Current Electricity Revision Notes - IIT JEE/NEET Preparation | Nucleon

Current Electricity

• ELECTRIC CURRENT
• Time rate of flow of charge through a cross sectional area is called
Current
if q charge flows in time interval ?t then average current is given by

• CONDUCTOR
• In some materials, the outer electrons of each atom or molecule are only weakly
bound to it. These electrons are almost free to move throughout the body of the
material and are called free electrons

• INSULATOR
• Another class of materials is called insulators in which all the electrons are tightly
bound to their respective atoms or molecules. Effectively, there are no free electrons.

• SEMICONDUCTOR
• In semiconductors, the behaviour is like an insulator at low levels of temperature. But at higher temperatures,
a small number of electrons are able to free themselves and they respond to the applied electric field. As the
number of free electrons in a semiconductor is much smaller than that in a conductor, its behaviour is in
between a conductor and an insulator and hence,

• MOVEMENT OF ELECTRONS INSIDE CONDUCTOR
• All the free electrons are in random motion due to the thermal energy and relationship in given by

• RELATION BETWEEN I & V IN A CONDUCTOR
• In absence of potential difference across a conductor no net current flows through a cross
section. When a potential difference is applied across a conductor the charge carriers
(electrons in case of metallic conductors) start drifting in a direction opposite to electric field
with average drift velocity. If electrons are moving with velocity vd, A is area of cross section
and n is number of free electrons per unit volume then,

• ELECTRICAL RESISTANCE
• The property of a substance by virtue of which it opposes the flow of electric current through it is termed
as electrical resistance. Electrical resistance depends on the size, geometry, temperature and internal
structure of the conductor

• Dependence of Resistance on various factors
• Dependence of Resistance on various factors

• Temperature Dependence of Resistivity and Resistance
• The resistivity of a metallic conductor nearly increases with increasing temperature. This
is because, with the increase in temperature the ions of the conductor vibrate with greater
amplitude, and the collision between electrons and ions become more frequent

• ELECTRICAL POWER
• Energy liberated per second in a device is called its power. The electrical power P delivered or
consumed by an electrical device is given by P = VI, where V = Potential difference across
the device and

• BATTERY (CELL)
• A battery is a device which maintains a potential difference across its
two terminals A and B. Dry cells, secondary cells, generator and
thermocouple are the devices used for producing potential difference in
an electric circuit. Arrangement of cell or battery is shown in figure.
Electrolyte provides continuity for current

• ELECTROMOTIVE FORCE
• Definition  : Electromotive force is the capability of the system to make the charge flow.
Definition II : It is the work done by the battery for the flow of 1 coloumb charge from lower potential
terminal to higher potential terminal inside the battery.

• RELATIVE POTENTIAL
• While solving an electric circuit it is convenient to chose a reference
point and assigning its voltage as zero, then all other potentials
are measured with respect to this point. This point is also called
the common point.

• KIRCHHOFF'S LAWS
• This law is based on law of conservation of charge. It states
that " The algebraic sum of the currents meeting at a point of
the circuit is zero " or total currents entering a junction equals
total current leaving the junction

• Kirchhoffs Voltage Law
• The closed loop can be traversed in any direction. While traversing
and if potential decreases put a negative sign. ?Assume sign
convention

• COMBINATION OF RESISTANCES
• A number of resistances can be connected and all the complicated combinations
can be reduced to two different types, namely series and parallel

• Equivalent of Resistors
• The potential difference across a resistor is proportional to the resistance. Power
in each resistor is also proportional to the resistance

• Resistances in Parallel
• In the figure (a) and (b) all the resistors are connected between points A and B so they are in
parallel

• WHEATSTONE NETWORK : (4 TERMINALNETWORK)
• The arrangement as shown in figure, is known as Wheat stone bridge
Here there are four terminals in which except two all are connected to each other
through resistive elements.

• SYMMETRICAL CIRCUITS
• Some circuits can be modified to have simpler solution
by using symmetry if they are solved by traditional
method of KVL and KCL then it would take much time

• GROUPING OF CELLS
• GROUPING OF CELLS
Cells in Series

• GALVANOMETER
• It consists of a pivoted coil placed in the magnetic field of a permanent magnet. Attached to the coil is
a spring. In the equilibrium position, with no current in the coil, the pointer is at zero and spring is
relaxed. When there is a current in the coil, the magnetic field exerts a torque on the coil that is
proportional to current. As the coil turns,

• AMMETER
• A shunt (small resistance) is connected in parallel with
galvanometer to convert it into ammeter; An ideal
ammeter has zero resistance
Ammeter is represented as follow

• VOLTMETER
• A high resistance is put in series with galvanometer. It is used to measure
potential difference across a resistor in a circuit

• Current sensitivity
• The ratio of deflection to the current i.e. deflection per unit current is

• POTENTIOMETER
• A potentiometer is a linear conductor of uniform cross-section with a steady current
set up in it. This maintains a uniform potential gradient along the length of the wire.
Any potential difference which is less than the potential difference maintained across
the potentiometer wire can be measured using this

• Application of potentiometer
• Application of potentiometer

• METRE BRIDGE (USE TO MEASURE UNKNOWN RESISTANCE)
• METRE BRIDGE (USE TO MEASURE UNKNOWN RESISTANCE)

• POST-OFFICE BOX
• Introduction. It is so named because its shape is like a box and it was originally designed to determine
the resistances of electric cables and telegraph wires. It was used in post offices to determine the
resistance of transmission lines

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