Olympiad Test: Heat And Temperature -2 - Class 7 MCQ

Convection of heat takes place in:
Introduction:
Convection is a mode of heat transfer in which heat is transferred through the movement of fluids. It occurs in liquids and gases due to the movement of molecules. Convection plays a significant role in various natural and man-made processes.
Explanation:
Convection of heat takes place in liquids and gases. Here's why:
Liquids:
- When a liquid is heated, the molecules gain energy and become less dense.
- The less dense hot liquid rises while the cooler, denser liquid sinks.
- This creates a circular flow called a convection current.
- Through convection, heat is transferred from the hot region to the cooler region in the liquid.
Gases:
- Similar to liquids, when a gas is heated, the molecules gain energy and become less dense.
- The less dense hot gas rises while the cooler, denser gas sinks.
- This movement of gases creates convection currents, transferring heat from one region to another.
- Convection is responsible for processes like atmospheric circulation, ocean currents, and ventilation systems.
Summary:
Convection of heat occurs in both liquids and gases. The movement of less dense, heated fluids creates convection currents, which transfer heat from hotter regions to cooler regions. This process is essential for natural phenomena and various engineering applications. Therefore, option (D) "liquids and gases" is the correct answer.

Incorrect Statement: C: Radiation requires medium for heat flow
Explanation:
Conduction, convection, and radiation are three different methods of heat transfer. Let's break down each statement to identify the incorrect one:
1. Convection current causes trade winds:
- This statement is correct. Convection currents play a significant role in the formation of trade winds, which are global wind patterns caused by the uneven heating of the Earth's surface.
2. Transmission of heat without actual movement of particles is called conduction:
- This statement is correct. Conduction is the transfer of heat through direct contact between particles or objects without the actual movement of particles.
3. Radiation requires medium for heat flow:
- This statement is incorrect. Radiation is a method of heat transfer that does not require a medium. It can occur through empty space, unlike conduction and convection. Radiation involves the emission of electromagnetic waves, such as infrared radiation, which carry thermal energy.
4. In convection, heat is transferred through the movement of fluid:
- This statement is correct. Convection is the transfer of heat through the movement of a fluid (liquid or gas). When a fluid is heated, it becomes less dense and rises, while the cooler fluid descends to take its place, creating a convection current.
Therefore, the incorrect statement is C: Radiation requires medium for heat flow.

Explanation:
When a wooden spoon is dipped in an ice cream cup, it undergoes a process called conduction, which is the transfer of heat from a hotter object to a colder object through direct contact. Here's a detailed explanation:
Conduction:
- Conduction is the transfer of heat energy through direct contact between two objects.
- In this scenario, the wooden spoon and the ice cream cup are in direct contact, allowing heat to transfer from the spoon to the ice cream cup.
- The spoon, being at a higher temperature than the ice cream, transfers its heat energy to the colder ice cream cup.
- As a result, the spoon loses heat and becomes colder.
Convection:
- Convection is the transfer of heat energy through the movement of a fluid (liquid or gas).
- In this scenario, there is no fluid movement involved, so convection is not the primary mechanism for heat transfer.
Radiation:
- Radiation is the transfer of heat energy through electromagnetic waves.
- In this scenario, radiation is not the primary mechanism for heat transfer as there is no significant emission or absorption of electromagnetic waves.
No Becomes Cold:
- The correct answer is that the wooden spoon does not become cold.
- While the spoon may lose some heat through conduction to the ice cream cup, it does not become cold as the ice cream itself is at a very low temperature.
- The spoon may cool down slightly, but it will not become as cold as the ice cream.
To summarize, when a wooden spoon is dipped in an ice cream cup, it becomes colder through conduction, but it does not become as cold as the ice cream itself.

Explanation:
The odd one out is option C, "We wear dark-coloured clothes in summers."


The other options are related to the concept of heat absorption and radiation, while option C is about personal preference or cultural practice. Let's break down each option to understand why it is the odd one out:
A: Good absorbers are good radiators
- This statement is based on the principle of thermal radiation. Good absorbers of heat also tend to be good radiators of heat. This means that materials that can absorb heat well can also release it efficiently.
B: Shiny surfaces are good reflectors of heat
- Shiny surfaces have a high reflectivity, meaning they reflect a significant portion of the incident heat radiation instead of absorbing it. This property makes shiny surfaces good reflectors of heat.
C: We wear dark-coloured clothes in summers
- This statement is more subjective and based on personal preferences or cultural practices. While it is true that dark-colored clothes can absorb more heat from sunlight compared to light-colored clothes, it is not a universally followed practice in all regions or cultures.
D: Dark colors are good absorbers of heat
- This statement is true and aligns with the concept of heat absorption. Dark colors, such as black, absorb a larger amount of heat compared to lighter colors. This is because dark colors have a higher absorption coefficient for thermal radiation.
Therefore, option C stands out as it is not directly related to the scientific concept of heat absorption and radiation, unlike the other options.

