Request a call back

Join NOW to get access to exclusive study material for best results

Current Electricity

Current Electricity PDF Notes, Important Questions And Synopsis

Current Electricity PDF Notes, Important Questions and Synopsis

SYNOPSIS

  • Electric current: The rate of flow of electric charges through a particular cross section. The SI unit of current is Ampere.
  • Ohm’s law: The current between the ends of a conductor is directly proportional to the potential difference applied across its end, provided all other factors remain constant.
    V = IR
  • Dependence of resistance on various factors:
    Resistance directly depends on length. Resistance is inversely proportional to area.
    With an increase in temperature, resistance increases.
  • Current density: The amount of current flowing per unit area around that point, provided the area is normal to the direction of current.
  • Drift speed, which is the magnitude of this velocity, is enormously small as compared to the thermal speed, which is not a vector and is much larger.
  • Metals have low resistivity: Range of ρ varies from 10–8 Ω m to 10–6 Ω m.
    Insulators such as glass and rubber have high resistivity: Range of r varies from 1022 to 1024 times more than metals.
    Semiconductors such as Si and Ge lie roughly in the middle range of resistivity on a logarithmic scale.
  • When a conducting substance is brought under the influence of an electric field begin mathsize 12px style straight E with rightwards arrow on top end style free charges (e.g. free electrons in metals) move under the influence of this field in such a manner that the current density begin mathsize 12px style straight J with rightwards arrow on top end style due to their motion is proportional to the applied electric field.

    begin mathsize 12px style straight J with rightwards arrow on top equals straight sigma straight E with rightwards arrow on top end style
    where σ is a constant of proportionality called electrical conductivity.
  • Temperature coefficient of resistivity:
    For pure metals, resistance varies linearly with the rise of temperature.

    begin mathsize 12px style straight R subscript straight T equals straight R subscript straight T subscript straight 0 end subscript open square brackets 1 + straight alpha open parentheses straight T minus straight T subscript straight 0 close parentheses close square brackets end style
  • Emf (electromotive force) is the name given to a non-electrostatic agency. Typically, it is a battery in which a chemical process achieves this task of doing work in driving the positive charge from a low potential to a high potential.
    The effect of such a source is measured in terms of work done per unit charge in moving a charge once around the circuit. This is denoted by ε.

  • Ohm’s law is obeyed by many substances, but it is not a fundamental law of nature. It fails if

    • V depends on I non-linearly. Example: ρ increases with I even if temperature is kept fixed.
    • The relation between V and I is non-unique. Example: GaAs
    • The relation between V and I depends on the sign of V for the same absolute value of V.

  • Kirchhoff's First Rule:
    At any junction of several circuit elements, the sum of currents entering the junction must equal the sum of currents leaving it.


    In the above junction, current I enters and currents I1 and I2 leave.
    Thus, I = I1 + I2.
    This is a consequence of charge conservation and assumption that currents are steady, i.e. no charge piles up at the junction.

  • Kirchhoff's Second Rule:
    The algebraic sum of changes in potential around any closed resistor loop must be zero.  This is based on the principle that electrostatic forces alone cannot do any work in a closed loop, because this work equals the potential difference, which is zero, if we start at one point of the loop and come back to it.

    When applied to a loop as shown above (which could be part of a larger circuit), this gives
    -(R1 + R2) I1 - R3 I3 - R4 I4 = 0.

  • Points to remember in the case of current loops:
    Choose any closed loop in the network and designate a direction (in this example, counter clockwise) to traverse the loop.

  • Go around the loop in the designated direction, adding emfs and potential differences. An emf is counted as positive when it is traversed from (−) to (+) and negative in the opposite case, i.e. from (+) to (−).

  • An IR term is counted negative if the resistor is traversed in the same direction of the assumed current and positive if in the opposite direction.

  • Equate the total sum to zero. 

