Chapter: 03. Physical Quantities And Measurement 2
Physical Quantities and Measurement - 2
LEARNING GOALS
By the end of this study material, you will be able to:
Understand what time and temperature are. Use the correct units and symbols for time and temperature. Learn how to measure time using different instruments like clocks, watches, and stop-watches. Connect the temperature of an object to how hot or cold it feels. Measure a person’s temperature using a clinical thermometer. Measure an object’s temperature using a laboratory thermometer. Convert physical quantities from one unit to another related unit.
INTRODUCTION
Accurately measuring time is very important for scheduling and organizing our daily activities. Similarly, measuring temperature is also a key part of our daily lives.
MEASUREMENT OF TIME
Time is defined as the interval or duration between two events. We often measure time using the “mean solar day.” A mean solar day is the amount of time it takes for the Earth to complete one full rotation around its own axis.
Units of Time
The standard unit of time in the SI (International System of Units) system is the second (s).
Larger units of time include minutes, hours, days, months, and years.
Here are some commonly used units of time and their symbols:
Here are the relationships between some commonly used units of time:
Instruments Used to Measure Time
In the past, people estimated time by observing the sun and the moon.
Measures time based on the position of the shadow cast by the sun. Examples include the Samrat Yantras at Jantar Mantar in Jaipur and Delhi. These were built by Maharaja Jai Singh II of Jaipur.
Fig. 3.1 The Samrat Yantra at Jantar Mantar, Jaipur Time was also measured using: Sand clocks (hourglasses) Clocks based on the principle of the pendulum.
Fig. 3.3 Pendulum clock Today, we use electronic or digital watches and clocks (Fig. 3.4 - image not provided in source, but implied), which are very accurate. Stopwatches are used to precisely record the time interval between two events. You might see them used in sports or laboratories. Important Terms (SCI-VOCAB)
Time: The interval between two events. Mean solar day: The time taken by the Earth to complete one rotation about its own axis. Timekeeping Conventions
There are two main ways to tell and write time: the 12-hour clock and the 24-hour clock.
12-hour Clock
This is the most common way to tell time. The 24-hour day is divided into two 12-hour periods: a.m. (ante meridiem): Means ‘before midday’, for times from midnight to just before noon. p.m. (post meridiem): Means ‘after midday’, for times from noon to just before midnight. When using this convention, you must include “a.m.” or “p.m.” to show whether the time is before or after noon. 6:20 a.m. means 6:20 in the early morning. 6:20 p.m. means 6:20 in the evening. 24-hour Clock
This convention does not use “a.m.” or “p.m.” Time is recorded from 0000 to 2400 (which is 0000 for the next day). Time from midnight to 12 noon is recorded as 0000 hours to 1200 hours. Time from 12 noon to midnight is recorded as 1201 hours to 2400 (0000) hours. 0750 hours means 7:50 in the morning. 1950 hours means 7:50 in the evening. Converting 24-hour to 12-hour format: If the hour value is greater than 1200 hours, subtract 1200 from the 24-hour format to get the 12-hour time (and add p.m.). For example, 1950 hours is 1950 - 1200 = 0750 hours, which is 7:50 p.m. The Necessity of the 24-hour Clock (SCIENCE TALK)
The 24-hour clock system is essential in many professions, such as aviation, military, and emergency services. It helps avoid confusion between a.m. and p.m., which is critical for coordination and safety in operations where precision and clarity are paramount. For example, “2:00” could mean 2 a.m. or 2 p.m., but “0200 hours” clearly means 2 a.m., and “1400 hours” clearly means 2 p.m. This reduces the risk of errors that could have serious consequences.
Applying Time Concepts (LIFE SKILLS Example)
Let’s consider an example of calculating travel time.
If a train journey is 5 hours 12 minutes long, leaves at 1500 hours, and stops for 15 minutes midway:
Departure Time: 1500 hours (which is 3:00 p.m. in 12-hour format). Journey Duration: 5 hours 12 minutes. Total Time Elapsed: 5 hours 12 minutes (travel) + 15 minutes (stop) = 5 hours 27 minutes. Arrival Time in 24-hour format: Add 5 hours to 1500 hours: 1500 + 500 = 2000 hours. Add 27 minutes to 2000 hours: 2000 + 0027 = 2027 hours. Convert to 12-hour format: Since 2027 hours is greater than 1200 hours, subtract 1200: 2027 - 1200 = 0827. So, 2027 hours is 8:27 p.m. Atomic Clocks (SCIENCE ALIVE)
Atomic clocks are the most precise devices for measuring time. They measure time based on the vibrations within atoms. Caesium, a soft silvery metal, is used in the most accurate atomic clocks. In India, atomic clocks are located at the National Physical Laboratory (NPL) in New Delhi and the Indian Space Research Organisation (ISRO) in Bengaluru.
