Electricity | Olympiad Preparation for Class 10 PDF Download

Introduction

The concept of electricity came into notice due to friction. It actually proves that charges can be produced and they can also be made to flow and the flowing of charges gives rise to electricity. So let us first learn about friction.

Friction

It comes into notice when two bodies are rubbed with each other.
For example, Rub a glass rod with silk. 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 in electrons and silk will acquire a negative charge as the amount of negative charge in it increases.

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

Electric Charge

So it was concluded that we have two types of charges:

• Positive charge
• Negative charge

The symbol used for it is (q) and a unit that is used to measure it is Coulomb which is denoted by C (unit).

Different experiments proved that “same charges repel each other and opposite charges attract each other.”

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

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

Coulomb’s law

There is always some force between two charges which is directly proportional to the product of the magnitude of their charges & inversely proportional to the square of the distance between them.

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 classified as:

• Conductors: They are those who 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: Static electricity is a type of electricity when electric charges develop but do not flow.
  For example: Charges developed by glass rods 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 is used in our homes.

Electrostatic Potential

It is the work done in moving the positive charge from infinity to a certain point in an electric field of another charge.
For example: A unit of positive charge is at a point at infinity and we try to bring it to point b which is in the electric field of some other charge, let’s say charge q². 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.
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.

Electric current

As we have understood that charges do move, here comes another physical quantity which 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 the wire in 1 second. The ammeter is an instrument used to measure the current. Now the connection of the ammeter in the circuit is that it is connected in series. It has low resistance and in series, the current remains constant.

Types of current

Electric circuits

A continuous conducting path between the terminals of a source of electric energy and other electrical components along which the electric current flows is called an electric circuit.

Types of electric circuits:

• Closed electric circuit
• Open circuit

• 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
By 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 to a minimum, and accordingly, when we switch on the current, the ammeter records the magnitude of the current and the voltmeter records the potential difference- we take their values as I₁ and V₁. Now the slider of the 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 the 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:

• 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 to the area of wire: Resistance is inversely proportional to the area of the wire. More is the area, less is the resistance as the number of collisions is less as they can pass easily.
• Resistance to 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 to temperature: Resistance is directly proportional to temperature for pure metals but for alloys it decreases. With the increase in the temperature, the number of collisions is more, therefore, resistance is more. Combining all factors we get
  R= rho L/A
• The specific resistance of resistivity: In the above formula of resistance, if we consider it for the wire of length 1m and area 1m2 then resistivity is defined as the measure of the resistance of wire 1m length and 1-meter square area.
  formula: R= rho L/A
  unit is ohm –meter

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 the figure:Derive expression to calculate equivalent resistance when different resistances are connected in series:
  Let’s say the same current ‘I’ passes through resistances R₁ and R₂ and a 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 the same current I₁ AND I₂ pass through resistances R₁ and R₂ and a potential difference are 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. The heat produced depends upon the square of current, resistance, and temperature.
i.e., H = I2RT 
Also called as joules heating effect.