  • The Wheatstone bridge is an arrangement of four resistances—R1, R2, R3 and R4. The null point condition is given by 

    begin mathsize 12px style therefore straight R subscript 1 over straight R subscript 2 equals straight R subscript 3 over straight R subscript 4 end style

    This is also known as the balance condition. If, for instance, R1, R2 and R3 are known, then R4 can be determined.
    begin mathsize 12px style straight R subscript straight 4 straight equals open parentheses straight R subscript straight 2 over straight R subscript straight 1 close parentheses straight R subscript straight 3 end style

  • In a balanced condition of the meter bridge,

    begin mathsize 12px style table attributes columnalign left end attributes row cell fraction numerator text R end text over denominator text S end text end fraction text = end text fraction numerator text P end text over denominator text Q end text end fraction text = end text fraction numerator text σl end text subscript text 1 end text end subscript over denominator text σ end text open parentheses text 100-l end text subscript text 1 end text end subscript close parentheses end fraction text = end text fraction numerator text l end text subscript text 1 end text end subscript over denominator open parentheses text 100-l end text subscript text 1 end text end subscript close parentheses end fraction end cell row cell text R= end text fraction numerator text Sl end text subscript text 1 end text end subscript over denominator open parentheses text 100-l end text subscript text 1 end text end subscript close parentheses end fraction end cell end table end style
    σ: Resistance per unit length of wire
    1: Length of wire from one end where the null point is obtained
    A potentiometer is a device to compare potential differences. Because the method involves a condition of no current flow, the device can be used to measure potential differences and the internal resistance of a cell and to compare the emfs of two sources.

Download complete content for FREE PDF
JEE Main - Physics
  Earthing provides a safety measure for electrical circuits and appliances. A thick metal plate is buried deep into the earth and thick wires are drawn from this plate; these are used in buildings for the purpose of earthing near the mains supply. Why only "thick" wire and "thick" metal plates are used?
Asked by aaryamanmodern | 21 Aug, 2023, 10:43: AM
ANSWERED BY EXPERT ANSWERED BY EXPERT
JEE Main - Physics
Sir the ans to this quesn is i3=-5/6 A so this means curretn flows from A to D (opp to current indicated in diagram) but through KVL we found PE at D is always greater than A, C,E hence it will always flow from D to other points. 1) How it is voilating Ienter=Iexit. 2)How current flows from A to D even though D is high PE?  
Question image
Asked by aaryamanmodern | 22 Jun, 2023, 03:10: PM
ANSWERED BY EXPERT ANSWERED BY EXPERT
JEE Main - Physics
what kind of instrument is used in suspension monitor of car and bikes?based on meter bridge.
Asked by ojasgarg96 | 26 Dec, 2022, 09:55: AM
ANSWERED BY EXPERT ANSWERED BY EXPERT
JEE Main - Physics
 do kirchoff current and junction rules apply on circuit containg bothcapacitors and resistors ?
Asked by ojasgarg96 | 25 Dec, 2022, 10:10: PM
ANSWERED BY EXPERT ANSWERED BY EXPERT
JEE Main - Physics
will the invention of room temperature superconductor will make the current understanding of register useless?
Asked by ojasgarg96 | 25 Dec, 2022, 10:26: AM
ANSWERED BY EXPERT ANSWERED BY EXPERT
JEE Main - Physics
why doesn't every metal becomes superconductor at low temperature?based on resistivity,how?
Asked by ojasgarg96 | 24 Dec, 2022, 01:28: AM
ANSWERED BY EXPERT ANSWERED BY EXPERT
JEE Main - Physics
Physics questions uploaded 
Question image
Asked by htrivedi94 | 23 Dec, 2022, 08:40: AM
ANSWERED BY EXPERT ANSWERED BY EXPERT
JEE Main - Physics
In the uploaded image, what is the potential differnce between the points x and y? Does current  flow through the 4 V battery in the middle, if not, can we remove it or will it bring change in the potential difference between x and y? Must the current flow for potential difference?
Question image
Asked by arjunsah797 | 21 Jan, 2022, 10:34: AM
ANSWERED BY EXPERT ANSWERED BY EXPERT
JEE Main - Physics
In the figure (in the uploaded image), will the current flow through the 4 V battery connected in the middle? Why?Why not? How will the current flow in the circuit?
Question image
Asked by arjunsah797 | 20 Jan, 2022, 11:51: AM
ANSWERED BY EXPERT ANSWERED BY EXPERT
JEE Main - Physics
Find equivalent resistance
Question image
Asked by ojastej235 | 29 Sep, 2021, 07:56: PM
ANSWERED BY EXPERT ANSWERED BY EXPERT

Be a Top Scorer in the Exam with Test Series and Test Generator!

Enrol Now
×