Lesson 12 - Electricity-Class 10 Science Notes

Electricity

electricity class X

 Concept of electric charges:- When a glass rod is rubbed with silk, electrons move from glass rod to silk therefore, the glass rod acquires a positive charge as it becomes deficient of electrons and silk will acquire negative charge as the amount of negative charge in it increases. 

            When an ebonite rod is rubbed with wool, electrons move from wool to ebonite rod. Therefore, the ebonite rod acquires a negative charge as it receives more electrons and wool acquires positive charge as it becomes deficient of electrons.

        So, due to this concept of friction, the concept of charges came in to notice. And further, coulomb was the one who gave more information about the charges.

Charge:- Like mass, the charge is the fundamental property of matter by which a substance become electrified.

    It is denoted by 'q'. Its S.I. unit is 'Coulomb'.

Units of charge: 

1.  In S.I. system of units, 'Coulomb' is unit of charge.

2. In C.G.S. system of units, 'State Coulomb' is unit of charge.

    1Coulomb = 3 x 10⁹  State coulomb

3. Smallest unit of charge- Franklin

4. Largest unit of charge- Faraday

            1 Faraday = 96500 Coulomb

5. Other units-    

1 mC (milli coulomb) = 10^-3  C

1 µC (micro coulomb) = 10^-6  C

1 nC (Nano coulomb) = 10^-9  C

1 pC (Pico coulomb) = 10^-12  C

Types of charge:- There are two types of charge

(i) Positive charge.

(ii) Negative charge.

Positive Charge:- The charge acquired by a glass rod when rubbed with silk is called a positive charge.

• When a substance losses 'electrons', it becomes 'Positively charged.

Negative Charge:- The charge acquired by an ebonite rod when rubbed with wool is called negative charge.

• When a substance receives 'electrons', it becomes 'Negatively charged'

Properties of Electric Charge:-

(i) Unlike charges attract each other.

(ii) Like charges repel each other.

(iii) Total charge is conserved.:  According to conservation law of charge, "Total charge of an isolated system remains constant."

            That means, Charge neither be created nor be destroyed only it can transfer from one body to another body.

Example:- If there is two bodies, one has +10C charge and other is neutral i.e. no charge. When we connect both bodies with each other then +5C charge transfers from charged body to neutral body. But total charge remains +10C i.e. constant.

(iv) Charge is additive.: Total charge of a system is algebraic sum of charges of elementary particles of that system separately

Example:- If in a body there is +15C positive charge and -10C negative charge. 

      then, Net charge on body will be +15C-10C=+5C

(v) Charge is quantized:- According to the quantization of charge, 'The electric charge is equal to the product of number of electrons in it and the magnitude of charge on the electron' i.e.

q = ne

where, q= Total charge on body

 n = Number of electrons and

e = Charge of a electron = 1.6 x 10^-19  C  

  

Q. Calculate number of electrons in 1C of charge.

Ans)    q= ne

q=1C

So, 1 = n x 1.6 x 10^-19

        n = 6.25 x 10¹⁸ electrons     

      

1 Coulomb:- 1C is that charge which contains 6.25 x 10¹⁸   number of electrons in it.

i.e. Total charge of  6.25 x 10¹⁸   number of electrons is called 1C of charge.

Coulomb’s law:- A French physicist Charles Augustin de Coulomb gives mathematical relationship for force of attraction or repulsion between two charged particles. 

            According to Coulomb's law,  "There is always some force between two charges which is directly proportional to the product of magnitude of their charges & inversely proportional to the square of the distance between them."

Let two charges q1 and q2 are kept at distance r from each other then,

Where K is proportional constant which is known as Coulomb's constant.

                                     K= 9 × 10⁹ N-m²/C² 

This law is also known as inverse square law.

Types of substances depending upon their ability to conduct current:-

We have different kinds of substances depending upon their power to conduct electricity. 

They are of two types:

(i) Conductors

(ii) Insulators

Conductors:- They are those which allow current to pass through them. They have free mobile electrons.

