RIVISION / Electricity.

Current and Voltage

Atomic Structure

  • An atom has a positively charged nucleus and negatively charged electrons revolving around it.
  • Valence electrons in metals are free to move within the conductor and constitute an electric current.

Charge

The charge is an intrinsic property of matter by virtue of which it can exert electromagnetic force.

Interaction between charges

Conductors and Insulators

A substance which offers comparatively less opposition to the flow of current is known as conductors and substances which offer larger opposition are insulators.

Electric Potential and Potential Difference

The electric potential at a point is defined as work done in bringing a unit positive charge from infinity to that point. The potential difference between two points is defined as the difference in electric potentials at the two given points.

 

Models of Electric Current

Electric Current(I)

Flow of electric charges is called electric current, i.e, I=Qt.

Electron sea model

Electric current in a solid conductor is due to drift of a ‘sea’ of free electrons, which are free to jump onto any neighbouring atom.

Electron sea model


Drift velocity of Electron

Average velocity which an electron attains inside a metallic conductor due to the application of an electric field due to the potential difference.

Drift velocity of an electron

Battery and its working

A cell is a source of potential difference, which is created inside it due to internal chemical reactions.

At anode:                                  Cu(s)⇌Cu2+(aq)+2e−
At cathode:                                 Ag(aq)+2e−⇌2Ag(s)
A combination of cells is called a battery.

 

Electric Circuit

Electric circuit and circuit diagram

  • A closed-loop path which a current take is called an electric circuit.
  • Representation of an electric circuit through symbols is called a circuit diagram.

Circuit Diagram

 

Resistance and Ohm's Law

Ohm's law

The current flowing through an ohmic conductor is directly proportional to the applied potential difference between the two ends of the conductor.

Ohm's law

Resistance

Resistance is a measure of the opposition offered to the current flow in an electric circuit.

Factors affecting Resistance & How they affect

Resistance is:
  • directly proportional to the length of the conductor.
  • directly proportional to nature of the conductor.
  • directly proportional to the temperature of the conductor.
  • inversely proportional to the cross-sectional area of the conductor.

Resistivity

The electrical resistance offered by a substance of unit length and unit cross-sectional area is called resistivity.

Ohmic and Non-Ohmic resistors

Resistors which follow Ohm’s Law are called Ohmic resistors, and those which do not follow it are called Non-Ohmic resistors.

Ohmic vs Non-Ohmic conductors

Superconductors

Conductors which offer zero resistance to the flow of current are called superconductors.

 

Combination of Resistors

Combination of resistors

  • Two resistors are said to be combined in series if they carry the same current.
  • Two resistors are said to be combined in parallel if the same potential difference is applied to them.

Equivalent Resistance of a system of resistors

The equivalent resistance of two resistors is given as:
  • In series, Req=R1+R2
  • In parallel, 1Req=1R1+1R2

EMF and Terminal Voltage

  • EMF: The potential difference between the two terminals of a cell, when there is no current flowing through the circuit.
  • Terminal voltage: The potential difference between the two terminals of a cell, when current is flowing through the circuit. ​​

EMF and Terminal voltage


Electric Power and AC

Heating Effect of Current

Joule's Law:
  • Heat (H) ∝  square of the current (I).
  • H ∝ Resistance of the given circuit.
  • H ∝  Time (t) for which current flows through the conductor.
When a potential difference is established, it causes electrons to move i.e. flow of current.

Electric Power

  • The rate of doing work or rate of consumption of electrical energy is called Electric Power. If W is work done in time t, then P=Wt.
  • S.I unit is Watt(W). One watt of power is consumed when 1 A of current flows at a potential difference of 1 V.
  • The commercial unit of electrical energy is kilowatt-hour (kWh).
  • 1kWh=3,600,000J=3.6×106J
  • Represented as P=I2R and P=V2R
  • One kilowatt-hour is defined as the amount of energy consumed when 1kW of power is used for 1 hour.

 KEY NOTES

  Current:
       I = Q/t, Where I = Current
                                    Q = Net charge flowing.
                                    t = time
                •  Unit:
                     The unit of current is Ampere.
                     Q = Coulomb(C)
                     I = Ampere(A)
                     t = Second(s) 1 A = 1C/1s
  Potential Difference:
       The potential difference between two separate points is defined as the work done to move
 a unit positive charge from one point to another.
       V = W/Q
                •  Unit:
                    The unit of potential difference is Volt
                    1 Volt = 1 Joule/ 1 Coulomb
  Ohm’s Law:
       Current ∝ potential difference
       V ∝ I
       V = I R where, R = Resistance
                •  Unit:
                     R = Ω(Ohm)
                     1Ω = 1V / 1 A
                •  Factors on which resistance depends:
                          ○  R ∝ 1, when A and material constant l = length
                          ○  R ∝ 1/ A, when l and material constant A = perpendicular cross-section
                              R ∝l / A,
                              R = ρl / A, where ρ = resistivity
                •  Resistivity: Resistivity of a substance is equal to the resistance of a unit
 square of that substance.
                     Its Unitis Ωm.
  Resistance in a series connection:
                     R = R1 + R2 + R3 + … + Rn
                     
  Resistance in parallel:
                     
  Heating Effect of current:
                     Electric energy = VIt
                     Heat, H = VIt = I2Rt
                •  Application:
                     Electric iron, toaster, fused wire, etc.
Fused wire: a low-melting point wire connected in series with electric devices for safety.
  Electric power:
                     P = VI = I2R = V2/ R
                •  Unit: 1 kWh = 3.6 × 106J
                     1 W = 1V × 1A
  Ohm’ law: Under constant physical conditions (i.e., constant temperature, pressure etc.),
 the current flowing through a conductor is directly proportional to the potential difference 
across the conductor.
  Potential difference (which is measured in Voltage) is the cause of current
 (which is measured in Ampere).
  In conductors, flow of electrons constitute current. In a circuit current flow from positive
 terminal of the battery to the negative terminal, but electrons travels from negative termina
l to the positive terminal. The negative terminal of a battery is said to be at lower potentia
l and the positive terminal is said to be at higher potential.
  When a battery is not connected to any circuit, the potential difference across the terminals
 of the battery is equal to the EMF of the battery. (EMF = Electro Motive Force). Electromotive
 force, also called EMF, (denoted and measured in volts), refers to voltage generated by a 
battery or by the magnetic forceaccording to Faraday's Law, which states that a time varying 
magnetic field will induce an electric current.

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