In which era human beings and dinosaurs lie ?

Ans: Human lies to Cenozoic era and dinosaurs lies in Mesozoic era.

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Explain about domestic electric current.

Ans: 
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)
2. The potential difference between neutral and live wire is 220V.
3. In household circuit system, two type of circuit are used. One of 15 A for appliances with higher power ratings and the other of 5 A ratings for bulbs etc.
4. 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.
5. The earth wire is used as a safety measure, especially for those appliances that have a metallic body.

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How Ceramic is prepared?

Ans:

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Q. Determine the amount of heat to be taken out a price of hot iron block of mass 300gm to cool it from 300 C to 20 C? Specific heat of iron = 480 J/kgC

Soln:

Mass of iron (m) = 300gm = 0.3kg

Sp. Heat Capacity (S) = 480 J/Kg⁰C

Change in temperature (dt) = 300 -20 = 280⁰C

Quantity of heat (Q) = ?

Using the formula,

Q = m*S*dt = 0.3*480*280 = 40320 J Ans.

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Q.In a house, four bulbs each of 60 watt glow for 5 hrs and an electric heater of 1200 watt is used 2 hrs every day. If the minimum charge up to 20 units is Rs. 80 and the charge for 21 units to 250 units is Rs. 7.30/unit, calculate the cost of electricity consumed in a month.

Soln:

For 4 bulbs:

Power (p) = 4 * 60 = 240 watt = 0.24 kw

Time (t) = 5 hrs

Power consumption (PC1) = p * t = 0.24 * 5 = 1.2 kwhrs

For electric heater:

Power = 1200 watts = 1.2 kw

Time = 2 hrs

Power consumption (PC2) = 1.2 * 2 = 2.4 kwhrs

Total power consumption in a day  = PC1 + PC2 = 1.2 + 2.4 = 3.6 kwhrs

Total power consumption in a month (30 days) = 3.6 * 30 = 108 kwhrs (unit)

The charge up to 20 units = Rs. 80

No. of extra units = 108 - 20 = 88

The cost of 88 units = 88 * 7.30 = Rs. 642.4

Total cost of electricity consumed in month = Rs. 642.4 + Rs. 80 = Rs. 722.4 Ans. 

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Q. A body weight 63 N on the surface of the earth. What is the gravitational force on it due to the earth at a height of 3200 Km from the surface of the earth? [Radius of the earth is 6400 Km]

Soln:

Weight on the earth's surface (We) = 63 N

Radius of the earth (R) = 6.4 * 10⁶ m

Height from earth's surface (h) = 3200 Km = 3200000 Km = 3.2 * 10⁶ m

Let M is the mass of the earth and G is the gravitational constant. Then, the weight of the body on the earth is

We = mg = m * (GM/R²)

When the body is at a height h from the earth's surface, the weight of the body is

Wh = mg' = m * [GM/(R + h)²]

∵ Wh/We = R² / (R + h)² = [R/(R + h)]²

Wh = [ 6.4 * 10⁶ / (6.4 * 10⁶ + 3.2 * 10⁶ ) ]² * 63 = 28 N Ans.

Hence, the required gravitational force on it due to earth is 28 N.

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Q. Unit 4 Heat

2. Give reason:

a. Water is used to cool the engines of vehicles.

This is because heat absorbed/released by a body is directly proportional to its specific heat capacity (QµS). That is, higher the value of specific heat capacity, higher the absorption of heat by the body. Water has very high specific capacity (4200 Jkg^-1oC^-1) as compared with other liquids. Thus, it will absorb a large amount of heat from the hot engine without appreciable rise in temperature of water. It thus prevents the engine from getting excessive hot.

b. Water pipes burst during winter season.

In winter season, the atmospheric temperature may fall below 0^oC. This causes water to freeze in water-supply pipes. Water expands and occupies larger volume when it freezes. This exerts force on these water pipes. As a result, water pipes burst during winter season.

c. The metal antimony behaves like water.

It expands on freezing and contracts at first when warmed above its melting-point. Because of such expansion, it makes sharp castings and is used as a constituent of type-metal.

d. When a beaker filled with water at 4^oC is cooled or heated, water overflows from the beaker.

It is because of anomalous expansion of water. Water has maximum density at 4^oC (i.e. least volume at 4^oC). Hence, whether water at 4^oC is heated or cooled, its volume increases in either case and there is overflow of water.

e. Water is suitable for heating systems.

The specific heat capacity of water is very high. It means that large amount of heat is required to raise its temperature than other liquids having same mass through the same range of temperature. When water is cooled by the same range of temperature, the same amount of heat is given out as it has absorbed. Hence, a large quantity of heat may be available by cooling the hot water. That’s why water is suitable for heating systems.

f. The night of desert is very cold and day of that is very hot.

Specific heat capacity of sand is less, so its temperature changes fast. At days, sand gets heated which results very hot day. At nights, sand gets cooled which results very cold night.

g. We sweat in summer. (Perspiration is necessary in summer.)

In summer, the atmospheric temperature goes up higher than our normal body temperature (37^oC). To maintain our body temperature fixed at 37^oC, our body loses the excess heat by excreting sweat. So, we sweat in summer.

h. We need warm clothes in winter.

In winter, the atmospheric temperature falls below our normal body temperature (37^oC). Due to this difference in temperature, our body loses heat and so our body cools. To prevent our body from excessive cooling, we need warm clothes in winter.

i. Water in the earthen pot is cooler than that in a metal pot.

In earthen pot, water oozes out through the pores and is evaporated. Evaporation of oozed water takes heat from water in the vessel and hence it cools the water. Such type of oozing does not occur in metal pots. So, water in the earthen pot is cooler than that in a metal pot.

j. Why does solid melt on heating?

When a solid is heated, the kinetic energy of vibration of its molecules increases. The kinetic energy overcomes the inter molecular force of attraction. Then, the solid is converted into liquid.

k. Water is used in hot bottles. Why?

Water has the highest specific heat among all the liquids. A liquid having higher specific heat can store more heat energy. Due to this reason, water is used in hot bottles.

l. Soda water (Coca-Cola) bottles often explode in summer? Explain.

During summer, the temperature increases. The increase in temperature expands the gas in the soda bottles. When the gas in a closed bottle expands, the pressure increases. The bottle explodes due to this increased pressure.

m. What is the effect of pressure on melting point of a substance?

For a substance which expands on melting, the melting point is increased when pressure is increased. e.g. wax.

For a substance which contracts on melting, the melting point is decreased when pressure is increased. e.g. ice.

n. What is the effect of pressure on boiling point of a liquid?

The boiling point of a liquid is increased when pressure is increased.

o. Dogs in summer keep their tongues out. Why?

In summer it is very hot. When a dog keeps its tongue out, water evaporates through its tongue. For the evaporation, heat is absorbed from the body of the dog. Since the dog has lost heat, it feels cool.

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Q. The back tyres of tractor are wider.

Ans: The tractor has to pass through muddy soil during ploughing in fields. If the rear (back) tyres have small area, the pressure on the ground will be greater and the wheels may descend into the muddy land. This makes moving difficult. So, to reduce the pressure on the ground in order to make moving faster, the rear wheels of tractor has large area.

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