DAV Class 7 Science Chapter 5 Heat Question Answer Notes
DAV Class 7 Science Chapter 5 Heat Question Answer Notes Solution by Expert Science Teacher. DAV Class 7 Science 5th Chapter Heat for Students of DAV Schools.
Lesson No 5
Heat
1.) Fill in the blanks
Answer: (1) energy (2) 35, 42 (3) poor / bad (4) temperature (5) radiation
B.) Write True or False for the following statements.
Answer: (1) False , heat is produced when work is done.
(2) False , gaps are left between tracks so that when they expand during seasons it does not cause any accident
(3) True
(4) True
(5) False, radiation does not require a medium.
C.) Tick the correct option.
Answer:
1.) there would be no transfer of heat between the copper ball and water.
2.) 180C
3.) as the thermometer is taken out from the mouth, the level of mercury, in it, immediately starts falling.
4.) conduction as well as convection
5.) the setting up of ‘convection currents, in the air inside the refrigerator.
D.) Answer the following questions in brief.
1.) In which direction does the transfer of heat normally take place?
Answer: Heat transfer takes place from a body at a higher temperature to body at a lower temperature
2.) What is the cause of heat generation in the following situations:
(a) We apply brakes on our fast-moving car.
(b) People often jump up and down to feel warmer in cold weather.
Answer:
(a) Frictional force
(b) Blood circulation rate increases
3.) State two types of physical changes that may take place when a substance is heated.
Answer: the substance tends to change its state that is may change from solid to liquid or even liquid to gas, the other change is that it might change in shape and size , an example would be metals expanding on being heated.
4.) In what way(s) does a clinical thermometer differ from an ordinary thermometer?
Answer: The clinical thermometer is calibrated from 35 degrees Celsius to 42 degrees Celsius.
it is slightly bent or has a king, he missed capillary tube .
5.) State the mode/different modes, of heat transfer, in the following situations.
(a) A paper cup, full of hot soup, lying on a table.
(b) Cooking vegetables in a pan.
(c) Melting of a chocolate bar, in the school bag, on a hot day.
(d) Cooking food in a microwave oven.
Answer:
(a) Convection
(b) Convection as well as conduction
(c) Conduction
(d) Convection
6.) What is meant by sea breeze? When does it occur?
In costal areas this phenomenon is observed.
E.) Answer the following questions.
Q1. In the arrangement shown in figure pins A, B, C and D are fixed to a circular metal loop with the help of wax. The circular metal loop is heated at the point A with the help of candle flame. In which order would the pins fall if AB< AD? Justify your answer.
Draw diagram
Answer: since the candle flame is placed at therefore the pins will fall in the order A, B, D and lastly C.
Q.) 2.) Give reasons for the following:
(a) Iron rims are heated red hot before ‘fixing’ them on cart wheels.
We know that on heating metals expand this is the reason why metal rims are heated before they are fixed onto cart wins so that they expand in size and can easily fit the wooden wheel. When the rim cools down it contracts and gets tightly fixed to the wheel.
(b) A clinical thermometer has a slight bend, or kink, in its capillary tube.
The kink helps the mercury thread from falling back Just after a person’s temperature has been recorded. this makes sure that one can take the reading properly every time with utter convenience.
(c) A new quilt is warmer than an old one.
Blankets keep us warm because air is trapped inside the fabric and acts as an insulator. In case of old blankets, due to wear and tear the air inside it becomes lesser as a result result it provides lesser insulation than blankets that are comparatively warmer.
(d) A brass tumbler feels much cooler than a wooden tray on a chilly day.
It is so because a wooden tray is an insulator and bad conductor of heat and therefore does not let much of the heat escape from it as well as a result it loses less heat than the brass tumbler keeping it warmer than it.
(e) The bottoms of cooking utensils are often kept black.
Black absorbs heat , thus the bottom of the utensil that is placed on the flame is painted black so that the utensil can absorb as much as heat possible.
Q.) 4.) Explain briefly how winds are caused.
Q.5.) Supriya, while doing an experiment in the Science Laboratory, kept a laboratory thermometer ‘P’ 10 cm away on one side of the flame of a candle. Her friend Riya kept a similar thermometer ‘Q’ 10 cm above the flame of the candle as shown in the figure. (see figure in book)
In which of the thermometers, ‘P’ or ‘Q’, the rise in temperature will be faster? Give reason for your answer .
