Contact Us
Contact
Need assistance? Contact us on below numbers

For Enquiry

9:00am - 9:00pm IST all days.

Business Inquiry (North)

Business Inquiry (West / East / South)

OR

or

Thanks, You will receive a call shortly.
Customer Support

You are very important to us

For any content/service related issues please contact on this number

022-62211530

Mon to Sat - 10 AM to 7 PM

Current Electricity

Share this:

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.

Show more

NEET Physics Current Electricity Video Solutions by Experts

NEET Tests & Papers Solutions

Physics syllabus

Purchase Our Experts Course Packages

Enroll now to crack NEET

Testimonials

Ask Experts for NEET

Queries asked on Sunday and after 7 pm from Monday to Saturday will be answered after 12 pm the next working day.

View More

Chat with us on WhatsApp