Class 10 Science Chapter 11 Important Questions

As compared to other forms of energy electricity has the following important advantages:
(i) Electricity is the most convenient sources of energy.
(ii) It can readily have transmitted over large distances with relatively small loss in energy.
(iii) It can be easily converted into other forms of energy like heat, light, sound, motion, etc.

Electricity has two main branches:
(i) Static electricity: It deals with the physical phenomena produced by the electric charge at rest.
(ii) Current electricity: It deals with the physical phenomena produced by electric charge in motion.

The flow of electric charges across a cross-section of a conductor constitutes an electric current. For example, a stream of electrons moving through a conducting wire constitutes an electric curre

A closed and continuous path along which an electric current flows is called an electric circuit.
In a torch a switch provides a conducting a link between a battery and a bulb. When the switch is turned ON an electric current flows through the bulb and it gives light. Such a continuous and closed path of an electric current is called an electric circuit. When the circuit is broken anywhere (or the switch is turned OFF), the current stops flowing and the bulb does not glow.
An electric circuit through which no current flows is called an open circuit.
An electric circuit through which current flows continuously is called a closed circuit.

A device which converts chemical energy into electric energy is called an electrochemical cell or voltaic cell or galvanic cell. It is a source used for producing a steady current.

Conventional current and electronic current: When a conductor AB is connected across the terminals of a cell, free electrons begin to drift or move from its end B (connected to the negative terminals of the cell) to the end A (connected to the positive terminal of the cell). The current constituted by flowing electrons is called electronic current. Clearly, the direction of electronic current is from negative terminal to positive terminal.
By convection the direction of motion of positive charges is taken as the direction of electric current. It is from positive terminal to negative terminal. As the electrons are negatively charged, the direction of conventional current in an electric circuit is taken as opposite to the direction of the flow of electrons.

Ammeter: An ammeter (ampere + meter) is a device used to measure electric current in a circuit.
An ammeter is always is always connected in series in a circuit so that entire current, which we wish to measure flows through it.

Just as heat flows from higher temperature to lower temperature, positive charges flow from higher potential to lower potential.

It is volt and has the symbol V.

Ohm’s law: This law states that the current (I) flowing through a conductor is directly proportional to the potential difference (V) applied across its ends, provided the temperature and other physical conditions remain unchanged.
Mathematically,
The proportional constant R is called the resistance of the conductor.

SI unit of resistance is ohm: The resistance of a conductor is said to be 1 ohm if a current of 1 ampere flows through it an applying a potential difference of 1 volt across its ends.
1 Ohm = (1 Volt )/(1 ampere )

When free electrons drift inside a conductor they get frequent collisions. This is the basic cause of resistance.

It changes the current in a circuit due to the change in its resistance.

Factors affecting the resistance: At a constant temperature the resistance of a conductor depends on the following factors:
(i) Length: Resistance R of a conductor is directly proportional to its length L, i.e., R ∝ L
(ii) Area of cross-section: Resistance R of conductor is inversely proportional to its area of cross-section i.e., R ∝ 1/A
(iii) Nature of the material: Resistance also depends on the nature of the material of which the conductor is made. The resistance of a copper wire is much less than that of a nichrome wire of same length and area of cross-section.
Combining the above factors, we get, R∝ L/A
The proportionality constant is called resistivity or specific resistance which depends on the nature of the material.

Nichrome wire is thicker than copper wire.

This is because, copper and Aluminium have low resistivity or high conductivity.

R ∝ 1/A
So, the resistance decreases when area of cross-section is increased.

Combination of resistance: In order to obtain a desired value of current in an electrical circuit, a number of resistance have to be used. Resistance can be combined together in the following three ways:
(i) Series combination.
(ii) Parallel combination.
(iii) Mixed combination.

Ordinary matter is made up of atom which have positively charged nuclei with negatively charged electrons orbiting them. So the change can be considered as a fundamental property of matter. During the process the net charge of an isolated system remains conserved charge can be neither be crated nor be destroyed but can only be transferred. This is law of conservation of electric current.

The amount of charge Q flowing through a particular area of Cross section in unit time t is called electric current. This leads to the basic formula for current which is I = Q/t. The electric current is a scaler quantity.

In a metal flow of electron constitutes the current. Direction of flow of electrons gives the direction of electronic current. By convention the direction of flow of positive charge is taken as the direction of current. This is called conventional current. Thus the direction is opposite to the direction of flow of electrons in the circuit. It should be remembered that in external circuit current always flow positive terminal of the cell to the negative terminal of that energy source.

