Chapter – 11

Electricity

In this post we have given the detailed notes of class 10 Science Chapter 11 (Electricity) in English. These notes are useful for the students who are going to appear in class 10 board exams.

Chapter 11: Electricity

Electric Charge ⚡

• Frictional Electricity: Electricity produced by rubbing or friction is called frictional electricity.
• Electric Charge: Electric charges are of two types.

1. Positive Charge: When a glass rod is rubbed with a silk cloth, the charge acquired by the glass rod is called positive charge.

2. Negative Charge: When an ebonite rod is rubbed with wool, the charge acquired by the rod is called negative charge.

• Positive charge arises due to a deficiency of electrons.
• Negative charge arises due to an excess of electrons.

Basic Law of Electrostatics:

• Like charges repel each other.
• Unlike charges attract each other.

Static Electricity: When electric charge is at rest, it is called static electricity.

Current Electricity: When electric charge is in motion, it is called current electricity.

Electric Current and Charge

• When electric charge flows through a conductor, we say that there is an electric current in the conductor.
• In other words, the flow of electric charge is called electric current.
• Electric current is expressed by the amount of charge flowing through a particular area in unit time.
  • Electric current flows through a conductor or wire.
  • Electric current is a vector quantity (conventionally treated with a direction).

Flow of Electrons 🔋

Electrons are repelled by the negative terminal of a battery and attracted towards the positive terminal. Hence, electrons flow from the negative terminal to the positive terminal. When these electrons reach the positive terminal, they get transferred inside the battery through a chemical reaction and reappear at the negative terminal. In this way, electrons keep flowing continuously in the circuit.

Conductors

Materials that allow electric charge to flow easily through them are called conductors. Examples: copper, silver, aluminium, etc.

• Good conductors offer very little resistance to the flow of current.
• Insulators offer very high resistance to the flow of current.

Insulators

• Materials that do not allow electric current to flow through them are called insulators. Examples: rubber, plastic, ebonite and glass, etc.

Conductance

• Conductance is the property of a conductor that allows electric charge to flow through it.

Superconductivity

• Superconductivity is a phenomenon in which a conductor offers exactly zero electrical resistance at very low temperatures.

Coulomb’s Law

• The force of attraction or repulsion between two point charges is directly proportional to the product of the charges (q₁q₂) and inversely proportional to the square of the distance (r) between them.

Mathematically,

F ∝ q₁q₂ ……………………. (i)

F ∝ 1/r² …………………….. (ii)

Combining (i) and (ii): F = k·q₁q₂/r²

Here, k is a constant, but the value of k depends on the nature of the medium between the two charges.

The value of k in vacuum/free space is 9 × 10⁹ Nm²/C².

Electric Circuit 🔌

• A continuous and closed path of an electric current is called an electric circuit.

Flow of Electricity: Charge is constituted by electrons. Electric current was assumed to be a flow of positive charges, and the direction of flow of positive charge was taken as the direction of current. As per convention, in an electric circuit, the direction of current is opposite to the direction of flow of electrons (which constitute negative charge).

If a net charge Q flows through the cross-section of a conductor in time t, then the current I through the cross-section is expressed as:

I = Q/t

The SI unit of electric charge is coulomb (C), which is equivalent to the charge contained in nearly 6 × 10¹⁸ electrons.

Coulomb: The SI unit of electric charge is coulomb (C), equal to the charge of about 6 × 10¹⁸ electrons.

Charge on one electron = −1.6 × 10⁻¹⁹ coulomb (C).

Charge on one proton = 1.6 × 10⁻¹⁹ coulomb (C).

Law of Conservation of Charge

Electric charges can neither be created nor be destroyed. Charge can only be transferred from one body to another.

Ampere: Ampere is the SI unit of electric current. When 1 coulomb of charge flows through a conductor in 1 second, the current is said to be 1 ampere.

1A = 1C/1s

• Small quantities of current are measured in milliampere.
• 1 mA = 10⁻³ A, and microampere: 1 μA = 10⁻⁶ A.

In an electric circuit, conventional current flows from the positive terminal (+) to the negative terminal (−) of a cell or battery.