For example Copper (Cu), Aluminum ( Al), Iron (Fe)  etc.

Insulators:- They are those which do not allow current to pass through them as they do not have free mobile electrons.

For example glass, wood, rubber, etc. 

•     Those substances which have free electrons are good conductors and those which don’t have free electrons are poor conductors or insulators.

Difference between conductor and insulator:

 Conductor                                                               

1. Electric current can flow through conductor.               

2. Electrons flows easily.                                                   

3. Conductivity is more.                                                   

4. Resistance is less.                                                         

5. Rate of heat transfer is more.                                        

 Insulator

 1. Electric current can not flow through insulator.

2. Electrones not flow easily. 

3. Conductivity is less or zero. 

4. Resistance is more. 

5. Rate of heat transfer is less.  

Types of electricity:- There are two types of electricity-

1.Static electricity

2.Current electricity

Static Electricity:- Static electricity is a type of electricity when electric charges develop but do not flow.

Example:- Charges developed on glass rod when rubbed with silk, and on ebonite rod when rubbed with wool. 

Current Electricity:- Current electricity is a type of electricity when the electronic charges that develop are also in motion.

Example:- electricity used in our homes.

Electric current:- The rate of flow of electric charges through a conductor is called electric current. It is represented by 'I'. S.I. unit of current is 'Ampere'.

I=q/t

Where, I = Electric current through conductor

            q = Charges flow through conductor

            t =  Time through which charge flow

1Ampere:-

We know, electric current,            I=q/t

1A=1C/1sec

So, When 1Coulomb of charge flows through a wire in 1second, it would be equal to 1 ampere.

Small Quantity of Electric Current:- Small quantity of electric current is expressed in milliampere and microampere. Milliampere is written as mA and microampere as µA.

1 mA (milliampere) = 10^-3  A

1 µA (microampere) = 10^-6  A

1 nA (nanoampere) = 10^-9  A

1 pA (picoampere) = 10^-12  A

Ammeter:- The ammeter is an instrument used to measure the current.

     the connection of ammeter in circuit is that it is connected in series. It has low resistance and in series, the current remains constant.

Electric Field:-  When the electric charges are kept, they create a small area around itself  where its effect can be felt called as the electric field.

Electrostatic potential:- 

It is the work done in moving the positive charge from infinity to a certain point in an electric field of other charge.

For example -  A unit of positive charge is at 'A' point at infinity and we try to bring it to point 'B' that is in the electric field of some other charge, let’s say charge q. So the work that we need to do in doing so is called electrostatic potential.

Potential difference:-

When we need to move the charge between two specified points, then the work done in doing so is called the potential difference. So it is defined as the work done in moving a charge from one point to another point.

                        V=W/q

Where, V =  Electrostatic potential

            W = Workdone

            q = Charge

It's SI unit is Volt.

1 Volt:- 

We know, potential difference,         V=W/q

1V=1joule/1Coulomb

So, Potential difference is said to be 1 volt when 1J of work is done in moving IC of charge from one point to another.

Voltmeter:- The instrument that is used to measure its magnitude is called Voltmeter.

      It is connected in parallel because it has a high resistance and also in parallel the potential difference remains constant.

Cell:- Cell is simple device used to maintain potential difference in a circuit.

Electric current always flow from a high potential difference to low potential difference. 

Electric circuit:- It is a continuous conducting path through which electric current flows when potential difference applied. 

Types of electric circuit:- There are two types of electric circuit-

(i) Closed electric circuit

(ii) Open electric circuit

(i) Closed electric circuit:- The circuit in which all the components are joined to one another such that a continuous current flows through them is called closed electric circuit.

(ii) Open electric circuit:- It is the circuit in which electric contact is broken at some point such that no current flows through the components of circuit.

Galvanometer:- It is a device used to measure very small amount of current. It has very low resistance.

Electric circuit diagram:- Electric circuit is with the help of electric cell, switch, electric elements and conducting wires etc.

