Current Electricity Flashcards
charge on an electron
-1.6 * 10^-19 C
1 coulomb of charge has a deficit of
6.25 * 10^18 electrons
define current
rate of flow of charge across a cross section of a conductor
define ampere
1 ampere is the current that flows in a circuit when a charge of 1 coulomb passes through it in 1 second
direction of current in a circuit
opposite to direction of motion of electrons
define potential
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
formula for potential
work done on getting charge from infinity to the point / charge
= W/Q
define potential difference
the p.d between two points is equal to the work done in moving a unit positive charge from one point on the other.
where is +ve of voltmeter connected
at higher potential point
define resistance
the obstruction offered to the flow of current by the conductor is called resistance
what is needed in a conductor for the continuous flow of current
a constant potential difference has to be maintained across its ends
statement of ohm’s law
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
resistance of a conductor is equal to
potential difference applied across its ends for the flow of unit current through it
define conductance
the reciprocal of resistance is called conductance (G)
unit is ohm^-1, or siemen (S)
slope of I/V graph
reciprocal of resistance of conductor
limitation of ohm’s law
applied only when temperature remains constant
slope of V/I graph
resistance of conductor
what are ohmic resistors
conductors which obey Ohm’s law are linear resistances
examples of ohmic resistors
all metallic conductors, nichrome, copper sulphate soln. with copper electrodes, dil. sulphuric acid
examples of non-ohmic resistors
LED, solar cell, junction diode, transistor, filament
(graph plotted for these is a curve V/I)
factors affecting resistance
- material
- length
- thickness
- temperature
relation of resistance with radius of wire
inversely proportional to square of radius
define specific resistance
resistance of a wire of that material of unit length and unit area of cross section
unit of specific resistance
ohm metre
factors affecting specific resistance
- material/substance (very low for metals, low of semiconductors, high for insulators)
- temperature (decreases with increase in temperature of semiconductors)
for what materials will specific resistance never change even on changing temp?
constantan, manganin
specific resistance and resistance changes
specific resistance- remains constant with change in length or area
resistance- does not remain constant
define conductivity
reciprocal of specific resistance is known as conductivity
material of resistance wires
manganin, constantan (effect of change in temp is negligible)
material of fuse wire
an alloy of lead and tin (because melting point is low, and resistance is more than connecting wire)
material of filament of bulb
tungsten (because it has high melting point, and glows)
material of heating element in appliances
nichrome wire (specific R is high, and resistance increases with increase in temperature)
define superconductor
a substance of zero resistance/ infinite conductance at a very low temperature
current persists even when there is no potential difference across the ends
(ex: mercury, lead, niobium)
what is electric cell?
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
define electromotive force emf
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
how can emf be measured
by connecting a voltmeter of high resistance across the cell
factors affecting emf
- material of electrodes
- electrolyte used in cell
factors NOT affecting emf
- shape of electrodes
- distance between electrodes
- amount of electrolyte
define terminal voltage V
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)
define voltage drop v
the work done per unit charge in carrying the positive test charge through the electrolyte
this voltage cannot be used
relationship bw emf and terminal voltage
emf= terminal voltage - voltage drop
terminal V depends on what
amount of current drawn from the cell
define internal resistance of a cell
the resistance offered by the electrolyte, inside a cell, to the flow of current. voltage drop occurs due to this.
relationship between emf, terminal V, internal r
emf of cell= Current(external resistance+internal resistance)
factors affecting internal resistance of a cell
- surface area of electrode (inverse)
- distance between electrodes (direct)
- nature and concentration of electrolyte (more electrolyte, more Ir)
- temp of electrolyte (inverse)
which combination to increase/decrease resistance
increase- connect in series
decrease- connect in parallel
facts about series connection
- current has single path for flow. potential difference across any resistor is directly proportional to resistance.
- p.d. is equal to sum of resistances
facts about parallel connection
- potential difference across all resistors is the same
- current is inversely proportional to its resistance.
relation between equivalent resistance and smallest resistance connected in parallel
equivalent resistance is always smaller than the smallest resistance connected in parallel
comparison between resistors joined in series vs parallel
equivalent resistance of series is more than when joined in parallel
work needed to move a charge Q through p.d. V is
W= QV
W= VI*t
formula for electrical energy supplied by the source
VIt= I^2Rt= V^2t/R
define electric power
the rate at which electrical energy is supplied by the source
formula for electric power
P= W/t
P= QV/t
P= VIt/t = VI
VI= IIR
SI unit of electric power
Joule per sec
define 1 watt of electric power
1 watt electric power is consumed when a circuit of 1 ampere flows through a circuit having potential difference 1 V
commercial units of electric energy
P= W/t
W= Pt
W= watt second
- watt hour
- killowatt hour
define watt hour
1 Wh is electrical energy consumed by an electrical appliance of 1 watt when used for 1 hour
from the rating of appliance, what can be calculated
- resistance of its filament when it is in use
- safe limit of current which can flow through the appliance when in use
formula for resistance of element of appliance when in use
R= (voltage rating)^2/ power rating on appliance
formula for safe current which can flow through appliance
safe I= power rating on appliance/ voltage rating on appliance
when will power consumed by a bulb be equal to power rating
only when bulb is connected exactly at the voltage rated on it.
formula for cost of electricity
electrical energy in kWh * cost per kWh
amount of heat produced in wire on passing current through it depends on
- amount of current passing through the wire (direct to square of current)
- resistance (direct)
- time (direct)
formula for heating effect of current
H= I^2*Rt joule
joule’s law of heating
H= I^2Rt joule
H= 0.24 * I^2Rt cal