Ammeter: The instrument used to measure electric current in a circuit is called an ammeter. It is always connected in series in the circuit in which the current is to be measured.

Galvanometer: A galvanometer is a device that detects the presence of current in an electric circuit.

Conventional Current: Conventionally, the direction of motion of positive charges is taken as the direction of current. The direction of conventional current is opposite to the direction of flow of electrons.

Electrostatic Potential: Electrostatic potential at a point in an electric field is defined as the amount of work done in bringing a unit positive charge of 1 coulomb from infinity to that point. Its SI unit is volt.

Potential Difference 🔋

Electrons move only when there is a difference of electric pressure, called potential difference, across the two ends of a conductor or circuit.

• This potential difference is maintained by connecting a battery, one or more cells, or a dynamo.
• A chemical reaction inside a cell generates a potential difference between its terminals, even when no current is being drawn from the cell.
• When a cell is connected to a conducting circuit element, the potential difference sets the charges in the conductor in motion, producing an electric current. To maintain the current, the cell consumes its stored chemical energy.

Voltmeter 📏

A voltmeter is an instrument that measures the potential difference between two ends of a conductor.

Definition: Potential difference is measured by an instrument called a voltmeter.

1. Volt (Definition): If 1 joule of work is done to move 1 coulomb of charge from one point to another in a current-carrying conductor, the potential difference between those two points is said to be 1 volt.

Connection of Voltmeter: A voltmeter is always connected in parallel across the two points between which the potential difference is to be measured.

In a circuit, if we want to measure the potential difference across the two ends of a resistor R₂, we connect a voltmeter in parallel across its two terminals, as shown in the circuit diagram. Such a connection is called a parallel connection.

Cell or Battery

A cell/battery is a device that helps maintain a potential difference across the two points of a conductor.

Cell: A cell is a device that uses its stored chemical energy to produce a potential difference across the two terminals of a conductor, which sets the charges in motion and produces an electric current.

Battery: A device made up of a combination of two or more cells is called a battery.

Ohm’s Law: “The electric current flowing through a metallic wire is directly proportional to the potential difference across its ends, provided the temperature of the wire remains constant.” According to this law,

V ∝ I

or, V = RI

Look at this potential-current (V–I) graph: When potential difference increases, the value of current also increases, and when potential difference decreases, current also decreases — that is, there exists a directly proportional relationship between them. This is called Ohm’s law.

Resistance: Resistance is the property of a conductor due to which it opposes the flow of electric current through it. This property of the conductor is called resistance. From Ohm’s law:

Resistance (R) = Potential difference (V) / Current (I)

• The SI unit of resistance is ohm (Ω).
• V/I = R, which is a constant for a given conductor at constant temperature.

1. Ohm (Definition): If the potential difference across the two ends of a conductor is 1 V and the current flowing through it is 1 A, then the resistance R of that conductor is 1 Ω.

When 1 ampere of current flows in a circuit and the potential difference is 1 volt, the resistance is said to be 1 ohm.

2. Variable Resistance (Rheostat): A component used to regulate current in a circuit without changing the voltage source is called a variable resistance or rheostat.

Current Regulator (Rheostat) 🎛️

A device used to change the resistance in a circuit is called a rheostat/current regulator.

Factors on Which the Resistance of a Conductor Depends:

• Directly proportional to the length of the conductor.
• Inversely proportional to the area of cross-section.
• Directly proportional to temperature.
• Also depends on the nature of the material.

Resistivity 📐

The resistance offered by a conductor of unit length (1 metre) and unit cross-sectional area (1 m²), i.e., a cube of side 1 metre, when current flows across its opposite faces, is called resistivity.

The SI unit of resistivity is Ωm.

• Resistivity does not change with the length or area of cross-section of the conductor, but it does change with temperature.
• The resistivity of metals and alloys ranges from 10⁻⁸ to 10⁻⁶ Ωm.
• The resistivity of alloys is usually higher than that of their constituent metals.
• Alloys do not oxidise (burn) readily at high temperature, so they are commonly used in electrical heating devices.
• Copper and aluminium are used for electrical transmission because they have low resistivity.

