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CBSE Questions for Class 7 Physics Heat Quiz 5 - MCQExams.com
CBSE
Class 7 Physics
Heat
Quiz 5
Find the good conductors of heat from the given options.
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Metals
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Plastic
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Paper
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Wood
Explanation
Metals are generally good conductors of heat.
Plastic, paper, and wood - all are bad conductors of heat.
In vacuum, heat ______ be transferred by convection.
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Can
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Cannot
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Both
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None
Explanation
In liquids
and gases, the heat is transferred by convection. No medium is required
for the transfer of heat by radiation
. In a vacuum, heat cannot be transferred by convection.
Why do insulators take larger time to heat up?
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They need more heat per every degree Celsius rise in temperature.
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They need less energy per every degree Celsius rise in temperature.
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They need more time to perform more work per every degree Celsius rise in temperature.
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They need less heat per every degree Farenheit rise in temperature.
Explanation
Substances that don't allow heat to easily pass through them are known as insulators. They cannot pass the heat easily through them. Therefore, the unit mass of an insulator requires more heat to raise its temperature by $$1^{o}$$ i.e. specific heat capacity of insulators is very high or we can say they need more heat per degree rise in temperature.
Substances which do not allow heat to easily pass through them are known as
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Conductors
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Insulators
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Semi Conductors
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None
Explanation
Substances which don't allow heat to easily pass through them are known as insulators. Insulators have crystal structure of molecules, molecules are far from each other, compared to conductors. Hence they cannot pass the heat easily through them, besides this insulators don't have free electrons to pass heat more quickly.
Substances which allow heat to pass through them easily are known as
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Insulators
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Conductors
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Semi Conductors
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None
Explanation
Substances which allows heat to pass through them easily are known as Conductors.
Substances which don't allow heat to pass through them easily are known as Insulators.
Substances which allows current to pass through them in some conditions are known as Semiconductors.
If needle is burnt then the heat transfer through the whole length of needle is faster because:
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needle is a bad conductor of heat
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needle is a good conductor of heat
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needle is an insulator
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none of the above
Explanation
If needle is burnt then the heat transfer through the whole length of needle is faster, because needle is made up of metal, which is a good conductor of heat. In the needle, heat passes along the whole length by conduction, in which heat is transferred by one atom to nearby atoms .
Water is poured into a pan and heated over a flame. After some time, the hot water begins rising to the top. Which type of heat transfer is involved in this ?
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conduction
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convection
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radiation
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thermal expansion
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specific heat
Explanation
Convection is heat transfer by mass motion of a fluid such as air or water when the heated fluid is caused to move away from the source of heat, carrying energy with it. Convection a
bove a hot surface occurs because hot air expands, becomes less dense, and rises.
Irons have much lower specific heat than:
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Steel
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Copper
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Water
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none of the above
Explanation
Specific heat of iron - $$460.548\ J/kg-K$$
Specific heat of steel - $$502.416\ J/kg-K$$
Specific heat of copper - $$376.812\ J/kg-K$$
Specific heat of water - $$4184\ J/kg-K$$
So from a simple comparison, we can see that irons have much lower specific heat than water.
Our body temperature cannot fall below:
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$$37 ^\circ C$$
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$$35 ^\circ C$$
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$$40 ^\circ C$$
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$$42 ^\circ C$$
Explanation
Our body temperature cannot fall below $$35^{o}C$$ or $$95^{o}F$$, because below this temperature body will suffer hypothermia.
When you approach fire, you feel heat on your body. Which of the following mode of heat transfer is involved in this ?
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Conduction
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Convection
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Radiation
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Thermal expansion
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Specific heat
Explanation
Heat radiation is the transfer of energy, in the form of electromagnetic waves, through space. The heat emitted from fire, is transmitted through space in the form of radiation. Hence, when you approach fire, you feel heat on your body.
.
This is the reason that, when facing the fire, only the front is warmed.
The backside is not warmed until the person turns around.
