Pre Boards

Practice to excel and get familiar with the paper pattern and the type of questions. Check you answers with answer keys provided.

Sample Papers

Download the PDF Sample Papers Free for off line practice and view the Solutions online.

CBSE Class 10 Science Solved Question Paper 2018

Short Answer Type

11.

a) Write Joule’s law of heating.

b) Two lamps, one rated 100 W; 220 V, and the other 60 W; 220 V, are connected in parallel to the electric mains supply. Find the current drawn by two bulbs from the line, if the supply voltage is 220 V

12.

Name the hormones secreted by the following endocrine glands and specify one function of each :

(a) Thyroid
(b) Pituitary
(c) Pancreas

13.

2 mL of sodium hydroxide solution is added to a few pieces of granulated zinc metal taken in a test tube. When the contents are warmed, a gas evolves which is bubbled through a soap solution before testing. Write the equation of the chemical reaction involved and the test to detect the gas. Name the gas which will be evolved when the same metal reacts with a dilute solution of a strong acid.

14.

The pH of a salt used to make tasty and crispy pakoras is 14. Identify the salt and write a chemical equation for its formation. List its two uses.

15.

(a) Why are most carbon compounds poor conductors of electricity?

(b) Write the name and structure of a saturated compound in which the carbon atoms are arranged in a ring. Give the number of single bonds present in this compound.

16.

Decomposition reactions require energy either in the form of heat or light or electricity for breaking down the reactants. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light and electricity.

17.

What is a dam? Why do we seek to build large dams? While building large dams, which three main problems should particularly be addressed to maintain peace among local people? Mention them.

18.

Students in a school listened to the news read in the morning assembly that the mountain of garbage
in Delhi, suddenly exploded and various vehicles got buried under it. Several people were also injured
and there was traffic jam all around. In the brain- storming session, the teacher also discussed this issue and asked the students to find out a solution to the problem of garbage. Finally, they arrived at two main points - one is self-management of the garbage we produce and the second is to generate lee
garbage at the individual level.

(a) Suggest two measures to manage the garbage we produce.
(b) As an individual, what can we do to generate the least garbage ? Give two points.
(c) List two values the teacher instilled in his students in this episode.

19.

(a) List the factors on which the resistance of a conductor in the shape of a wire depends.
(b) Why are metals good conductors of electricity whereas glass is a bad conductor of electricity? Give reason.
(c) Why are alloys commonly used in electrical heating devices ? Give reason.

20.
(a) State Fleming’s left-hand rule.
(b) Write the principle of working of an electric motor.
(c) Explain the function of the following parts of an electric motor.
(i) Armature (ii) Brushes (iii) Split ring

The direction of the force which acts on the current carrying conductor placed in a magnetic field is given by Fleming's left-hand rule.

Fleming's right-hand rule stretch the thumb, forefinger and the central finger of the right hand mutually perpendicular to each other. If the forefinger points in the direction of the magnetic field, thumb in the direction of the motion of the conductor, then the central finger points in the direction of current induced in the conductor.

(b) Electric Motor: An electric motor converts electrical energy into mechanical energy.

Principle: It is based on the fact that when a coil carrying current is held in a magnetic field, it experiences a torque, which rotates the coil.

Construction: An electric motor consists of the following main parts:

1. Field magnet: It is a strong horse shoe type magnet with concave poles.

2. Armature: It is a rectangular coil ABCD having a large number of turns of thin insulated copper wire wound over a soft iron core.

The armature is placed between the poles of the field magnet and it can be rotated about an axis perpendicular to the magnetic field lines.

3. Split ring commutator: It consists of a cylindrical metal ring split into two halves S₁and S₂. The two ends A and D of the armature coil are connected to the split rings S₁ and S₂ respectively. As the coil rotates, the split rings also rotate about the same axis of rotation. The function of the split ring commutator is to reverse the direction of current in the coil after every half rotation.

4. Brushes: Two graphite or flexible metal rods maintain a sliding contact with split rings S₁ and S₂, alternately.

5. Battery: A battery of few cells is connected to the brushes. The current from the battery flows to the armature coil through the brushes and the split rings.

Working: Initially, suppose the plane of the coil is horizontal. The split ring S₁ touches the brush B₁ and split ring S₂ touches the brush B₂. The current in coil flows in the direction ABCD from the source battery, as shown in Fig.(a).

Clearly, the currents in arms AB and CD are in opposite directions. The force acting on arm AB pushes it downwards while the force acting on the arm CD pushes it upwards. Thus the armature coil alongwith the axle rotates anticlockwise. After half a rotation, as shown in Fig.(b), the split ring comes in contact with brush B₂, and S₂ in contact with brush B₁. Therefore, the current in the coil gets reversed and flows along the path DCBA. A device that reverses the direction of flow of current through a circuit is called commutator. Thus the arm AB is now pushed up and the arm CD is pushed down. Therefore, the coil and the axle rotate half a turn more in the same direction. The reversing of the current is repeated at each half rotation, giving rise to a continuous rotation of the coil and to the axle.

Split ring acts as commutator in electric motors.