CBSE NOTES CLASS 10 SCIENCE CHAPTER 12

ELECTRICITY

Positive and negative charges

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

Coulomb

It is the S.I. unit of charge. One coulomb is defined as that amount of charge which repels an equal and similar charge with a force of 9 x 109 N when placed in vacuum at a distance of 1 meter from it.

Charge on an electron = - 1.6 × 10-19 coulomb.

Other units – 1 C = 106 micro Coulomb (μC)

1 C = 3 × 10-9 stat coulomb or statC

Static and current electricity

Static electricity deals with the electric charges at rest while the current electricity deals with the electric charges in motion.

Conductor

A substance which allows passage of electric charges through it easily is called a conductor. A conductor offers very low resistance to the flow of current. For example silver, copper, aluminium etc.

Insulator

A substance that has infinitely high resistance does not allow electric current to flow through it. It is called an insulator. For example rubber, glass, plastic, ebonite etc.

Electric current

It is defined as the rate of flow of the electric charge through any given cross section of a conductor.

Electric current = $\frac{\mathrm{Total charge flown}}{\mathrm{Time take}}$

$\mathrm{Or I =}\frac{\mathrm{Q}}{\mathrm{t}}$

• Electric current is a scalar quantity.

Conventional current

Conventionally, the direction of motion of positive charges is taken as the direction of current. Actually the current in a metal conductor flows due to flow of electrons. The direction of conventional current is opposite to that of motion of the negatively charged electrons.

Ampere

It is the S.I. unit of current.

If one coulomb of charge flows through any section of a conductor in one second, then current through it is said to be one ampere.

1 ampere = $\frac{\mathrm{1 coulomb}}{\mathrm{1 second}}$ or 1 A = = 1Cs-1

1 milliampere = 1 mA = 10-3 A

1 microampere = 1µA = 10-6 A

Electric circuit

The closed path through which electric current flows is called an electric circuit. It consists of electric devices, switching devices, source of electricity, etc. that are connected by conducting wires.

Symbols of some commonly used components in circuit diagrams

 S. No. Componet Symbol 1 Electric Cell 2 Battery or combination of cells 3 Plug key or switch (open) 4 Plug key or switch (closed) 5 A wire joint 6 Wires crossing without joining 7 Electric bulb 8 A resistor of resistance R 9 Variable resistance or rheostat or 10 Ammeter 11 Voltmeter

Electrical cell

An electrical cell is a device that is used to generate electricity or one that is used to make chemical reactions possible by applying electricity.

Galvanometer

It is device to detect current in an electric circuit.

Ammeter

It is device to measure current in a circuit. It is always connected in series in a circuit.

Voltmeter

It is a device to measure potential difference. It is always connected in parallel to the component across which the potential difference is to be measured.

Electric field

It is the region around a charged body within which its influence can be experienced.

Electrostatic potential

Electrostatic potential at any point in an electric field is defined as the amount of work done in bringing a unit positive charge from infinity to that point.

Its SI unit is volt (V).

Positive charges move from higher to lower potential regions.

Electrons, being negatively charged, move from lower to higher potential regions.

Potential difference between two points: The Potential difference between two points in an electric field is the amount of work done in bringing a unit positive charge from one to another.

Potential difference =

Or V = $\frac{\mathrm{W}}{\mathrm{Q}}$

One volt potential difference: The Potential difference between two points in an electric field is said to be one volt if one joule of work has to be done in bringing a positive charge of one coulomb from one point to another.

1 volt = $\frac{\mathrm{1 Joule}}{\mathrm{1 Coulomb}}$

Or V = $\frac{\mathrm{1 J}}{\mathrm{1 C}}$ = 1 J C-1

Ohm’s law: This law states that the current passing through a conductor is directly proportional to the potential difference cross its ends, provided the physical conditions like temperature, remain unchanged.

V ∝ I

Or V = RI

$\frac{\mathrm{V}}{\mathrm{I}}$ = constanct = R

 Resistance: It is a property of a conductor by virtue of which it opposes the flow of current through it. It is equal to the ratio of the potential difference applied across its ends and the current flowing through it. ⇒ R = $\frac{\mathrm{V}}{\mathrm{I}}$

Ohm: It is the S.I. unit of resistance. A conductor has a resistance of one ohm if a current of one ampere flows through it on applying a potential difference of one volt across its ends.

1 ohm = $\frac{\mathrm{1 volt}}{\mathrm{1 ampere}}$

⇒ 1Ω = $\frac{1\mathrm{V}}{1\mathrm{A}}$

Factors on which resistance of a conductor depends

The resistance R of a conductor depends

i) Directly on its length L i.e.