Symbols used in electric circuit diagram:- 

Ohm's law:- It is a generalization made by Georg Simon Ohm on the basis of experiments. 

According to this law, "At a constant temperature, electric current flowing in a circuit is directly proportional to the applied potential difference across ends of the conductor."

            let in a circuit 'V' is potential difference and 'I' is current flowing through it. then,    

Where R is proportional constant which is known as Resistance

Resistance:-  Resistance is the property of a conductor to resist the flow of electric current through it.

                        It is represented by R. Its SI unit is ohm (Ω)

1 Ohm:- By ohm's law,  R = V/I

                                         1 Ω = 1 V/ 1 A

So,  When applied potential difference between two points is 1V and current flow through it is 1A, then resistance is 1 ohm.

Variable Resistance:-  The component of an electric circuit which is used to regulate the current, without changing the voltage from the source, is called variable resistance.

Rheostat:- This is a device which is used in a circuit to provide variable resistance without changing the voltage source.

Factors on which resistance of a conductor depends:-  Resistance of uniform metallic conductor is

(i) directly proportional to the length of a conductor.

------------(i)

(ii) inversely proportional to the area of cross section.

------------(ii)

(iii) directly proportional to temperature.

(iv) depends on the nature of conductor.

             From (i) and (ii) 

-------------(iii)

Where ρ is proportionality constant. It is known as electrical resistivity of conductor. Its SI unit is ohm-meter.

Resistivity or Specific resistance:-   

From (iii)   

            if l=1m  and A=1m²

                ^{then  R=}ρ 

So, if we consider it for wire of length 1m and area 1m2 then resistivity is defined as the measure of resistance of wire 1m length and 1 meter square area .

• Resistivity does not change with change in length or area of cross section but it changes with change in temperature. 
• Range of resistivity of metal and alloy is 10^-8 to 10^-6 Ωm

• Range of resistivity of insulators is 10¹² to 10¹⁷ Ωm

• Resistivity of alloy is generally higher than that of  its constituent metals. 
• Alloy do not oxidize (burn) readily at high temperature, that's why they are commonly used electrical heating devices. 
• Copper and Aluminum are commonly used in electrical transmission lines as they have low resistivity. 

Combination of resistors:- Resistors can combined in two ways-

(i) Series combination

(ii) Parallel combination

Series combination:- When resistors are joined from end to end, it is called in series. In this case, the total resistance of the system is equal to the sum of the resistance of all the resistors in the system.

In series, the current remains constant and potential difference changes.

   Let, three resistors R1, R2, and R3 are connected in series.
Potential difference across A and B = V
Potential difference across R1, R2 and R3 = V1, V2 and V3
Current flowing through the circuit = I
We, know that
V= V1 + V2 + V3                     -------------(i)
According to Ohm’s Law :
V1 = IR1                                          -------------(ii)

V2 = IR2                                      ---------------(iii)

V3 = IR3                                         -------------(iv)

Let, equivalent resistance = R
Then, V = IR                                   -------------(v)
put value from equations (ii), (iii), (iv) and (v) in equation (i)
IR = IR1 + IR2 + IR3
R = R1 + R2 + R3
So, in series combination, "Equivalent resistance is equal to the sum of resistances connected in circuit."

Parallel combination:- When resistors are joined in parallel, the reciprocal of the total resistance of the system is equal to the sum of reciprocal of the resistance of resistors.

In parallel, Potential Difference remains constant and  the current changes.

Let three resistors R1, R2 and R3 connected in parallel.
Potential difference across point A and B = V
Total current flowing between point A and B = I
Currents flowing through resistors R1, R2 and R3 = I1, I2 and I3 respectively.
We, know that,
I = I1 + I2 + I3                                          ------------(i)
Since, the potential difference across R1, R2, and R3 is the same = V
According to Ohm’s Law,

So, in parallel combination, "Reciprocal of equivalent resistance is equal to sum of reciprocals of resistances connected in circuit."

Advantage of  parallel combination over series combination:- 

(i) In series combination, when one component is fails the circuit is broken and none of the component work.