Electric cell or battery is the device for generating or storing electrical energy by means of electrochemical reaction.
(i) Within a cell a chemical reaction occurs that transfer an electron from one terminal to the another terminal.
(ii) The chemical energy stored in the cell is responsible for maintaining the current in the external circuit by creating a difference of electric pressure called potential difference.

Electric potential: Electric potential at a point in an electric field is equal to the work required to transfer a unit positive charge from an infinite distance to a given point against an electric field. It is commonly known as voltage.
Electric potential (V) at any point in the electric field is given by
V = (Work done (W))/(Charge (Q))
SI unit of electric potential is volt named in honour of the Italian physicist Alessandro Volta.
1V= 1JC⁻¹.
Electric potential Difference: It measure the work done per unit charge. It is defined as the difference in the electric potential between two points in an electric field equal to the work done per unit quantity of charge in moving it from one point to another in an electrostatic field,
Thus Potential difference = (Work done )/(Quantity of charge moved )
P.D = V x B – V x A = (W x AB)/(Q)
One volt: The potential difference between two points in an electrostatic field is said be 1 volt if one joules work is done in moving 1 coulomb of electrical charge from one point to another in the same electrostatic field.
Negative charge always moves from lower potential to higher potential but reverse is true for current.
If both the ends across the conductor are at the same potential, no current flows through it.

(i) Ammeter: It is used to measure the current in a circuit and is always connected in series. It should have a low resistance.
(ii) Voltmeter: It measures potential difference between two points of the circuit and is connected in parallel. It should have a high resistance.
(iii) Galvanometer: It must be connected in series to detect the current in the circuit.

Insulator: (i) Those material which cannot conduct electricity as they have high electrical resistance are called insulator.
(ii) Electrical conductivity is either very small or negligible.
(iii) The electron is tightly bound with nucleus.
(iv) Glass, rubber, wood, ebonite, Bakelite, etc. are some examples of insulators.
Conductor: (i) A substance through which an electric charge can flow easily is called conductor.
(ii) It has high electrical conductivity and low electrical resistance. It has large number of free electrons
(iii) Silver, copper and Aluminium are the good conductor of electricity.

Resistance is the property of a conductor which opposite or restrict the flow of current through it. It is measured in Ohm’s. The SI unit of resistance is named in the honour of George Simon Ohm.
According to Ohm’s law, R= V/I
1 Ohm = (1 Volt )/(1 ampere )

The resistance of a conductor depends on
(i) its area of cross section (A) as R ∝ 1/A,
(ii) its length as R ∝ L
For constant temperature,
R ∝ 1/A or R = ρ 1/A
Where ρ(rho) is a constant of proportionality and is known as specific resistance or resistivity of the material of the conductor. It depends upon the nature of material. It increases with increases in temperature.

An alloy is a homogenous mixture of two or more metals or a metal and a non-metal in a fixed proportion. Example Nichrome is an alloy of nickel, chromium, manganese and iron metal and bronze is an alloy of Cu and Sn.
Properties of an alloy
(i) Resistivity of an alloy is generally higher than that of its constituent metals. So it has a higher resistance.
(ii) Alloy do not oxides readily even at easily at high temperature.
(iii) Change in resistivity of alloy due to change in temperature is much smaller as compared to its constituent metals.
Uses of alloys
(a) Due to reason (i) and (ii) alloys are commonly used in electrical heating devices such as room heater, water heater, electric iron, toaster, cooking range etc.
(b) On account of negligible effect of temperature alloys are used to make wire of standard resistance, resistance box, rheostat, etc.

When an end of one resistor is connected to the beginning of the next and so forth, the resistor are said to connected in series.
Characteristic of series circuit:
1. Total potential difference across the series combination is equal to the sum of the potential difference across the individual resistors V= V₁ + V₂ + V₃.
2. In series combination of resistors, same current passes through each resistor.
3. Potential difference across each resistor is directly proportional to the resistor V ∝ R.
4. Equivalent resistance of series combination is equal to the sum of their individual resistance R₁, R₂ and R₃
Rₛ = R₁ + R₂ + R₃
5. The equivalent resistance is greater than any individual resistance.
Uses of series circuit:
(i) It is used when large resistance in the circuit is required
(ii) current in the circuit is to be reduced
(iii) less potential difference across a particular resistance is needed.

When two or more resistor are connected between two common points whose one end will be at higher potential and other at lower potential in a circuit the resistors are said to be parallel.
Characteristics of parallel circuit:
1. Voltage across each resistor is same and is equal to the voltage applied across he combination.
2. Current which passes through individual branch of the circuit is inversely proportional to the resistance of that branch I ∝ 1/R
3. Total current I which flows in the circuit is equal to the sum of the current passing through the individual resistor of the combination. I = I₁ + I₂ + I₃
4. The reciprocal the equivalent resistance is the algebraic sum of the inverse of individual resistances
1/Rₚ = (1 )/R₁ + 1/R₂ + 1/R₃
5. Equivalent resistance of the parallel combination is less than the least resistance of any resistor in the circuit.
Uses of parallel circuit:
1. It is used when resistance of the circuit is to be decreased.
2. current in the circuit is to be increased.
Therefore, all the electrical appliances for a household purpose are connected in parallel combination.