MEASUREMENT OF TEMPERATURE
We often talk about how hot or cold things are, like the weather, milk, or juice. To understand these clearly, we measure temperature. Temperature is defined as the degree of hotness or coldness of a body.
Units of Temperature
The standard unit of temperature in the SI system is kelvin (K). This unit is primarily used in scientific measurements.
However, the commonly used units of temperature are degree Celsius (°C) and degree Fahrenheit (°F).
Here are the units of temperature and their symbols:
Instruments Used to Measure Temperature
An instrument used to measure temperature is called a thermometer. There are different types of thermometers, classified based on their specific uses.
Important Terms (SCI-VOCAB)
Temperature: The degree of hotness or coldness of a body. Thermometer: An instrument used to measure temperature. Laboratory Thermometer (Mercury Thermometer)
Purpose: Used to measure room temperature or the temperature of solids and liquids in a laboratory setting. Consists of a very fine glass tube called a capillary tube with a tiny hole (bore). At one end, there is a very thin glass bulb filled with mercury. The other end of the capillary tube is sealed. The capillary tube is protected by a thick glass tube called the stem. The stem usually has markings (graduations) in degrees Celsius, typically ranging from -10° C to 110° C.
Fig. 3.5 Laboratory thermometer To measure an object’s temperature, the thermometer’s bulb is placed inside or on the object. The mercury inside the bulb expands when it gets hotter and rises up the capillary tube. The level to which the mercury rises indicates the temperature of the object in degrees Celsius. Mercury is easy to see in the capillary tube because it looks like a shiny thread. Experiment: Measuring the Temperature of Hot Water
Aim: To measure the temperature of hot water. Materials required: A laboratory thermometer, a beaker filled with hot water, and an iron stand. Suspend the laboratory thermometer from the iron stand. Place the beaker with hot water so that the thermometer’s bulb is completely immersed in the water.
Fig. 3.6 Measuring the temperature of hot water Observation: You will see the mercury level rise steadily, become constant for a while, and then start to fall if the water cools. Conclusion: When the mercury level becomes constant, that reading is the temperature of the hot water. Make sure your eye is level with the mercury to avoid “parallax error” (reading incorrectly due to viewing angle). Responsible Disposal (ECO-SOLDIER)
Mercury is a harmful substance. If a mercury thermometer breaks, the mercury inside should not be thrown directly into the dustbin. Discussing how to dispose of mercury safely aligns with the United Nations’ Sustainable Development Goal 12: Responsible management of chemicals and waste.
Clinical Thermometer (Doctor’s Thermometer)
Purpose: Used by doctors to measure a person’s body temperature. Range: Usually graduated from 35° C to 42° C (or 95° F to 108° F). This specific range covers the typical variation of human body temperature. Normal Body Temperature: The normal body temperature of a person is 37° C or 98.6° F. If a reading is above this, the person has a fever.
Fig. 3.7 Clinical thermometer Key Feature: Constriction: A clinical thermometer has a slight bend or kink called a constriction in its capillary tube, just above the bulb. Purpose of Constriction: This constriction prevents the mercury level from falling back into the bulb immediately after the thermometer is removed from the patient’s mouth. This allows you enough time to read the temperature accurately before the mercury drops. Before Re-use: Before using it again, the thermometer must be washed with room temperature water and then shaken firmly until the mercury level falls below the constriction. Why a Smaller Range? (REASON CORNER)
A clinical thermometer has a smaller range of measurement (35° C to 42° C) compared to a laboratory thermometer (e.g., -10° C to 110° C) because the human body temperature only varies within a narrow range. A wider range is unnecessary for medical use and could make the scale less precise for small changes in body temperature.
Measuring Body Temperature (ACTIVITY)
Aim: To measure the body temperature of a person with a clinical thermometer. Materials required: A clinical thermometer. Wash the bulb of the thermometer in running water. Check the mercury position. If it’s above the constriction, give the thermometer a few firm jerks to make the mercury fall below the constriction into the bulb. Place the bulb gently below the tongue or under the armpit.
Fig. 3.8 Measuring temperature using clinical thermometer Remove the thermometer after about two minutes. Observation: The mercury level will be above the constriction. Conclusion: Carefully read the mercury level. A reading of 98.6° F (or 37° C) means normal body temperature. A reading above 98.6° F indicates a fever. Digital Thermometer
Popularity: Becoming more popular for measuring body temperature today. Mercury-Free: They do not use mercury. Mechanism: They have a sensor that measures temperature and displays it on a digital screen. Indication: A beep sound is usually produced when the thermometer has measured the maximum body temperature. Scenario Application (Implied Question Response)
If a child is having chills and shivering and needs their body temperature measured:
Which thermometer: A clinical thermometer (or a digital thermometer) should be used. Where to place: Below the tongue or under the armpit. 
Self Study
Here are the relationships between some commonly used units of time:
