Electricity Class 10 Science Notes

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  • Electricity : Flow of electric charge constitute electric current. It is Rate of flow of electric charge. It is denoted by I. Unit is Ampere ‘A’.

 

    • I=Qt
    • One Ampere Current :  “One Coulomb of charge flows through a given cross sectional area in one second.” Current flows from positive to negative terminal.
    • Current is measured by connecting ammeter in series.
  • Electric Circuit : It is a continuous closed path through which electric current flows. It consists of battery, metallic wires, switch and various electrical devices. Direction of current flow specified by arrow pointing towards negative terminal.
  • Potential Difference : The difference in potential levels of two terminals in the circuit is called potential difference. P.D.=W/Q. Its unit is Volt (V). Potential difference of 1 Volt is the work done of 1 joule to move a charge of 1 Coulomb from one terminal to other. Voltmeter is a device connected in parallel to the device.
  • Circuit Diagram: Electrical circuit can be represented on paper using various electrical symbols, such diagram is called circuit diagram.

Electrical Cell    

Battery        

Closed Switch

Open Switch  

A Joint Wire  

Wire crossing without joining

Electric Bulb

Resistor  

Variable Resistance

Ammeter

Voltmeter

 

 

    • Ohm’s Law: “At constant temperature potential difference is directly proportional to current.”

 

  • V ∝ I
  • The graph of potential difference versus current is a straight line passing through the origin.

 

  • Resistance: The constant of proportionality in ohm’s law i.e. the slope of V vs I graph is resistance. Resistance opposes the current. Its unit is ohm (Ω).
    • V/I = Constant = R.
    • One Ohm Resistance : Current of one ampere flows due to potential difference of 1 V.

 

Good Conductors

Resistors

Insulators

Low Resistance and high conductivity

High resistance and moderate conductivity

Very high resistance and no conductivity

All Metals are good conductors

Metal alloys are resistors

Rubber, wood, plastic are insulators

Have resistivity ⍴ between 10-8 Ωm to 10-6 Ωm

have ⍴ between 10-6Ωm to 1012Ωm

have ⍴ between 1012Ωm to 1017Ωm

 

  • Resistance of a conductor depends on:
    • Resistance of a conductor is directly proportional to its length and inversely proportional to cross sectional area.
    • ∴ R ∝ l/A
    • Resistance also depend on nature of material. It is given by resistivity ⍴. Unit of resistivity is Ωm.
    • ∴ R=⍴lA
    • ∴ ⍴=R Al
    • Resistivity is the resistance per unit length per unit cross sectional area.
    • Resistivity of metals varies directly with temperatures.

 

  • Equivalent Resistance in Series:

 

    • Rs = R1 + R2 + … + Rn.
  • Properties of Series Circuit:
    • Current remains same, voltage get divided.
    • The equivalent resistance of a series circuit is the sum of individual resistances.
    • The equivalent resistance of series circuit is always greater than individual resistances.
    • From a given set of resistors highest resistance can be achieved by connecting resistors in series.
    • Damage of one resistor breaks the circuit so we can’t connect appliances in series.
  • Equivalent Resistance in Parallel :
      • ∴ 1/ RP = 1/R1 + 1/R2  + 1/R3.
    • Properties of Parallel circuit:

 

  • Current get divided, voltage remains same.
  • The equivalent resistance of parallel combination of resistors is equal to sum of reciprocals of individual resistances.
  • The equivalent resistance is always less than lowest individual resistance.
  • From available resistances we can get minimum possible resistance by connecting all resistors in parallel.
  • Failure of one device does not damage other devices.

 

  • Heating Effect of Electric Current:
    • Conversion of electrical energy to heat is called heating effect of electric current.
    • In a current carrying circuit having resistance R the work done to move a charge Q through potential difference of voltage V is given by W=VQ.
    • The power P = W/t = VQ/t = VI.
    • Energy spent i.e. heat produced = power ‘P’ × time ‘t’

∴  H=VIt .

∴ H =I2Rt.    

    • This is called Joule’s Law of Heating, which states that, “heat produced in a resistor is equal to square of current flowing through it, resistance of the resistor and the time for which it flows”.
      • In Electric bulbs, tungsten filament having high melting point get heated and produces light. To minimize the chances of melting, bulbs are filled with nitrogen and argon gases.
      • Fuse are the metal wires which have comparatively low melting point than the circuit elements. Fuse is placed in series with the circuit element. In case of short circuit situation large current flows through the circuit. This causes heating of fuse wire thus fuse wire breaks and protect the circuit from damage.
      • How to choose fuse:  Typical fuse wires have different ratings e.g. 1A, 2A, 3A 5A 10A and hence must be selected appropriately. Suppose we use electric iron that consumes 1kW of power. The supply voltage is 220 V then current required for it is 1000/220 i.e. 4.54A hence we should choose 5A fuse.
    • Electric Power:

 

  • Electric Power can be defined as rate of doing work. Or it can also be defined as rate of consumption of energy.
  • It is given by P = W/t = VQ/t = VI = I2R=V2/R
  • SI unit of power is Watt. 1 Watt is defined as the power consumed when  current of 1A flows through a circuit under 1V potential difference. Higher powers can be expressed by 1 kilowatt.
  • Electric Energy:
  • Electrical energy is the product of electrical power and time for which it is used.
  • It is given by E=P×t its unit is watt hour (W h).
  • 1 W h is the energy consumed by using 1 watt power for 1 hour.
  • Commercially we use kW h as unit of electrical energy.

 

1kW h=3.6 × 106 Joule (J).