel

47
Q

facts about series connection

A
  1. current has single path for flow. potential difference across any resistor is directly proportional to resistance.
  2. p.d. is equal to sum of resistances
48
Q

facts about parallel connection

A
  1. potential difference across all resistors is the same
  2. current is inversely proportional to its resistance.
49
Q

relation between equivalent resistance and smallest resistance connected in parallel

A

equivalent resistance is always smaller than the smallest resistance connected in parallel

50
Q

comparison between resistors joined in series vs parallel

A

equivalent resistance of series is more than when joined in parallel

51
Q

work needed to move a charge Q through p.d. V is

A

W= QV
W= VI*t

52
Q

formula for electrical energy supplied by the source

A

VIt= I^2Rt= V^2t/R

53
Q

define electric power

A

the rate at which electrical energy is supplied by the source

54
Q

formula for electric power

A

P= W/t
P= QV/t
P= VIt/t = VI
VI= IIR

55
Q

SI unit of electric power

A

Joule per sec

56
Q

define 1 watt of electric power

A

1 watt electric power is consumed when a circuit of 1 ampere flows through a circuit having potential difference 1 V

57
Q

commercial units of electric energy

A

P= W/t
W= Pt
W= watt second

  1. watt hour
  2. killowatt hour
58
Q

define watt hour

A

1 Wh is electrical energy consumed by an electrical appliance of 1 watt when used for 1 hour

59
Q

from the rating of appliance, what can be calculated

A
  1. resistance of its filament when it is in use
  2. safe limit of current which can flow through the appliance when in use
60
Q

formula for resistance of element of appliance when in use

A

R= (voltage rating)^2/ power rating on appliance

61
Q

formula for safe current which can flow through appliance

A

safe I= power rating on appliance/ voltage rating on appliance

62
Q

when will power consumed by a bulb be equal to power rating

A

only when bulb is connected exactly at the voltage rated on it.

63
Q

formula for cost of electricity

A

electrical energy in kWh * cost per kWh

64
Q

amount of heat produced in wire on passing current through it depends on

A
  1. amount of current passing through the wire (direct to square of current)
  2. resistance (direct)
  3. time (direct)
65
Q

formula for heating effect of current

A

H= I^2*Rt joule

66
Q

joule’s law of heating

A

H= I^2Rt joule
H= 0.24 * I^2Rt cal

Physics (12 decks)

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