NCERT Solutions for Class 10 Science Chapter 12

Resistance: The resistance of a conductor is its property by virtue of which it opposes the flow of current through it. It is equal to the ratio of the potential difference applied across the conductor to the current flowing through it.
Resistance = Potential difference / Current Or R = V/I.

A closed and continuous path along which an electric current flows is called an electric circuit.
In a torch, a switch provides a conducting link between a battery (a number of cells placed in proper order) and a bulb. When the switch is turned ON, an electric current flow through the bulb and its gives light. Such a continuous and closed path on an electric current is called an electric circuit. When the circuit is broken anywhere (or switch is turned OFF), the current stops flowing ant 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.

Potential difference: The potential difference between two points in an electric field is the amount of work done in bringing a unit positive charge from one point to the other.
V = W/Q or Potential difference = Work done / Charge
The SI unit of potential difference is joule/coulomb or volt.

In most of the household heating appliance, nichrome element is used because of the following reasons:
(i) Its melting point is high.
(ii) Its resistivity is large.
(iii) It can be easily drawn into thin wire.
(iv) It is not easily oxidised by the oxygen of the air when heated.

To measure the potential difference, a voltmeter should be connected in parallel.

Total resistance of resistors when connected in series is given by R = R1 + R2 + R3 + R4 + R5 = ⟹ R= 0.2 Ω + 0.3 Ω + 0.4 Ω + 0.5 Ω + 12 Ω =13.4 Ω According to Ohm’s law, V = IR I = V/R = 9/13.4 = 0.67 A There is no current division occurring in a series circuit. So, the current through the 12 Ω resistor will be same as 0.67 A.

Energy consumed by an electrical appliance is given by H = Pt For the TV set: Power W = 250 W and time t = 1 hour = 3600 seconds So, energy consumed H = 250 × 3600 = 900000 J For the toaster: Power W = 1200 W and time t = 10 minutes = 600 seconds So, energy consumed H = 1200 × 600 = 720000 J Hence, TV set uses more energy than toaster.

The melting point and resistivity of tungsten are very high. It does not burn readily at a high temperature. The electric lamps glow at very high temperatures. Hence, tungsten is mainly used as heating element of electric bulbs.

The conductors of electric heating devices such as bread toasters and electric irons are made of alloy because resistivity of an alloy is more than that of metals. It produces large amount of heat and do not burn easily.

There is voltage division in series circuits. Each component of a series circuit receives a small voltage for a large supply voltage. As a result, the amount of current decreases and the device becomes hot. Hence, series arrangement is not used in domestic circuits.

Copper and aluminium wires have low resistivity. They are good conductors of electricity. Hence, they are usually employed for electricity transmission.

An electric circuit is the pathway in which current can flow. It consists of electric devices, switching devices, source of electricity, etc. that are connected by conducting wires.

A cell, battery, power supply, etc. helps to maintain a potential difference across a conductor.

When 1 J of work is required to move a charge of 1 C from one point to another, then it is said that the potential difference between the two points is 1 V. V = W/Q 1 V = 1 j / 1 C

The resistance of a conductor depends upon the following factors: Length of the conductor Cross-sectional area of the conductor Material of the conductor Temperature of the conductor.

The resistivity of an alloy is higher than the pure metal and it does not corrode easily. Moreover, even at high temperatures, the alloys do not melt readily. Hence, the coils of heating appliances such as electric toasters and electric irons are made of an alloy rather than a pure metal.

In parallel there is no division of voltage among the appliances. The potential difference across each appliance is equal to the supplied voltage and the total effective resistance of the circuit can be reduced by connecting electrical appliances in parallel.

The heating element of an electric heater is a resistor. According to Joule’s law of heating, the amount of heat produced by it is proportional to its resistance. H = I^2 Rt The resistance of the element of an electric heater is very high. As current flows through the heating element, it becomes too hot and glows red. On the other hand, the resistance of the cord is low. It does not become red when current flows through it.

The rate of consumption of electric energy in an electric appliance is called electric power. Hence, the rate at which energy is delivered by a current is the power of the appliance.