Saturday, June 27, 2009

2. CURRENT ELECTRICITY

2. CURRENT ELECTRICITY
1.DEFINE “CURRENT ELECTRICITY”.
The branch of Physics which deals with the study of motion of
electric charges is called current electricity.
2.DEFINE “EMF” (OR) “ELECTRO MOTIVE FORCE”
The external energy necessary to drive the free
electrons in a definite direction is called electromotive force (emf).
3.DEFINE ELECTRIC CURRENT.WRITE UNIT,EXPRESSION,QTY.
The current is defined as the rate of flow of charges across any
cross sectional area of a conductor. If a net charge q passes through
any cross section of a conductor in time t, then the current I = q / t,
where q is in coulomb and t is in second.
The current I is expressed in ampere.Current is a scalar quantity.
4.DEFINE “DRIFT VELOCITY”.WRITE EXPRESSION.
Drift velocity is defined as the velocity with which free electrons
get drifted towards the positive terminal, when an electric field is
applied. vd =aτ
5.DEFINE MOBILITY. WRITR EXPRESSION.
The mobility is defined as the drift velocity acquired per unit electric field. μ = Vd / E. It takes the unit m2V–1s–1.
6.DEFINE “CURRENT DENSITY”. WRITE EXPRESSION,UNIT,QTY.
Current density at a point is defined as the quantity of charge
passing per unit time through unit area, taken perpendicular to the
direction of flow of charge at that point.
The current density J for a current I flowing across a conductor
having an area of cross section A is J =(q/t) /A = I/ A
Current density is a vector quantity. It is expressed in A m–2
7.STATE OHM’S LAW.
Ohm’s law states that, at a constant temperature, the steady
current flowing through a conductor is directly proportional to the
potential difference between the two ends of the conductor.
(i.e) I α V (or)V = IR .
8. DEFINE RESISTANCE OF A CONDUCTOR.
Resistance of a conductor is defined as the ratio of potential difference across the conductor to the current flowing through it. R =V/I
The unit of resistance is ohm (Ω)
9.DEFINE ELECTRICAL RESISTIVITY.
The electrical resistivity of a material is defined as the resistance offered to current flow by a conductor of unit length having unit area of cross section ρ. =R.A/L.The unit of ρ is ohm−m (Ω m).
10.DEFINE “CONDUCTANCE” AND CONDUCTIVITY”.
The reciprocal of resistance is conductance. ρ. = 1/R.
Its unit is mho (Ω–1).
The reciprocal of electrical resistivity, is called electrical
conductivity, σ = 1/ρ. The unit of conductivity is mho m-1 (Ω–1 m–1)
11.DEFINE CONDUCTORS BASED ON RESISTIVITY.
The resistivity of a material is the characteristic of that particular
material. The materials can be broadly classified into conductors and
insulators. The metals and alloys which have low resistivity of the order
of 10−6 – 10−8 Ω m are good conductors of electricity. They carry
current without appreciable loss of energy. Example : silver,
aluminium, copper, iron, tungsten, nichrome, manganin, constantan.
12.DEFINE INSULATORS (OR) NON-CONDUCTORS BASED ON RESISTIVITY.
Insulators are substances which have very high resistivity of the order
of 108 – 1014 Ω m. They offer very high resistance to the flow of current
and are termed non−conductors. Example : glass, mica, amber, quartz,
wood, teflon, bakelite.
13.DEFINE SEMICONDUCTORS BASED ON RESISTIVITY.
In between these two classes of materials lie the semiconductors .They are partially conducting. The resistivity of semiconductor is 10−2 – 104 Ω m. Example : germanium, silicon.
14.What are superconductors,superconductivity?
Ordinary conductors of electricity become better conductors at
lower temperatures. The ability of certain metals, their compounds and
alloys to conduct electricity with zero resistance at very low
temperatures is called superconductivity. The materials which exhibit
this property are called superconductors.
15.Define transition temperature (or) critical temperature.
The temperature at which electrical resistivity of the material suddenly drops to zero and the material changes from normal conductor to a superconductor is called the transition temperature or critical temperature TC.
16.What are the changes observed at transition temperature?
At the transition temperature the following changes are observed :
(i) The electrical resistivity drops to zero.
(ii) The conductivity becomes infinity
(iii) The magnetic flux lines are excluded from the material.
17.STATE FOUR APPLICATIONS OF SUPERCONDUCTORS
(i) Superconductors form the basis of energy saving power systems, namely the superconducting generators,
(ii) Superconducting magnets have been used to levitate trains above its rails. They can be driven at high speed with minimal expenditure of energy.
(iii) the current in a superconducting wire can flow
without any change in magnitude, it can be used for transmission
lines. (iv) Superconductors can be used as memory or storage
elements in computers.
18.WHAT ARE CARBON RESISTORS ?
Carbon resistor consists of a ceramic core,on which a thin layer of crystalline carbon is deposited. These resistors are cheaper, stable and small in size.
19.WHAT ARE COLOUR CODES IN CARBON RESISTORS?
The resistance of a carbon resistor is indicated by the
colour code drawn on it. Black 0, Brown 1, Red 2,Orange 3,Yellow 4
Green 5, Blue 6,Violet 7,Grey 8,White 9.