The more you go closer to the fire, more the amount of radiation is received.
Which of the following is an example of convection?
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The heat of the sun warming our planet
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The heat from an electric stove warming a frying pan
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Ice cubes cooling a drink
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A microwave oven cooking a meal
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An overhead fan cooling a room
Explanation
Convection is a form of heat transfer where a large number of molecules move from one place to another. An overhead fan works precisely by this method: it sends cooler air molecules down into a hot room, cooling the temperature of the room. The heat of the sun and the cooking action of a microwave are both forms of radiation, while the heat on a frying pan and the cooling action of ice cubes are both forms of conduction.
Insulators have very high:
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boiling point
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melting point
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specific heat capacity
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freezing point
Explanation
The specific heat capacity of a substance is the amount of heat required to raise the temperature of unit mass of substance through $$1^{o} \ C$$.
The insulators don't allow the heat to pass through them easily. A large amount of heat is required to raise their temperature even by $$1^{o} \ C$$ because insulators have high specific heat capacity.
If you touch the outside surface of the metal container, your hand will become warmer directly by which of the above choices?
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I only
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I and II only
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I and III only
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II only
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III only
Explanation
When you touch the outer surface of the container, there is a direct contact between the hand and the container. So, heat is conducted through the metal to your hand and causing the sensation of hotness.
As the air around the base of a candle flame is heated, it rises and is replaced by cooler air due to which of following type of heat transfer?
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Conduction
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Convection
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Radiation
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Diffraction
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Latent heat
Explanation
Whenever the temperature of any region of a liquid or gas becomes greater than other region ,density of hotter region becomes low for that the particles from high density region (colder region) come there ,that is convection. in the given incident it is happening so it is convection.
In conduction particles don't leave their positions so it not conduction .
In radiation heat is transferred from one point to other without heating the medium so it is also not radiation.
Latent heat is related to state change but there is no state change.
Diffraction is the property of waves , having no relation in this particle system.
The heat transfer in air is poor because
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it takes place mainly due to convection
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it takes place mainly due to conduction
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it takes place mainly due to radiation
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all of the above
Explanation
Air is a gas and its molecules are far apart from each other. So, this is not a good condition for conduction. Also, air being a non-metal doesn't have free electrons for conduction. Hence option B is not applicable.
C is also not applicable because radiation doesn't need any medium. Air being a fluid, convection can take place through it. But convection is not a very fast mode of transfer of heat.
In the convection process rate of heat transfer is compared in the option between gas and liquid, choose the correct statement
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Gas convects more as compared to other
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Liquid convects more as compared to other
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Liquid convects more as compared to other only if certain factors assists water
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None of the above
Explanation
In liquids, particles are neither too close nor too far but comparatively closer than gas; so liquid convects more as compared to gas. At $$0^{\circ}C$$ and $$4^{\circ}C$$ when density is more; the rate of convection decreases. But when the density of water is normal i.e. at room temperature, the convection rate of water is pretty good. Density only affects the rate of convection.
Using a heating blanket to get warm is an example of-
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Conduction
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Convection
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Radiation
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Insulation
An insulator is a non-metallic substance which :
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allows heat to pass through it
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has extremely low resistance to the flow of charge through them
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has extremely high resistance to the flow of charge through them
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increases current in the circuit
Explanation
An electrical insulator is a material whose internal electric charges do not flow freely because of extremely high resistance. Very little electric current will flow through it under the influence of a potential difference applied.
Metals are good conductors of electricity than non-metals because:
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Metals have free electrons which can move from atom to atom easily
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Non-metals have free electrons which can move from atom to atom easily
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Metals have bound electrons which cannot move easily
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Non-metals have bound electrons which cannot move easily
Explanation
Metals have free electrons which can move from atom to atom easily, whereas non-metals do not, their electrons are 'bound' meaning they cannot move easily. That's why metal are usually good conductors than non-metals.
Which of the following is/are bad conductors of heat?
