ELECTRICITY CLASS 10 TERM 2 NOTES

               ELECTRICITY CLASS 10 TERM 2

by krishan kumar

Electricity: Electric current, electric circuit, voltage or electric potential, resistance and (Ohm’s law).

Electric Current: The flow of electric charge is known as Electric Current, Electric current is carried by moving electrons through a conductor.
By convention, electric current flows in the opposite direction to the movement of electrons.

Electric Circuit: Electric circuit is a continuous and closed path of electric current.

Expression of Electric Current: Electric current is denoted by the letter ‘I’. Electric current is expressed by the rate of flow of electric charges. Rate of flow means, the amount of charge flowing through a particular area in unit time.

If a net electric charge (Q) flows through a cross-section of a conductor in time t, then,

Where I is electric current, Q is a net charge and t is a time in second.

S.I. Unit of Electric Charge and Current: S.I. unit of electric charge is coulomb (C).

One coulomb is nearly equal to 6 × 1018 electrons. S.I. unit of electric current is ampere (A). Ampere is the flow of electric charge through a surface at the rate of one coulomb per second. This means, if 1 coulomb of electric charge flows through a cross section for 1 second, it would be equal to 1 ampere.
Therefore, 1 A = 1 C/1 s

Small Quantity of Electric Current: Small quantity of electric current is expressed in milliampere and microampere. Milliampere is written as mA and microampere as pA.
1 mA (milliampere) = 10-3 A
1 pA (microampere) = 10-6 A

Ammeter: An apparatus to measure electric current in a circuit.,

Charge: Like mass, the charge is the fundamental property of matter. There are two types of charge
(i) Positive charge.
(ii) Negative charge.

Positive and Negative Charge: The charge acquired by a glass rod when rubbed with silk is called a positive charge and the charge acquired by an ebonite rod when rubbed with wool is called negative charge.

Properties of Electric Charge:
(i) Unlike charges attract each other and like charges repel each other.

Coulomb’s law

(ii) The.force between two charges varies directly as the product of two charges and inversely as the square of the distance (r) between both charges (q1 and q2).

S.I. unit of charge is coulomb (C).
1 coulomb = 1 ampere × 1 second.
1C = 1A × 1s
Thus, the quantity of charge which flows through a circuit when one ampere of current flows through it in one second is known as a 1-coulomb charge.

when the electric charges are kept, they create a small area around itself where its effect can be felt called as the electric field.

The electric charge is equal to the number of electrons in it and the magnitude of charge on the electron i.e.
q = ne
where n = number of electrons and
e = charge of electron
= 1.6 x 10-19 C

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

For example Cu, Al, etc. Like in the given figure, the wire conducts electricity but the plastic insulation is an insulator.

 

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

For example glass etc. Those substances which give free electrons are good conductors and those which don’t have free electrons are poor conductors.

Types of electricity

 

• Static electricity
• Current electricity

 

 

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

For example charges developed by glass rod are rubbed with silk.

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

For example electricity used in our homes.

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.

i.e. V= W (from one point to another)
q
The unit that is used to measure it is volt and we can define 1 volt as :1v = 1 joule
1 coulomb

So it is defined as:

The potential difference is said to be 1 volt when 1J of work is done in moving IC of charge from one point to another. The instrument that is used to measure its magnitude is called Voltmeter.
There is an important feature that how we connect it. It is connected in parallel because it has a high resistance and also in parallel the potential difference remains constant.

Therefore, Voltage = WorkdoneCharge
Voltage or electric potential difference is denoted by V’. Therefore, V = WQ
Where, W = Work done and Q = Charge

S.I. Unit of Electric Potential Difference (Voltage)
S.I. unit of electric potential difference is volt and denoted by ‘V’ This is named in honour of Italian Physicist Alessandro Volta.

Electric current

As we have understood that charges do move, so here comes another physical quantity that is the rate of flow of electric charge through a conductor.

I = Q
T
The Unit that is used to measure it is ampere.
1A = 1C
1 sec
We can define 1 ampere current as 1A, when 1Coulomb of charge flows through wire in 1second.
The ammeter is an instrument used to measure the current. Now the connection of ammeter in circuit is that it is connected in series. It has low resistance and in series, the current remains constant.

Types of current:

Conventional	Electric
Flow of protons	Flow of electrons
More from +ve terminal  to –ve terminal in outer circuit.	Move from – ve terminal to +ve terminal.
practically not important	Practically important

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 the circuit.

Closed electric circuit: The circuit in which all the components of a circuit are joined to one another such that a continuous current flows through them is a closed electric circuit. Some electronic devices with symbols used are:

Rheostat – It is defined as the variable resistance. It is connected to change the current in the circuit without changing the voltage source like cell etc.

Galvanometer– It is a device used to measure a very small amount of current as it has very low resistance.

Ohm’s law

It is the generalization made by ohm on the basis of the experiments he conducted. According to it, at a constant temperature, the current flowing through the circuit is directly proportional to the potential difference applied across the ends of the conductor.
i.e. V x I
Removing proportionately sign, we have
V= I

 

Constant Resistance.