R ∝ L

ii) Inversely on its area of cross-section A i.e.

iii) On the nature of material of the conductor on. On combining the above factors, we get

The proportionality constant ρ (Rho) is called resistivity of conductor.

iv) Temperature of the conductor.

Limitations of Ohm's Law

The Ohms law is not obeyed if,

(i) V depends on I non-linearly, example Diode, Triode etc..

(ii) The relationship between V and I depends on the sign of V for the same absolute value of V.

(iii) The relation between V and I is non-unique, that is, for the same current I, there is more than one value of voltage V.

Resistivity

It is defined as the resistance offered by a cube of a material of side 1 m when current flow perpendicular to its opposite faces. It is a characteristic property of the material. Its S.I. unit is ohm-meter (Ωm).

Resistivity, ρ =

The metals and alloys have very low resistivity in the range of 10–8 Ωm to 10–6 Ωm. They are good conductors of electricity.

Insulators like rubber and glass have resistivity of the order of 1012 to 1017 Ωm.

The substances with resistivity from 10-6 Ωm to 1012 Ωm are called semi-conductors.

• Both the resistance and resistivity of a material vary with temperature.

• Resistivity of an alloy is generally higher than that of its constituent metals. Alloys do not oxidise (burn) readily at high temperatures. Hence they are used in electrical heating devices, like electric iron, toasters etc.

• Tungsten is used almost exclusively for filaments of electric bulbs.

• Copper and aluminium are used for electrical transmission lines, due to their low resistivity and reasonable cost.

Equivalent resistance

If a single resistance which can replace the combination of resistances in such a manner that the current in the circuit remains unchanged, that single resistance is called the equivalent resistance.

Laws of resistances in series

(i) Current through each resistance is same (I)

(ii) Total voltage across the combination = Sum of the all voltage drops.

⇒ V = V1 + V2 + V3

(iii) Voltage drop across any resistor is proportional to its resistance.

⇒ V1 = IR1, V2 = IR2, V3 = IR3

(iv) Equivalent resistance = Sum of the individual resistances,

⇒ IRS = IR1 + IR2 + IR3

⇒ RS = R1 + R2 + R3

(v) Equivalent resistance is larger than the largest individual resistance.

Laws of resistances in parallel

(i) Voltage across each resistance in parallel is same and is equal to the applied voltage.

(ii) Total current = Sum of the currents through the individual resistances.

I = I1 + I2 + I3

(iii) Currents through various resistances are inversely proportional to the individual resistances.

(iv) Reciprocal of equivalent resistance = Sum of reciprocals of individual resistances.

$\frac{1}{{\mathrm{R}}_{\mathrm{P}}}=\frac{1}{{\mathrm{R}}_{1}}+\frac{1}{{\mathrm{R}}_{2}}+\frac{1}{{\mathrm{R}}_{3}}$

(v) Equivalent resistance is less than the smallest individual resistance.

Heating Effects of Current

A battery or a cell is a source of electrical energy. The chemical reaction within the cell generates the potential difference between its two terminals that sets the electrons in motion to flow the current through a resistor or a system of resistors connected to the battery.

A part of the source energy in maintaining the current may be consumed into useful work (like in rotating the blades of an electric fan). Rest of the source energy may be expended in heat to raise the temperature of gadget.

If the electric circuit is purely resistive, the source energy continually gets dissipated entirely in the form of heat. This is known as the heating effect of electric current.

Joule’s law of heating

It states that the heat produced in a conductor is directly proportional to

(i) the square of the current I through it

(ii) proportional to its resistances R and

(iii) the time t for which current is passed.

Mathematically, it can be expressed as

H = I2 R t Joule

H = V I t Joule

H = V2/R t Joule

H = V Q Joule

Electric energy

It is the total work done in maintaining an electric current in an electric circuit for given time.

W = V I t = I2 R t Joule

Electrical Power

Electrical power is the rate at which electric energy is consumed by an appliance.

P = $\frac{\mathrm{W}}{\mathrm{t}}$

P = V I

P = I2 R

P = $\frac{{\mathrm{V}}^{2}}{\mathrm{R}}$

Watt

It is the S.I. unit of power.

The power of an appliance is 1 watt if one ampere of current flows through it on applying a potential differences of 1 volt across its ends.

