Temperature and Effects of Heat - Class 7 Notes & Olympiad Questions

Heat and Temperature

Heat and temperature are related to each other, but they are not the same thing.

Heat

a) Heat is a form of energy. When something has heat, it means that it has energy that can make it feel hot or warm.
b) Heat energy flows from something that is hotter to something that is cooler. When you touch something that feels hot, it means that heat energy is transferring from that object to your skin.
c) Heat can be produced in many ways, such as by burning fuel, using electricity, or even through the warmth of the Sun.

Temperature

a) Temperature is a way to measure how hot or cold something is. It tells us how much thermal energy (heat) is present in an object or a system.
b) Think of temperature as a scale that goes from low to high. When something has a high temperature, it means it is very hot. And when something has a low temperature, it means it is very cold.

Difference between Heat and Temperature

a) The main difference between heat and temperature is that heat is the energy being transferred, while temperature is the measure of how hot or cold something is. They are related because heat is the energy that causes changes in temperature.
b) Heat is the cause, and temperature is the effect.
c) Heat always flows from a warmer object to a cooler one until both objects reach the same temperature.
d) An example to understand this is to imagine two cups of water—one filled with hot water and the other with cold water. When you pour hot water into the cold water cup, heat transfers from the hot water to the cold water until they reach the same temperature. This equalisation of temperature is known as thermal equilibrium.
e) It's important to note that different objects can have the same temperature but different amounts of heat. For example, a cup of hot water and a swimming pool can both be at 50 degrees Celsius, but the cup of hot water has less heat compared to the entire pool because it has less water.

Importance of Heat and Temperature

Heat and temperature are important in many aspects of our lives. Understanding heat and temperature helps us in various ways:

1. Cooking: Heat is crucial for cooking food. Different temperatures are needed for boiling, frying, baking, and grilling, and understanding temperature allows us to cook food properly and safely.

2. Weather: Temperature plays a big role in weather conditions. It determines whether it will be hot or cold outside, and it helps meteorologists predict weather patterns and phenomena like rain, snow, or storms.

3. Energy Transfer: Heat is involved in the transfer of energy. For example, we use heat energy to produce electricity, power our vehicles, and provide heating and cooling for our homes.

4. Material Properties: Heat and temperature affect the behaviour of materials. For instance, metals expand when heated and contract when cooled. This knowledge is important in engineering and construction.

Different Units of Temperature and Temperature Scales

Temperature can be measured and expressed in different units, including Celsius (°C), Fahrenheit (°F), and Kelvin (K). Each temperature scale has its own way of measuring and representing temperature.

1. Celsius (°C)

a) The Celsius scale is used in many countries for everyday temperature measurements.
b) It is based on the freezing and boiling points of water.
c) On this scale, the freezing point of water is set at 0°C, and the boiling point of water is set at 100°C.

2. Fahrenheit (°F)

a) The Fahrenheit scale is commonly used in the United States and a few other countries.
b) It is also based on the freezing and boiling points of water, but the scale is divided into smaller increments compared to Celsius.
c) On this scale, the freezing point of water is set at 32°F, and the boiling point of water is set at 212°F.

3. Kelvin (K)

a) The Kelvin scale is used in scientific and engineering applications.
b) It starts from the coldest temperature possible, which is called absolute zero and is defined as 0 Kelvin (0 K).
c) The size of one Kelvin degree is the same as one Celsius degree, so the Kelvin scale has the same increments as Celsius.

Temperature Conversions

Sometimes we need to convert temperatures from one scale to another. For example, if you're used to measuring temperature in Celsius and someone tells you a temperature in Fahrenheit, you might want to know what it is in Celsius, or vice versa.

There are some formulas that can be used to do these conversions.

1. Celsius to Fahrenheit

The formula that can be used to convert Celsius to Fahrenheit is:
°F = (°C × 9/5) + 32

Example: Let's say you have a temperature of 20 degrees Celsius (20°C).
Solution: You can convert it to Fahrenheit by multiplying 20 by 9/5 (which is 1.8) and then adding 32.

So, the conversion would be:
°F = (20 × 9/5) + 32
°F = 36 + 32
°F = 68°F

2. Fahrenheit to Celsius

The formula that can be used to convert Fahrenheit to Celsius is:
°C = (°F - 32) × 5/9

Example: Let's say you have a temperature of 68 degrees Fahrenheit (68°F).
Solution: You can convert it to Celsius by subtracting 32 from 68 and then multiplying the result by 5/9.