 

 

 

Verification of ohm’s law: It can be verified as in this we connect the circuit as shown in the figure.
In this, a rheostat is introduced whose resistance can be increased or decreased. In the first case, the resistance of rheostat is kept minimum and accordingly when we switch on the current, the ammeter records the magnitude of the current and voltmeter records the potential difference- we take their values as I1 and V1. Now the slider of rheostat is moved such that its resistance increases slightly and then again ammeter and voltmeter take the reading as I2 and V2. Likewise, we take 5 consecutive readings and after comparing them we concluded that V1/I1=V2/I2=V3/I3 and so on.
Resistance– It is the obstruction to flow of charge.
Formula: R=V/I
Unit: ohm
We can define 1ohm as – resistance is said to be 1 ohm when 1volt of potential difference is applied which allows 1 ampere current to flow through it.
Factors on which resistance depends :

1. Dependence of resistance on length of wire: Resistance is directly proportional to the length of the wire. More is the length of wire, more is the number of collisions and more is the resistance.

Resistance on area of wire: Resistance is inversely proportional to the area of the wire. More is the area, less is the resistance as number of collisions is less as they can pass easily.

Resistance on nature of wire: Depending upon the material used for making wires we have:

Conductors: have almost nil resistance
Semiconductors: have resistance
Insulators: have high resistance
Resistance on temperature: Resistance is directly proportional to temperature for pure metals but for alloys it decreases. With increase in the temperature, number of collisions are more, therefore, resistance is more. Combining all factors we get :
R= rho L/A
Specific resistance or resistivity: In the above formula of resistance, 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 .

formula : R= rho L/A
unit is ohm –metre
Resistance can be introduced in a circuit in 2 ways:

Resistance in series: In series, the resistances are attached as shown

 

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

Resistance in parallel: They are connected as shown in figure :

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

 

Derive expression to calculate equivalent resistance when different resistances are connected in series :
Let’s say same current ‘I’ passes through resistances R1 and R2 and potential difference is V1 and V2.
Then we can write :
V=V1 +V2
IR=IR1 +IR2
Or we can write
R=R1+R2

 

Expression to calculate equivalent resistance when different resistances are connected in parallel. Let’s say same current I1 AND I2 passes through resistances R1 and R2 and potential difference is V.

Then we can write :

I=I1+I2
V/R=V/R1 +V/R2
V(1/R=1/R1+1/R2)V
Or we can write
1/R=1/R1+1/R2 —–

 

Joules heating effect of current

 

When current flows, the electrons move and when they move, they collide with each other. When they collide, heat is produced. Heat produced depends upon the square of current, resistance and temperature.

i.e.,:  H=I2RT

Also called as joules heating effect.

You know that nichrome wire is used in heating devices and the reason being
because

1. It has high resistance
2. It does not burn even at high temperature
3. It has high coefficient of linear expansion

Power

We come across ‘power’ that is mentioned on all electrical appliances. So, if we define, it is the rate of doing work.

Formula : P=w/t
Unit : joule/sec
Bigger units : kilowatt=1000watt
megawatt=1000000 watt
gigawatt=1000000000watt
horsepower =746 watts
Further derived formulas of power :
P=v.i
P=v2/r
P=i2/r

Electric Motor : Principle, Construction, Working and                                                  Uses

Electric Motor:-

An electric motor is a rotating device that converts electrical energy to mechanical energy.

Principle:-

It is based on the principle that when a current carrying conductor is placed in a magnetic field perpendicular to it, it experiences a force.

Construction:-

Consider a rectangular coil ABCD placed in a magnetic field such that AB and CD are perpendicular to the direction of magnetic field. The ends of coil are connected to two halves split rings. The inner sides of split rings are insulated and attached to an axle. The external conducting edges of split rings touch two carbon brushes. The opposite sides of brushes are connected with battery and key as shown in figure.

Commutator:- A device that reverses the direction of flow of current through a circuit is called a commutator.

➢ In electric motor, split rings act as commutator.

Armature:- The soft iron core, on which the coil is wound, plus the coils is called an armature.

Working:-

According to the figure, the current in the coil ABCD enters from brush X. Since current carrying coil ABCD is placed in a magnetic field perpendicular to it. So by Fleming’s left hand rule, we can say that AB moves in downward and CD moves in upward direction. In next half rotation, CD comes towards north pole and AB comes towards south pole. Again by applying Fleming’s left hand rule, we see that AB moves in upward and CD moves in downward direction. This process continues and the coil starts rotating.

In commercial motors,

• an electromagnet is used in place of permanent magnet.
• large number of turns of coil is used.
• a soft iron core is used on which the coil is wound.

Uses of Electric Motor:-

It is used in electric fans, refrigerator, mp3 player, washing machine etc.

➢ DC motor consists of split rings while AC motor consists of two slip rings

hope that this will really helps u alot in understanding chapter electricity

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