Notes of Chap 12
Electricity
Covered Topics
- Introduction
- Electric Current
- Electric Circuits
- Electric Potential
- Potential Difference
- Circuit Diagram
- Ohm's Law
- Factor/Cause of Resistance in a Conductor
- The resistance of a System of Resistors
- Resistors In Series
- Resistors In Parallel
- Heating Effect of Electric Current
- Applications of Heating Effect of Electric Current
- Electric Power
- 👉 The phenomenon of attraction produced by rubbing of the two substances with each other is called electricity.
- Example: Rubbing comb with our hair generate a charge.
- 👉 Electric Charge(Q): Basic property of electrons, protons and other subatomic particles are known as electric charge.
- 👉 Types of charge: There are two types of charges.
- (i) Negative charge: Charge Carried by-elections is called a negative charge.
- (ii) Positive Charge: Charge carried by protons is called a positive charge.
- 👉 Properties of Charge:
- (i) Like charge repel each other.
- (ii) Unlike charge attract each other.
- 👉 Charge on 1 electron = 1.6 x 10⁻¹⁹ C
- 👉 S.I unit of Charge is Coulomb(C)
- 👉1 Coulomb Charge = 6 x 10¹⁸ electrons
- 👉 Q=ne
- Where, "Q" is Charge, "n" is the number of electrons, "e" is charge present on 1 electron.
- The flowing rate of electric charge is called electric current.
- 👉 I= Q/t or Current=Charge/time
- S.I unit of Current= Coulomb/second or Ampere(A)
- 1Ampere = 1C/s [1 Coulomb/second]
- 1mA = 10⁻³A [1 Mili Ampere]
- 1µA = 10⁻⁶A [1 Micro Ampere]
- Ammeter: An instrument that measures the electric current of an electric circuit is called an ammeter. It is denoted by capital "A"
- ⇒ It has less resistance.
- ⇒ An ideal ammeter has zero resistance.
- ⇒ It is always connected in series.
- Galvanometer: An instrument that is used to detect electric current in an electric circuit is called a galvanometer. It is denoted by "G".
- ⇒It measured current in microamps.
- A closed electrical path in which charge flows from a source point to the whole path and completes the electric path called electric circuit.
- The amount of work needed to move a unit of charge from a reference point to a specific point inside the field without producing an acceleration is called electric potential.
- ⇒S.I unit of electric potential is "Volt"
- ⇒It is denoted by capital "V".
- The amount of work done to bring a charge from one point to another in the electric field is called the potential difference.
- ⇒It is denoted by "V"
- ⇒ V = W/Q or [P. difference = Work/Charge ]
- ⇒S.I unit of potential difference is "Volt".
- ⇒1V = 1 Joule/Coulomb [ 1 Joule work done to bring 1 Coulomb charge].
- Voltmeter: An instrument that is used to measure electric potential or potential difference is called a voltmeter. It is denoted by capital "V".
- ⇒It has high resistance power.
- ⇒It is always connected in parallel.
- ⇒Charge always flows from higher potential to lower potential.
- The system in which different components of the circuit are represented by the symbols is called a circuit diagram.
- At a constant temperature current passing through a conductor is directly proportional to its potential difference.
- ⇒V ∝ I
- ⇒V = IR [ R= Constant]
- Resistance: Resisting property of conductors which prevent the flow of electric charge is called Resistance. It is denoted by capital "R".
- ⇒S.I unit of resistance is Ohm (symbol: Ω).
- ⇒ lΩ = 1Volt/1Ampere
- ⇒ R= V/I
- ⇒ 1 Ohm Resistance is equal to 1 Volt potential difference per 1 ampere electric current.
- Resistor: The components of an electric circuit which prevent the flow of electric charge through a conductor are called Resistor.
- Variable Resistance: Electric circuit's component which regulates the current without changing source voltage is called variable resistance.
- Rheostat: It is a variable resistor that is used to regulate the electric current in the electric circuit.
- Factors on which resistance of a conductor depend:
- (i) Nature of material: Resistance is directly proportional to temperature and nature. For example, Plastic is a bad conductor.
- (ii) Length of material: Resistance is directly proportional to the length of a conductor............... R ∝ l …
- (iii) Cross-section Area of material: Resistance is inversely proportional to its Cross-section area of a conductor......... R ∝1/A ….
