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Explain the following. (a) Why is the tungsten used almost exclusively for filament of electric lamps? (b) Why are the conductors of electric heating devices, such as bread-toasters and electric irons, made of an alloy rather than a pure metal? (c) Why is the series arrangement not used for domestic circuits? (d) How does the resistance of a wire vary with its area of cross-section? (e) Why are copper and aluminum wires usually employed for electricity transmission?

(a) Tungsten is used as filament in electric bulbs The melting point and resistivity of Tungsten are very high. At high temperatures, it does... View Article

Compare the power used in the 2 Ohm resistor in each of the following circuits: (i) a 6 V battery in series with 1 Ohm and 2 Ohm resistors, and (ii) a 4 V battery in parallel with 12 Ohm and 2 Ohm resistors.

Answer: We need to compare the power consumed in the given two combinations. A 6 V battery in series with 1 Ohm and 2 Ohm resistors Potential... View Article

While teaching the topic of physical and chemical changes, Mahima’s teacher listed the conditions that take place during a change. a) Change in state b)Change in colour c) Evolution of a gas d)Change in temperature He then asked Mahima to identify the conditions based on which it can be said with certainty that a particular change is chemical in nature. Among the changes that are listed by the teacher, which changes should be selected by Mahima?

Any change that is accompanied by evolution of gas is known as a chemical change. Therefore, whenever a gas is evolved in a process, it can be... View Article

Calculate the amount of ice which is required to cool 150 g of water contained in a vessel of mass 100 g at 30°C, such that the final temperature of the mixture is 5°C. Take specific heat capacity of material of vessel as 0.4 J g1°C-1 ; specific latent heat of fusion of ice = 336 J g-1; specific heat capacity of water = 4.2 J g-1°C-1.

Heat lost by the vessel and water= [150 x 4.2 x (30 - 5) J + 100 x 0.4 x (30-5)] J = (150 x 4.2 x 25 + 100 x 0.4 x 25) J = (15,750 + 1000) J=... View Article

Explain the process of synthesis of food in green plants.

The process by which plants make food is named photosynthesis. Photosynthesis is the process in which green plants use sunlight to make their own... View Article

In a reaction, 5.3 g of sodium carbonate reacted with 6 g of ethanoic acid. The products were 2.2 g of carbon dioxide, 0.9 g water and 8.2 g of sodium ethanoate. Show that these observations are in agreement with the law of conservation of mass. sodium carbonate + ethanoic acid → sodium ethanoate + carbon dioxide + water.

In the given reaction, sodium carbonate reacts with ethanoic acid to produce sodium ethanoate, carbon dioxide, and water sodium carbonate +... View Article

On a hot day, Aslam takes a bottle of cold-drink from the refrigerator and places it on a table. Within minutes, some drops of water appear on the outer surface of the bottle. As the water drops accumulate, they trickle down the surface of the bottle. Name the processes of the water cycle that are demonstrated in the events observed by Aslam.

Answer: The process by which water vapour is cooled into water is called condensation. The temperature outside the refrigerator is warmer... View Article

Smita studied about the Modern Periodic Table. She placed hydrogen, lithium, sodium, and calcium in the same group in the periodic table. However, her teacher told her that she has placed one element incorrectly in the periodic table.(Atomic numbers of H, Li, Na, and Ca are 1, 3, 11, and 20 respectively) Which element has been placed incorrectly by Smita?

Elements containing same number of valence electrons are present in the same group of the modern periodic table. The number of valence electrons... View Article

How would these advantages differ from the advantages of using a long-term perspective in managing our resources?

If resources are used in accordance with short term aims, the present generation will be able to utilize the resources properly for growth and... View Article

Consider a circular loop of wire lying in the plane of the table. Let the current pass through the loop clockwise. Apply the right-hand rule to find out the direction of the magnetic field inside and outside the loop.

Inside the loop = Pierce inside the table Outside the loop = Appear to emerge out from the table For downward direction of current flowing in the... View Article

What are the different ways in which individuals with a particular trait may increase in a population?

An individual trait may increase in a population in the following ways The trait is beneficial to the population: if a trait is beneficial to... View Article

In the case of real gases, the equation of state, PV = RT (where P, V and T are respectively the pressure, volume and absolute temperature), is strictly satisfied only if corrections are applied to the measured pressure P and the measured volume V. When does the corrections for P and V arise?

In kinetic theory of gases it is assumed that there is no force between molecules But there is actually intermolecular attraction which reduces... View Article

Give two reasons to justify: a) Water at room temperature is a liquid. b) An iron almirah is a solid at room temperature.

Water at room temperature is liquid due to the presence of hydrogen bond. These bonds hold water molecules together for small fractions of a... View Article

Which hormone causes dilation of blood vessels, increased oxygen consumption and glucogenesis? a)Adrenaline b)Serotonin c)Insulin d)Thyroid

Adrenaline is secreted by the adrenal medulla. It mainly acts on smooth muscles, cardiac muscles, blood vessels, skeletal muscles and fat cells.... View Article

A scooter waiting on a signal has its mirror vibrating violently. Why?

This is because of resonance. When a body is subjected to forced vibration with a frequency which equal to or an integral multiple of the natural... View Article

Name the physical quantity which we get from a S-t graph.

Speed-Time Graphs look much like Distance-Time graph. Time is plotted on the X-axis. Speed or velocity is plotted on the Y-axis A straight... View Article

In the following example, try to identify the number of times the velocity of the ball changes: “A football player kicks a football to another player of his team who kicks the football towards the goal. The goalkeeper of the opposite team collects the football and kicks it towards a player of his own team”. Also identify the agent supplying the force in each case.

The velocity of football changes four times. First, when a football player kicks to another player, second when that player kicks the football to... View Article

Why is it difficult to hold a school bag having a strap made of a thin and strong string?

It is difficult to hold a school bag having a thin strap because the pressure on the shoulders is quite large. This is because the pressure is... View Article

What is an induced movement?

Induced movement or induced motion is an illusion of visual perception in which a stationary or a moving object appears to move or to move... View Article

Which of the following molecules show the presence of intramolecular hydrogen bonding? (a) Cellulose (b) HCN (c) Concentrated Acetic Acid (d) H₂O₂

Option (a) Cellulose Cellulose shows the presence of intra molecule hydrogen bonding in them. This is due to the fact that the Glucose monomers... View Article

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