Draw
Answer: In the thermometer Q temperature will rise more since it was kept above the flame. Since hot air rises in an upward direction it is therefore more likely that the temperature in Thermometer Q will rise.
Q. 6.) In the two ‘set-ups’ X and Y, shown on the next page, the wires AB and PQR are made of the same material and have equal ‘thickness. The length of the wire AB, (in the set-up ‘X’) is equal to the diameter (= PR) of the semi-circle, formed by the wire PQR, (in the set-up ‘Y’). Pins, P1 and P2, are attached, to wires AB and PQR respectively, with the help of wax. Which of the two pins, P1 or P2, will fall off later? Give reason for your answer.
Answer : P2 will come off later because pqr is longer thus will take longer time to be heated uniformly..
More Extra Questions for Preparation
Q.1) Fill in the blanks.
1.) Expansion is greater in gases and least in solid.
2.) When you heat a solid it melts or change into a liquid.
3.) The heat energy received by an object through radiation is called radiant energy.
4.) Melting point of ice is 0o C.
5.) Boiling point of water is 100o C.
Q.2) True or false.
1.) Metals are bad conductors of heat.
Answer= False
2.) Good conductors of heat are also known as insulators.
Answer= False
3.) Heat can be converted into other forms of energy.
Answer= True
Q.3) Match the following.
1. Thermometer a) Cyclic movements of molecules |
2. Fahrenheit b) Clinical thermometer |
3. Doctors thermometer c) o F |
4. Convection d) Measure the temperature |
Answer= 1 – d), 2 – c), 3 – b,) 4 – a).
Q.4) Choose the correct option.
1.) Device used to measure temperature of human body.
a) Clinical thermometer
b) Laboratory thermometer
c) Both a and b
d) None of these
Answer= a) Clinical thermometer
2.) The breeze that flows from sea surface to land surface during day time.
a) Land breeze
b) Sea breeze
c) Water breeze
d) None of these
Answer=b) Sea breeze
3.) Range of laboratory thermometer is from
a)35oC-42o C
b) -15o C-25oC
c)-10oC- 110oC
d)82oC-150oC
Answer= c)-10oC- 110oC
Q.5 ) Differentiate Good and Bad conductors of heat from the following.
Air, copper, wood, gold, glass, rubber
Answer Good conductor – Copper, Gold, and Rubber
Bad conductor- Air, Glass, Wood
Q.6) Define the following.
1.) Insulators with example.
Answer – Materials that do not allow the electricity to pass through them are called insulators.
Example – wood, air, plastic.
2.) Radiant energy.
Answer – The heat energy by an object through radiation is called radiant energy.
3.) Methods of transfer of heat.
Answer –
a.) Conduction
b.) Convection
c.) Radiation
Q.7) Answer the questions.
1.) How we measure temperature?
Answer – Temperature is measured by thermometer.
2.) What are two commonly scales for measuring temperature?
Answer – Celsius scale and Fahrenheit scale.
3.) How can oxygen be prepared in laboratory?
Answer – We can prepare oxygen in laboratory by heating potassium chlorate over a flame ( along with manganese oxide as a catalyst).
Q.8) What are physical changes of heat?
Answer –
a.) Changes in temperature – Addition of heat to a body raises it’s temperature and removal of heat from body lowers temperature.
b.) Change of state – Under appropriate condition, addition or removal of heat to or from body bring about a change in its state.
9) Give reason
1.) Mercury is often used in thermometer.
Answer – Mercury is often used in thermometer because mercury expands in a uniform way and also remains in its liquid state over a wide temperature range. It does not stick to the walls of glass capillary tube and is a shining silvery liquid.
Q.10) What are monsoon winds?
Answer – 1.) In summer at places near the equator the land warms up faster.
2.) Hence, most of time land temperature is higher than that of water in oceans.
3.) The air over the load gets heated up and rises.
4.) This causes the wind to flow from the oceans towards the land.
5.) These winds carry water and bring rain.
6.) Hence, these winds are called monsoon winds.