🌡️ Temperature Conversion
Class 7 ICSE Physics
Temperature can be measured using different scales. The three commonly used scales are Celsius (°C), Fahrenheit (°F), and Kelvin (K).
In this lesson, we will learn how to convert temperature from one scale to another using simple formulas and step-by-step methods.
🌟 A Small Story: Zara Becomes a Temperature Detective
One morning, Zara was getting ready for school. Her mother looked at the thermometer and said,
“The temperature today is 25°C.”
Later, Zara spoke to her friend who lived in another country. Her friend said,
“It is 77°F here today.”
Zara was surprised.
“Is 77°F much hotter than 25°C?” she wondered.
Her Physics teacher smiled and explained,
“No, Zara! They represent the same temperature.”
Zara became curious. How could two different numbers represent the same temperature?
Her teacher explained that different temperature scales use different reference points. Therefore, we need conversion formulas to change a temperature from one scale to another.
Zara decided to become a Temperature Detective.
Her mission was to learn all six temperature conversions!
🌡️ What is Temperature?
Temperature tells us how hot or cold an object or place is.
The three commonly used temperature scales are:
- Celsius – °C
- Fahrenheit – °F
- Kelvin – K
The important reference points given in the notes are:
| Temperature | Celsius | Fahrenheit | Kelvin |
|---|---|---|---|
| Freezing point of water | 0°C | 32°F | 273 K |
| Boiling point of water | 100°C | 212°F | 373 K |
🔄 The Six Temperature Conversions
There are six important conversions we need to learn.
1️⃣ Celsius → Fahrenheit
Example: Convert 25°C into Fahrenheit.
°F = (9/5 × 25) + 32
= 45 + 32
= 77°F
Answer: 77°F
Practice
- Convert 30°C into Fahrenheit.
- Convert 60°C into Fahrenheit.
Answers: 86°F and 140°F
2️⃣ Fahrenheit → Celsius
Example: Convert 95°F into Celsius.
°C = 5/9 × (95 − 32)
= 5/9 × 63
= 35°C
Answer: 35°C
Practice
- Convert 68°F into Celsius.
- Convert 104°F into Celsius.
Answers: 20°C and 40°C
3️⃣ Kelvin → Celsius
Example: Convert 300 K into Celsius.
°C = 300 − 273
= 27°C
Answer: 27°C
Practice
- Convert 273 K into Celsius.
- Convert 323 K into Celsius.
Answers: 0°C and 50°C
4️⃣ Celsius → Kelvin
Example: Convert 25°C into Kelvin.
K = 25 + 273
= 298 K
Answer: 298 K
Practice
- Convert 10°C into Kelvin.
- Convert 80°C into Kelvin.
Answers: 283 K and 353 K
5️⃣ Fahrenheit → Kelvin
This conversion is completed in two steps.
°C = 5/9 × (°F − 32)
K = °C + 273
Example: Convert 68°F into Kelvin.
Step 1:
°C = 5/9 × (68 − 32)
= 20°C
Step 2:
K = 20 + 273
= 293 K
Answer: 293 K
Practice
- Convert 32°F into Kelvin.
- Convert 86°F into Kelvin.
Answers: 273 K and 303 K
6️⃣ Kelvin → Fahrenheit
This conversion is also completed in two steps.
°C = K − 273
°F = (9/5 × °C) + 32
Example: Convert 303 K into Fahrenheit.
Step 1:
°C = 303 − 273
= 30°C
Step 2:
°F = (9/5 × 30) + 32
= 86°F
Answer: 86°F
Practice
- Convert 273 K into Fahrenheit.
- Convert 313 K into Fahrenheit.
Answers: 32°F and 104°F
📋 Quick Formula Chart
| Conversion | Formula / Method |
|---|---|
| °C → °F | °F = (9/5 × °C) + 32 |
| °F → °C | °C = 5/9 × (°F − 32) |
| K → °C | °C = K − 273 |
| °C → K | K = °C + 273 |
| °F → K | First °F → °C, then °C → K |
| K → °F | First K → °C, then °C → °F |
🧠 Easy Memory Tricks
C → F: Multiply by 9/5, then add 32.
F → C: Subtract 32, then multiply by 5/9.
C → K: Add 273.
K → C: Subtract 273.
🔬 Temperature in Daily Life
Temperature conversion is useful when temperature is expressed using different scales.
For example, the same temperature may be written in Celsius, Fahrenheit or Kelvin depending on the situation.
Scientists and students must therefore understand how the different scales are related.
💡 Think Like a Temperature Detective
Suppose one thermometer shows 68°F while another temperature reading is given as 20°C.
Are they different temperatures?
Let us check.
°C = 5/9 × (68 − 32)
= 5/9 × 36
= 20°C
Therefore,
68°F = 20°C
So, different numbers on different scales can represent the same temperature.
🌱 Moral of the Story
Zara learnt an important lesson from temperature conversion:
“Different ways of expressing something do not always mean that they are different.”
We should understand the relationship between things before making a judgement.
Science teaches us to observe carefully, understand the concept and then reach a conclusion.
Value: Think Before You Judge.
🌈 Rainbow Revision
READ → Read the temperature carefully.
IDENTIFY → Identify the starting and required scale.
UNDERSTAND → Recall the correct conversion formula.
CONNECT → Connect the two temperature scales.
MAP → Arrange the conversion steps correctly.
APPLY → Substitute the value and calculate.
EXPLAIN → Write the final answer with the correct unit.
⭐ Important Points to Remember
- 0°C = 32°F = 273 K
- 100°C = 212°F = 373 K
- Celsius to Fahrenheit: multiply by 9/5 and add 32.
- Fahrenheit to Celsius: subtract 32 and multiply by 5/9.
- Celsius to Kelvin: add 273.
- Kelvin to Celsius: subtract 273.
- Fahrenheit to Kelvin requires two steps.
- Kelvin to Fahrenheit requires two steps.
- Kelvin is written as K, not °K.
🎯 Interactive MCQ Challenge
Test your understanding of Temperature Conversion. Select one option for each question and click Submit Quiz.
🎉 Well Done, Temperature Detective!
If you can convert temperature confidently from one scale to another, you have successfully completed your Temperature Conversion mission!
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