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Glass
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Aluminium
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Iron
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Wood
Explanation
All metals are good conductors while non-metals are bad conductors of heat. Hence, the correct options are A and D.
Air is a bad conductor of heat because:
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it's molecules are in continuous contact with each other.
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it's molecules are not in continuous contact with each other
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it's molecules absorb all the heat
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it's molecules are moving randomly due to Brownian motion
Explanation
Air is a bad conductor of heat because it's molecules are not in continuous contact with each other.
Air molecules are too far to disperse heat energy to one another efficiently. Heat is conducted by molecules and atoms that are very closely bonded together and vibrating at high frequency.
In conductors, outer electrons :
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are not tightly bound to the nucleus
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are tightly bound to the nucleus
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never becomes free electrons
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are always free to move
Explanation
Conductors can easily conduct electricity because outer electrons are loosely bound to the nucleus.
Overhead cables are insulated from pylons through
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plastic.
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ceramics and glass.
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rubber.
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PVC.
Explanation
Examples of ceramics are clay, bricks, tiles, glass, and cement.
Overhead cable is insulated from pylons through ceramics and glass.
The overhead power lines that are supported by pylons are not covered in insulating materials. However, ceramics or glass insulators are fitted between pylons and wires. This stops electricity flowing down the metal pylons to the ground, where it could hurt or even kill people or animals.
Why are electrical plugs made of plastic?
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Plastic is stronger than all other materials.
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Plastic is a good electrical conductor.
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Plastic protects people from electricity.
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Only plastic can be made into the right shape for a plug
Explanation
Electrical plugs are made up of plastic because plastic protects people from electricity. As we know that plastic is an electrical insulator readily available and cheap. So as to make the plugs shock free, they are made up of plastic.
Why is electrical wiring usually covered with a layer of plastic?
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To make it look pretty
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To help electricity flow along the wire
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To make it safe
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none of these
Explanation
Electrical wiring is usually covered with layer of plastic to make it safe. As we know that plastic is an electrical insulator and it is readily available and cheap. So as to insulate the electricity carrying wire, to make it shock free. It is covered with layer of plastic.
Which of these is a good electrical conductor?
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Water
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Sweat
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The human body
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All of the above
Explanation
Water is a good electrical conductor but not distilled water. Tap water, Sea water, rain water etc. conduct electricity as they consist of impurities; salts of $$Ca, \; Mg,\; Na$$ which behave as charge carriers.
Sweat conducts electricity although it is mainly water, it still consists of lot of minerals, lactic acid, urea; in short, it is an ionic liquid (electrolyte)
Human body consists of $$75 $$% of water and also consists of many minerals and salts. So human body also conducts electricity.
A material that does NOT let electricity pass through it is called ...
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an electrical conductor
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an electrical insulator
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an electrical appliance
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none of these
Explanation
A material that does not allow to electricity to pass through, it is called an electrical insulator.
They do not have free electrons present in their outer most shell which would work as charge carriers for conduction of electricity.
Easy to shape materials like ceramics are used as
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conductors.
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insulators.
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electrodes.
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catalysts.
Explanation
Glass, tiles, pottery, porcelain, brick, cement etc. are ceramics.
They are obviously insulators of heat as they conduct heat.
Examples of non-conductors include
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sulphur.
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phosphorus.
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wood.
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all of these.
Explanation
Sulphur, Phosphorus, Wood are non-conductors of electricity due to no presence of free electrons in outermost shell.
Poor conductors of electricity consists of
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silver
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aluminum
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distilled water
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copper
Explanation
Pure water or distilled water does not conduct electricity but tap water, rainwater, or seawater conduct electricity. For electricity to travel through a liquid, movement of charges must take place through the liquid. In tap water, rainwater or seawater, there are impurities such as a salt of $$Na,Mg \; and \; Ca$$. They behave as charge carriers for the conduction of electricity. But in distilled water, there are no impurities i.e. no ions. There are only neutral water molecules. So, distilled water does not conduct electricity.
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