1 watt =

1 watt =1 volt × 1 ampere

Or 1 W = 1 Js -1 = 1 VA

And 1 kilowatt = 1000 W

Kilowatt hour

It is the commercial unit of electrical energy. One kilowatt hour is the electric energy consumed by an appliance of 1000 watts when used for one hour. 1 kilowatt hour (kWh) = 3.6 x 106 J

Harmful Effects of Electrical Heating

• Heating due to electric current is undesirable as it converts useful electrical energy into heat.

• In electric circuits, the unavoidable heating can increase the temperature of the components and alter their properties.

Practical Applications of Heating Effect of Electric Current

• Heating effect of electric current has many useful applications.

• The electric laundry iron, electric toaster, electric oven, electric kettle and electric heater are some of the familiar devices based on Joule’s heating.

• The electric heating is also used to produce light, as in an electric bulb.

Mechanism

The filament must retain as much of the heat generated as is possible, so that it gets very hot and emits light. It must not melt at such high temperature. A strong metal with high melting point such as tungsten (melting point 3380°C) is used for making bulb filaments. The filament should be thermally isolated as much as possible, using insulating support, etc. The bulbs are usually filled with chemically inactive nitrogen and argon gases to prolong the life of filament. Most of the power consumed by the filament appears as heat, but a small part of it is in the form of light radiated.

Due to wastage of energy in the form of heat, incandescent bulbs are no longer used now. Either fluorescent tubes or LED lights are used instead.

• A fuse consists of a piece of wire made of a metal or an alloy of appropriate melting point, for example aluminium, copper, iron, lead etc. It protects circuits and appliances by stopping the flow of any unduly high electric current. The fuse is placed in series with the device. If a current larger than the specified value flows through the circuit, the temperature of the fuse wire increases. This melts the fuse wire and breaks the circuit. The fuse wire is usually encased in a cartridge of porcelain or similar material with metal ends. The maximum current that can flow through the fuse wire before it melts, is called current rating of the fuse. The fuses used for domestic purposes are rated as 5A, 10A, 16A, 32A etc. These days miniature circuit breakers (MCB) are used instead of the fuse.

Domestic Electric Circuts

• Household electric supply consists of 3 types of wire. Live wire (red insulation cover), neutral wire (black insulation cover) and earth wire (green insulation cover)

• The potential difference between neutral and live wire is 220V.

• In household circuit system, two type of circuit are used. One of 15A for appliances with higher power ratings and the other of 5A ratings for bulbs etc.

• In domestic circuit, different appliances are connected in parallel combination. This ensures that if one appliance is switched ‘on’ or ‘off’, the others are not affected.

• The earth wire is used as a safety measure, especially for those appliances that have a metallic body.

Benefits of Parallel Circuit over Series

(i) Each appliance can be switched on and off independently.

(ii) Easy to maintain.

(iii) Parallel circuits ensure all components in the circuit have the same voltage as the source

(iv) Parallel circuits also allow components to be added in the circuit without changing the voltage.

(v) Total resistance of domestic circuit will be quite less hence electrical losses will less.

Working of earth wire

A conductor that provides a low resitance path from a circuit or appliance to the ground (or earth) is called earth wire. The metallic body of an appliance is connected to the earth wire, which provides a low resistance-conducting path for the current. It ensures that any leakage of current to the metallic body of the appliance will flow to the earth only and the user may not get a severe shock.

Short circuit

It means that the two wires, live and neutral, have come in contact with each other. This may happen either due to their insulation have been damaged or due to a fault in the appliance. In such a case, the resistance of the circuit decreases to a very small value. According to Ohm’s law, the current increases enormously. It may result in spark at the place of short circuit, which may even cause fire. Sometimes, the current also increases due to overloading of the circuit.

It is a situation when overall current (sum of all the current used by all appliances) exceeds the current carrying capacity of the connecting wires.

The wires cannot withstand such a high current and melt and may cause fire.

• Too many appliances are connected in the same circuit.

• Short circuit.

The electric fuse

Electric fuse is used as safety device for the protection of electric circuits and appliances due to short circuiting or overloading of the circuit.

The electric fuse is a piece of wire having a very low melting point and high resistance.

When a high current flows through the circuit due to short circuit or overloading, the fuse wire gets heated and melts. The circuit is broken and current stops flowing thus save the electric circuit and appliance form damage.

Ratings of fuse wire

The fuses for domestic purposes are rated as 1A, 2A, 3A, 5A, 10A and 15A

Characteristics of fuse wire

(i) Low melting point

(ii) High resistance

Fuse wire are made up of pure tin or made of an alloy of copper and tin.