Reflecting solar films are used on the top of the car to prevent heating by radiation.

Here are the reasons why reflecting solar films are used on the top of the car:

• Reflecting solar films are designed to reflect and block a significant amount of the sun's heat and radiation.


• By preventing the heating of the car's interior, the films help to maintain a cooler temperature inside the car, especially during hot summer days.


• Reducing the amount of heat radiation absorbed by the car's surface helps to minimize the need for air conditioning, thus saving energy and improving fuel efficiency.


• The films also help to protect the car's interior from the harmful effects of UV rays, which can cause fading and damage to upholstery, dashboard, and other components.


• By reducing the heat buildup inside the car, the films also contribute to a more comfortable driving experience for the occupants.


• Reflecting solar films are often transparent or tinted, allowing for visibility while still providing the desired heat and UV protection.

In summary, reflecting solar films are used on the top of the car primarily to prevent heating by radiation, improve energy efficiency, protect the car's interior, and enhance the comfort of the occupants.

The instrument used to detect radiation of heat is a thermoscope.

• A thermoscope is a device used to detect and measure the presence and intensity of heat radiation.


• It consists of a glass bulb with a thin stem that contains a liquid or gas.


• When heat radiation falls on the bulb, the liquid or gas in the stem expands and rises.


• This expansion and rise in the liquid or gas level indicates the presence and intensity of heat radiation.


• Thermoscopes are commonly used in various fields, including physics, meteorology, and medical science.


• They are particularly useful in measuring and monitoring temperature changes and heat flow.


• Unlike a thermometer, which measures temperature directly, a thermoscope measures the radiation of heat.

In summary, a thermoscope is the instrument used to detect radiation of heat.

Reasons for leaving gaps between railway tracks:
1. Expansion and Contraction:
- Railway tracks are made of metal, usually steel, which expands and contracts with changes in temperature.
- During hot summer months, the tracks can expand due to the heat, and if there were no gaps, they would buckle, bend or even break.
- By leaving gaps between railway tracks, there is room for the tracks to expand without causing any damage.
2. Thermal Stress:
- Heat causes thermal stress in railway tracks, which can lead to warping or distortion.
- The gaps between the tracks allow them to absorb the thermal stress without causing any significant damage.
3. Maintenance Access:
- Leaving gaps between railway tracks allows maintenance personnel to access the tracks more easily.
- It provides space for inspection, repair, and maintenance activities, such as checking for wear and tear, replacing damaged components, or adjusting the track alignment.
4. Noise Reduction:
- The gaps between railway tracks contribute to reducing noise levels produced by train movements.
- The small gaps create a rhythmic sound when the train wheels pass over them, which can be more pleasant compared to a continuous noise.
5. Safety:
- The gaps between tracks also serve as a safety measure.
- They allow for the drainage of rainwater, preventing the accumulation of water on the tracks, which could potentially cause slipping or sliding of the train wheels.
In conclusion, the gaps between railway tracks are left primarily to accommodate the expansion and contraction of the tracks in response to temperature changes, ensure safety, allow for maintenance access, and reduce noise levels.

Human body temperature is normally
The normal human body temperature is typically considered to be 98.6°F (37°C). This temperature can vary slightly depending on factors such as age, gender, time of day, and activity level.
Explanation:
There are several key points to understand about human body temperature:
1. Normal range: The normal range for human body temperature is typically between 97°F (36.1°C) and 99°F (37.2°C). However, the average temperature is often cited as 98.6°F (37°C).
2. Variations: It's important to note that body temperature can vary throughout the day. It tends to be lower in the morning and higher in the late afternoon or evening. Additionally, body temperature can be influenced by factors such as physical activity, stress, and illness.
3. Measurement: Body temperature can be measured using different methods, including oral, rectal, ear, and forehead thermometers. The most common method is oral measurement, which involves placing a thermometer under the tongue.
4. Fever: A fever is generally considered to be a body temperature above 100.4°F (38°C). It is a sign that the body is fighting off an infection or illness.
In conclusion, the normal human body temperature is typically considered to be 98.6°F (37°C), although it can vary slightly depending on various factors.