- From above statements R = ρl / A
- Resistivity(ρ): Resistivity is the resistance per unit length and cross-sectional area and It is the property of the material that opposes the flow of charge or the flow of electric current. Or ρ = RA/l , {ρ=rho}
- 👉S.I unit of Resistivity is Ohm meter (Ωm)
- When two or more Resistors combined in series or in parallel to perform a specific effect in an electric circuit is called a combination of Resistors. There two methods to combine Resistors (i) Resistors in Series (ii) Resistors in Parallel.
- When two or more resistors combine together end by end in an electric circuit is called resistors in series or series combination of the resistor.
- Total Resistance in Series:
- V = V₁ + V₂ + V₃ .........(i)
- By ohm's Law,
- V= IR, Putting the value of V in eq.(i)
- IR = IR₁ + IR₂ + IR₃
- IR = I(R₁ + R₂ + R₃)
- R = R₁ + R₂ + R₃
- R = R₁ + R₂ + R₃ +.......+ Rₙ
- Advantages of series combination:
- Series combination of resistors doesn't overheat easily.
- A series combination of resistors is easy to learn and understand.
- In the series combination of resistors, all resistors carry an equal amount of current.
- In the series combination of resistors has a higher output of resistance.
- Disadvantages of series combination:
- In the series combination of resistors if one Resistor breaks the whole circuit will break.
- As the number of resistors increases in series combination, greater will be the circuit resistance.
- When two or more resistors combine together in parallel in an electric circuit is called resistors in parallel or parallel combination of a resistor.
- Total Resistance in Parallel
- I = I₁ + I₂ + I₃ .........(i)
- By ohm's law, I = V/R, Putting the value of I in eq.(i)
- V/R = V/R₁ + V/R₂ + V/R₃ , or
- V/R = V(1/R₁ + 1/R₂ + 1/R₃), or
- 1/R = 1/R₁ + 1/R₂ + 1/R₃, or
- 1/R = 1/R₁ + 1/R₂ + 1/R₃ +....+ 1/Rₙ
- Advantages of parallel combination:
- All resistors in parallel combination get equal quantity of voltage.
- In the parallel combination of resistors simple to attach or disconnect brand new resistors while not poignant alternative parts of the circuit.
- In the parallel combination of resistors, if one resistor stopped operating, It doesn't affect other components of the circuit because current still passing through the remaining resistors.
- Disadvantages of parallel combination:
- In the parallel combination of resistors, it requires a lot of wire to connect it.
- In the parallel combination of resistors, we can't increase the voltages of the circuit.
- In the parallel combination of resistors not possible to pass exactly the same amount of current through the circuit units.
- When an electrical current is well-versed a conductor, it becomes hot after some time and produces heat due to the conversion of electric energy into heat energy, this effect of electric
current is understood as the heating impact of electrical current.
- P = VQ/t or
- P = VI [ I = Q/t]
- E = Pt [Electric energy (E) = Heat energy (H)]
- H = Pt
- H = VIt ........(i) [Pt ∝ VIt]
- By Ohm's Law, V = IR putting in eq.(i),
- H = (IR)It
- H = I²Rt
- Joule's Law of heating: The law implies that heat production in a resistor is-
- Directly proportional to the sq. of current for a given resistance. H ∝ I²
- Directly proportional to Resistance for a given current. H ∝ R
- Directly proportional to the time that the present flows through the resistance. H ∝ t
- From the above statements:
- H = I²Rt.
- Appliances based on heating Effect:-
- Electric Iron
- Electric Bulbs
- Electric Heater
- Electric Fuse
- Filaments of Electric Bulbs: It is made of tungsten metal because it has-
- High melting point [ 3380° C]
- Low Resistance
- it doesn't Oxidise rapidly at High temperature.
- Electric bulbs filled with chemically inert gases like Nitrogen and Argon.
- Electric Fuse: It is an electrical safety device that operates to provide overcurrent protection of an electrical circuit.
- It has high resistance.
- It has a low melting point.
- It is made up of an alloy of copper and tin.
- It is always connected in series with the live wire.
- The rate per unit time at which electrical energy is consumed by any electric appliance in an electric circuit is called electric power.
- P = VI , or
- P = V(V/R) [I = V/R By Ohm's Law]
- P = V²/R
- S.I unit of electric power is Watt (W).
- 1 Watt = 1 Volt x 1 Ampere.
- 1 Horse Power = 746 Watts.
- 1 Kilowatt = 1000 Watts.
- Commercial unit of electric energy is Kilowatt-hours ( KWh).
- 1 KWh = 1000 W × 3600 s , or
- 1 KWh = 3.6 × 10⁶ Joules.
- 1 KWh = 1 Unit.
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