If there is no branch at the common point then it is a series circuit.
If there are branches between two common points, then it is a parallel connection.

Practical applications of series circuit: Series circuit are used for dependent operation such as
1. Decorative light string on festivals.
2. Thermostats in hating devices to control the temperature.
3. Light switches, fuse with live wire in household wiring.
4. Batteries to get higher voltage.
5. Ammeter to measure the current.
6. Light emitting diodes are usually connected in series in electronic devices.

Practical application of parallel circuit:
1. Parallel circuit are used for independent operations of all the electrical devices such as light bulbs and other electrical devices in office, homes, etc. connected with the main supply.
2. Voltmeter measure the potential difference across the load.
3. Batteries give more current.
4. In a chandelier all light bulbs are in parallel.

1. More the components circuit has the greater will be its resistance. Example: On adding a light bulb in decorative string the bulb does not shine as brightly as before.
2. If the circuit is broken due to some defect in one device the entire circuit stops working because circuit become an open circuit.
3. The electrical appliances need current of widely different values to operate properly but in series sae amount of current flows through each of them.
4. All electrical appliances do not get the same voltage to operate properly because the voltage drop across each of them directly depends on their resistance.

(a) Heating effect: When an electric current passes through a wire the wire gets heated and its temperature rises. This is known as heating effect of current.
Example Light bulbs, electric heater, electric iron, electric welding etc.
(b) Magnetic effect: When an electric current flow through a wire it produces a magnetic field around it. This effect is known as magnetic field of current.
Example electromagnet, electric bell, electric fans etc.
(c) Chemical effect of current: When the current passes through the liquid it decomposes it into iits components. This effect of electric current is called chemical effect of current.
Example: Hydrolysis of water, electroplating process.

The heat produced in a resistor is directly proportional to the
(i) the square of the current for a given resistance. (H∝I²)
(ii) the resistance for a given current. (H ∝ R)
(iii) the time for which current flows through the resistor. (H ∝ t)
Thus the heat produced in the wire by current in time t is
H ∝ I²Rt or H = KI²Rt

A fuse is a safety device that does not allow any unduly high electric current to flow through an electric circuit.
(i) It works on the principle of heating effect of current.
(ii) It is made up of a metal and an alloy of Aluminium, copper, iron, lead and tin.
(iii) It should have high resistivity high resistance per unit length and low melting point and should be of a suitable rating corresponding to the load in the circuit.
(iv) It is available in various shape. It is usually encased in cartridge of porcelain and similar material sealed in a glass tube with metals ends.
(v) Use of different rating of fuse such as 1A, 2A, 3A, 5A, 10A, 15A, 32A, etc. depend on the current carrying capacity of the circuit and rating of an appliance. A fuse of different rating has different thickness. The radius of fuse wire would be larger if it is used in the circuit which carry high current.
Working of fuse: The fuse wire is always connected in series with the live wire or electrical devices. If due to some reason the flow of current exceeds the specified preset value, the heat produced in it melts it and disconnected the entire circuit or that device from the main supply. Thus fuse wire prevents the electrical circuit from fire and appliances from any possible damage due to excessive current flowing through it.

Electric power is the rate of doing work by an energy source or the rate at which the electrical energy it dissipated or consumed per unit time in an electric circuit.
Power P = (Work done )/(time ) = VI = V²/R = I²R
Unit of electric power: SI unit of electric power is watt (W).
Watt: 1 Watt is the power consumed by an electrical device that carry 1 ampere current when operated at a potential difference of 1 volt 1 watt = 1 volt × 1amp.
Bigger unit of power are 1kilowatt = 1Kw = 1000 W
1mega watt = 1 MW = 10⁶
The relationship between watt and Joules is given by (using P =(W )/(t ))
1 watt = (1J )/(1Sec )
Thus the electrical energy consumed by an electrical device at the rate of 1J/s
Using P = (Electrical energy dissipated )/(time )
E= P × t

1 unit means 1kWh
No. of units of electrical energy consumed in a month= Total no. of kWh = ( Watt ×hours )/1000
Total electrical energy consumed in a month = Total no. of units = ( Watt ×hours×days )/1000 = No. of units × No. of days in a month.
Total cost of electricity consumed = Total units × cost per unit of electricity.