Resistors in Series 🔗

• Series Combination: When two or more resistors are connected end-to-end, one after another, the combination is called a series combination.
• Total Resistance in Series: R_S = R₁ + R₂ + R₃
  • V = V₁ + V₂ + V₃
  • V₁ = IR₁, V₂ = IR₂, V₃ = IR₃
  • V₁ + V₂ + V₃ = IR₁ + IR₂ + IR₃
  • V = I(R₁ + R₂ + R₃) [since V₁ + V₂ + V₃ = V]
  • IR = I(R₁ + R₂ + R₃)
  • R = R₁ + R₂ + R₃

Therefore, the equivalent single resistance in a series combination is greater than the largest individual resistance in the combination.

Resistors in Parallel 🔀

Parallel Combination: When two or more resistors are connected together between the same two points X and Y, the combination is called a parallel combination.

In a parallel combination, the potential difference across each resistor is equal to the potential difference applied, while the total current is equal to the sum of the currents flowing through each individual resistor.

• I = I₁ + I₂ + I₃
• The reciprocal of the equivalent resistance of a parallel combination
• is equal to the sum of the reciprocals of the individual resistances: 1/R = 1/R₁ + 1/R₂ + 1/R₃

Advantages of Parallel Combination over Series Combination

• In a series combination, if one component fails, the circuit breaks and none of the components work.
• Different components may need different currents to operate — this is not suitable in a series combination since the current is the same throughout in series.
• The resistance of a parallel combination is lower than that of the individual resistors.

Heating Effect of Electric Current 🔥

If an electric circuit is purely resistive, the energy of the electric supply is completely dissipated as heat. This is called the heating effect of electric current.

• Energy = Power × time
• H = P × t
• H = VIt (since P = VI)
• H = I²Rt (since V = IR)
  • H = heat energy produced
• Hence, the heat produced, H = I²Rt (Joule’s Law of Heating).

Joule’s Law of Heating — According to this law:

• Heat produced in a resistor is directly proportional to the square of the current through it.
• Directly proportional to the resistance.
• Directly proportional to the time for which the current flows.
• The heating effect is desirable in devices like heaters, electric presses, etc., but undesirable in devices like computers, mobile phones, etc.
• In an electric bulb, most of the power is dissipated as heat, and only a small part is emitted as light.
• The filament of an electric bulb is made of tungsten because:
  • It does not oxidise readily at high temperatures.
  • It has a very high melting point (3380°C).
• Bulbs are usually filled with chemically inactive gases like nitrogen and argon, which increase the life of the filament.

Electric Power: The rate at which work is done is called power. The rate at which energy is consumed is also called power. In an electric circuit, the rate of consumption/dissipation of electric energy is also known as electric power. Power P is expressed as: P = VI

• The SI unit of power is watt (W).
• 1 watt = 1 volt × 1 ampere.
• The commercial unit of energy is kilowatt-hour (kWh).
• 1 kWh = 3.6 × 10⁶ J.
• 1 kWh is called one “unit” of electrical energy.

Key Points to Remember

• Electric current is the rate of flow of electric charge: I = Q/t, measured in ampere (A). ⚡
• Ohm’s Law states V = IR, valid at constant temperature; the V–I graph is a straight line through the origin. 📏
• Resistance R = V/I, SI unit ohm (Ω); resistance depends on length, area of cross-section, nature of material and temperature.
• Resistivity is a property of the material itself and does not depend on the length or area of cross-section (unlike resistance). Its SI unit is Ωm.
• For resistors in series: R = R₁ + R₂ + R₃ (current is the same in all resistors, voltages add up).
• For resistors in parallel: 1/R = 1/R₁ + 1/R₂ + 1/R₃ (voltage is the same across all resistors, currents add up).
• Heat produced by current is given by Joule’s law: H = I²Rt.
• Electric power P = VI = I²R = V²/R, SI unit watt (W); commercial unit of energy is kWh (1 kWh = 3.6 × 10⁶ J). 💡
• Ammeters are always connected in series (nearly zero resistance) and voltmeters are always connected in parallel (very high resistance).
• Fuse wires and heating elements use alloys because alloys have higher resistivity and do not oxidise easily at high temperature. 🔌

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