(ii) Different appliances have different requirement of current. This can not satisfied in series as current remains same.

(iii) The total resistance in parallel circuit is decreased while in series total resistance is increased. 

 

Heating Effect of Electric Current:- When electric current is supplied to a purely resistive conductor, the energy of electric current is dissipated entirely in the form of heat and as a result, resistor gets heated. The heating of resistor because of dissipation of electrical energy is commonly known as Heating Effect of Electric Current. 

Examples:- When electric energy is supplied to an electric bulb, the filament gets heated because of which, it gives light. The heating of electric bulb happens because of heating effect of electric current.

Cause of Heating Effect of Electric Current:- Electric current generates heat to overcome the resistance offered by the conductor through which it passes. Higher the resistance, the electric current will generate higher amount of heat. Thus, generation of heat by electric current while passing through a conductor is an inevitable consequence. 

This heating effect is used in many appliances, such as electric iron, electric heater, electric geyser, etc.

Nichrome wire is used in heating devices and the reason being
because

• It has high resistance
• It does not burn even at high temperature
• It has high coefficient of linear expansion

Joule’s Law Of Heating: Let, an electric current I is flowing through a resistor having resistance = R.
The potential difference through the resistor is = V.
The charge, Q flows through the circuit for the time, t
Thus, work done in moving of charge (Q) of potential difference (V),
W = V × Q
Since this charge, Q flows through the circuit for time t
Therefore, power input (P) to the circuit can be given by the following equation :
P = W/t                                   …..(i)
Substituting Qt W= VQ in equation (i), we get,
P = VQ/t                                         ...…(ii)

We know, electric current, I = Q/t
i.e., P = VI
Since, the electric energy is supplied for time t, thus
P × t = VI × t = VIt                     ……(iii)
H = VIt                    { H=P × t }

H = I2Rt                    { by ohm's law, V = IR } 

H  = V2R V²t/R                   { by ohm's law, I = V/RVR }

So, Joule's law states that

(i) Heat produced in a resistor is directly proportional to the square of currrent.

(ii) Heat produced is directly proportional to the resistance for a given current.

(iii) Heat is directly proportional to time for which current flows through a given conductor.

Electric Power:- The rate of consumption or dissipation of energy is called electric power.

  or Workdone per unit time is called power.

P = WT

We know that , W = V × Q

P = V × Qt
We know, electric current, I = Qt

so, P = VI 

P = I2R                    { by ohm's law, V = IR } 

P  = V2R                     { by ohm's law, I = VR }

Units of power:- 

SI unit of power is Watt or Joule/seconds

Other units-

                        kilowatt=1000watt

                        megawatt=1000000 watt
                        gigawatt=1000000000watt

                        horsepower =746 watts                            

Commercial unit of electricity:-  Commercial unit of electricity is Kilowatt0-hour (KWH). It is also known as unit. It is used to measure domestic electricity.

1 unit = 1 KWH

1 unit = 1000 W  × 3600 sec

1 unit = 3.6 ×10⁶ Joule

Number of units = Power ( in Watt) × Time ( in hour)/1000

Electric Bulb: In an electric bulb, the filament of bulb gives light because of the heating effect of electricity. Tungsten material is used as filament due to its high melting point, it has high resistance to heat and can be easily made into thin wire. The filament of bulb is generally, made of tungsten metal, having melting point equal to 3380°C.

Electric fuse:- It is a safety device that protects us from fire. It is shown in figure. It has two terminals T1 and T2 to which fuse wire is attached. The property of fuse wire is that it has very very low melting point. It is made up of an alloy consisting of lead and tin.

It works under two conditions:

1. Overloading: When many appliances are running together at the same time, they draw a large amount of current  which produces a lot of heat that can even lead to fire.
2. Short circuiting: When live wire and neutral wire touch each other, the current exceeds the normal value and it can also cause fire.
3. In both the conditions, when current exceeds the normal value, excess heat is produced which melts the fuse wire and hence, fire is caused.

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