20.WHAT IS TOLERANCE?GIVE THE VALUES.
The silver or gold ring at one end corresponds to the tolerance. It is
a tolerable range ( + ) of the resistance. The tolerance of silver,
gold, red and brown rings is 10%, 5%, 2% and 1% respectively. If
there is no coloured ring at this end, the tolerance is 20%.

21.WHAT IS THE EFFECTIVE RESISTANCE OF RESISTORS IN SERIES?
RS = R1 + R2 + R3
Thus, the equivalent resistance of a number of resistors in series
connection is equal to the sum of the resistance of individual resistors.
22.WHAT IS THE EFFECTIVE RESISTANCE OF RESISTORS IN PARALLEL?
When a number of resistors are connected in parallel, the sum of the reciprocal of the resistance of the individual resistors is equal to the reciprocal of the effective resistance of the combination
1/RP = 1/R1+1/R2+1/R3.
23.DEFINE THE TEMPERATURE DEPENDENCE OF RESISTANCE.
The temperature coefficient of resistance is defined as the ratio of
increase in resistance per degree rise in temperature to its resistance at
0o C. Its unit is per oC. Rt = Ro (1 + αt)
24.DEFINE INTERNAL RESISTANCE OF A CELL
The electric current in an external circuit flows from the positive
terminal to the negative terminal of the cell, through different circuit
elements. In order to maintain continuity, the current has to flow
through the electrolyte of the cell, from its negative terminal to positive
terminal. During this process of flow of current inside the cell, a
resistance is offered to current flow by the electrolyte of the cell. This
is termed as the internal resistance of the cell.
25. STATE KIRCHOFF’S FIRST LAW (CURRENT LAW)
Kirchoff’s current law states that the algebraic sum of the currents meeting at any junction in a circuit is zero. The current flowing towards a junction is positive and the current flowing away from the junction
is negative. The sum of the currents entering the junction is equal to the sum of the currents leaving the junction. This law is a consequence of
conservation of charges.
26.STATE KIRCHOFF’S SECOND LAW (VOLTAGE LAW)
Kirchoff’s voltage law states that the algebraic sum of the
products of resistance and current in each part of any closed circuit is
equal to the algebraic sum of the emf’s in that closed circuit. This law
is a consequence of conservation of energy.
27.DEFINE “ELECTRIC ENERGY” AND “ELECTRIC POWER”.
Electric power is defined as the rate of doing electric work.
Power = Work done/ time =VIt/t = VI
Electric power is the product of potential difference and current
strength. Since V = IR, Power = I2R
Electric energy is defined as the capacity to do work. Its unit is
joule. In practice, the electrical energy is measured by watt hour (Wh)
or kilowatt hour (kWh). 1 kWh is known as one unit of electric energy.
1 kWh = 1000 Wh = 1000 × 3600 J = 36 × 105 J

28.WHAT IS WATTMETER?
A wattmeter is an instrument used to measure electrical power
consumed i.e energy absorbed in unit time by a circuit. The wattmeter
consists of a movable coil arranged between a pair of fixed coils in the
form of a solenoid. A pointer is attached to the movable coil. The free
end of the pointer moves over a circular scale. When current flows
through the coils, the deflection of the pointer is directly proportional
to the power.
29.STATE FARADAY’S FIRST LAW OF ELECTROLYSIS
The mass of a substance liberated at an electrode is directly proportional to the charge passing through the electrolyte.
If an electric current I is passed through an electrolyte for a time
t, the amount of charge (q) passed is I t. According to the law, mass of
substance liberated (m) is m α q or m = zIt
where Z is a constant for the substance being liberated called as
electrochemical equivalent. Its unit is kg C–1.
30.DEFINE ELECTROCHEMICAL EQUIVALENT.
The electrochemical equivalent of a substance is defined as the
mass of substance liberated in electrolysis when one coulomb charge
is passed through the electrolyte. m = zIt
where Z is a constant for the substance being liberated called as
electrochemical equivalent. Its unit is kg C–1.
31.STATE STATE FARADAY’S SECOND LAW OF ELECTROLYSIS
The mass of a substance liberated at an electrode
by a given amount of charge is proportional to the *chemical equivalent
of the substance. If E is the chemical equivalent of a substance, from the second law m α E
32.STATE THE APPLICATIONS OF SECONDARY CELLS.
The secondary cells are rechargeable. They have very low internal
resistance. Hence they can deliver a high current if required. They can
be recharged a very large number of times without any deterioration in
properties. These cells are huge in size. They are used in all
automobiles like cars, two wheelers, trucks etc. The state of charging
these cells is, simply monitoring the specific gravity of the electrolyte.
It should lie between 1.28 to 1.12 during charging and discharging
respectively.

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