So, the conversion would be:
°C = (68 - 32) × 5/9
°C = 36 × 5/9
°C = 20°C

3. Celsius to Kelvin

To convert temperature from Celsius to Kelvin, simply add 273.15 to the Celsius value.
K = °C + 273.15

Example: If it's 20°C, the conversion would be:
Solution: K = 20 + 273.15
K = 293.15 K

4. Kelvin to Celsius

To convert temperature from Kelvin to Celsius, subtract 273.15 from the Kelvin value.
°C = K - 273.15

Example: If the temperature is 293.15 K, the conversion would be:
Solution: °C = 293.15 - 273.15
°C = 20°C

5. Fahrenheit to Kelvin

The formula that can be used to convert Fahrenheit to Kelvin is:
K = (°F + 459.67) × 5/9

Example: Let's say you have a temperature of 68 degrees Fahrenheit (68°F).
Solution: You can convert it to Kelvin using the formula:

K = (68 + 459.67) × 5/9
K = 527.67 × 5/9
K = 292.59 K

6. Kelvin to Fahrenheit

The formula that can be used to convert Kelvin to Fahrenheit is:
°F = (K × 9/5) - 459.67

Example: Let's say you have a temperature of 300 Kelvin (300 K).
Solution: You can convert it to Fahrenheit using the formula:

°F = (300 × 9/5) - 459.67
°F = 540 - 459.67
°F = 260.33°F

Measuring Temperature

Temperature can be measured using various devices, including thermometers. Different types of thermometers use different principles to measure temperature. Some common thermometers are:

1. The Laboratory Thermometer

a) A laboratory thermometer is a specialized thermometer used in scientific laboratories to measure temperature with high accuracy.
b) It typically consists of a glass tube filled with a liquid (such as mercury) that expands or contracts with temperature changes.
c) The tube has markings indicating the temperature values.
d) The laboratory thermometer is calibrated to measure temperatures within the range of -10°C to 110°C.

2. The Clinical Thermometer

a) A clinical thermometer is a type of thermometer designed for measuring human body temperature.
b) It is commonly used in medical settings, such as clinics and hospitals.
c) A clinical thermometer typically has a narrow, sealed glass tube with a bulb at one end containing a temperature-sensitive liquid (usually mercury or alcohol).
d) The temperature is measured by placing the bulb in the mouth or under the armpit depending on the type of clinical thermometer.
e) The thermometer has markings indicating the temperature values, allowing healthcare professionals to monitor body temperature accurately.

Effects of Heat

Heat is a form of energy, and when it is added to something, it can cause different effects.

Here are a few important effects of heat:

1. Change in Temperature

a) When heat is added to an object or substance, it can make it hotter.
b) It causes the particles inside the object to move faster and have more energy. This increase in energy makes the object's temperature rise.
c) On the other hand, if heat is taken away from an object, it can make it cooler and lower its temperature.

2. Change in Size (Expansion and Contraction)

a) Heat can also make things expand or contract, which means they can get bigger or smaller.
b) This happens because when objects are heated, their particles move faster and take up more space. So, they expand or get bigger.
c) When objects are cooled, their particles slow down and come closer together, causing them to contract or get smaller.

Expansion and Contraction of Water

a) Water behaves a little differently compared to many other substances when it comes to temperature changes. Normally, when things get colder, they contract or get smaller. Water does the same thing at first, but when it gets really cold and turns into ice, it actually expands or gets bigger.
b) When water gets colder, its particles slow down and come closer together, making it contract just like other substances. But when it reaches a certain point, close to freezing, something interesting happens. The particles in water arrange themselves in a special pattern to form ice crystals, and this pattern needs more space. So, when water freezes, it expands and takes up more room.
c) That's why you might have noticed that when you freeze water in a container, it can sometimes crack or burst. The expanding ice pushes against the sides of the container, which can cause it to break.
d) Another interesting thing about water is that when it freezes and expands, the ice is actually less dense than liquid water. That's why ice cubes float in your drink or why icebergs float in the ocean. The expanded ice is lighter and less heavy than the liquid water, so it stays on the surface.

3. Change in State

a) When you heat something, like ice, it turns into water, and if you heat it more, it can turn into steam.
b) This happens because heat makes the particles in the substance move faster, causing them to change from solid to liquid to gas.
c) Cooling the substance does the opposite, turning gas into a liquid and then into a solid.

Bimetallic Strip

a) A bimetallic strip is a special type of strip made by joining two different metal strips together. These metal strips are usually made of two metals with different expansion properties, such as brass and steel.
b) Each metal in the strip expands or contracts differently when it is heated or cooled. When the bimetallic strip is heated, the two metals expand at different rates. Since they are joined together, this difference in expansion causes the strip to bend or curve.
c) For example, imagine you have a bimetallic strip with brass on one side and steel on the other side. When you heat the strip, the brass side expands more than the steel side. As a result, the strip curves with the brass side on the outside of the curve.
d) This bending or curving of the bimetallic strip can be useful in various applications. One common application is in fire sprinklers. In a fire sprinkler, there is a tiny bimetallic strip that responds to high temperatures. When the temperature rises due to a fire, the strip bends and releases a valve, allowing water to flow and extinguish the fire.