PhysicsChapter 1: Physical Quantities and Measurement
1.7.4 Physical Balance
A physical balance
Explanation level
While instruments like the metre rule and vernier caliper are used to measure length, a different type of instrument is needed to measure mass. The physical balance is one of the oldest and most sensitive instruments for this purpose. It is a common laboratory tool used to measure the mass of an object with high precision, often down to the milligram level.
Definition
A physical balance (also called a balance scale, beam balance, or laboratory balance) is a very sensitive common balance used to measure mass. It consists of a vertical pillar, a horizontal beam resting on a knife edge, two pans, and a long pointer attached to the middle of the beam. It can measure masses in the milligram order.
Main Idea
The main idea is comparison by balancing. A physical balance does not directly measure mass; instead, it compares the unknown mass of an object with known standard masses (weights). When the beam is perfectly horizontal and the pointer is centered, the mass of the object is equal to the total mass of the standard weights placed on the other pan. This is a direct application of the principle of moments, where the turning effect of the object's weight on one side is balanced by the turning effect of the known weights on the other side.
Core Concept
- Vertical Pillar: The central support structure.
- Horizontal Beam: The beam that rests on a knife edge on top of the pillar.
- Two Pans: Suspended from each end of the beam. One pan holds the object to be measured, and the other holds standard weights.
- Pointer: A long needle attached to the middle of the beam. It moves along a scale at the base, indicating when the beam is balanced.
- Leveling Screws: Used to ensure the balance is perfectly horizontal before use. The pointer is set at the center of the scale by adjusting these screws.
- Protective Glass Case: The entire balance is enclosed in a glass case to protect it from dust and air currents, which could affect the accuracy of the measurement.
- Weight Box: A box containing standard weights comes with the balance.
- Placing the body in one pan.
- Placing some standard weights in the other pan.
- Calculating the mass from the standard weights placed and the resting point of the pointer. When the pointer is exactly at the center of the scale, the beam is horizontal, and the mass of the object equals the sum of the standard weights.
Detailed Explanation
The physical balance is a classic example of a precision measuring instrument. Its sensitivity comes from the knife edge on which the beam rests. A knife edge provides a very low-friction pivot point, allowing the beam to swing freely and respond to even tiny differences in mass.
The Importance of the Glass Case The text specifically mentions that the balance is placed in a protective glass case. This is crucial. Even a small draft of air or a speck of dust settling on one pan can cause the balance to tip, leading to an incorrect reading. The glass case isolates the delicate mechanism from these environmental disturbances, ensuring the accuracy of the measurement.
The Leveling Screws Before any measurement is taken, the balance must be leveled. This ensures that the beam is perfectly horizontal and the pointer is centered at the zero mark when both pans are empty. If the balance is not level, the readings will be systematically wrong. The leveling screws are adjusted until the pointer rests exactly at the center.
The Weighing Process The process of measuring mass with a physical balance is a careful comparison. You start with the object in one pan. Then, you add standard weights to the other pan, starting with the largest and working down to the smallest. When the pointer is close to the center, you use smaller weights and the rider (a small weight that slides on the beam) to achieve a perfect balance. The mass of the object is the sum of all the weights placed on the other pan.
Intermediate Level
The physical balance is a highly sensitive instrument that relies on the principle of moments. The beam acts as a lever with the knife edge as the fulcrum. The weights on each pan create a torque (moment) about the fulcrum. When the beam is balanced, the clockwise moment equals the anticlockwise moment. Since the distance from the fulcrum to each pan is the same, the balance condition simplifies to: mass of object = mass of standard weights. The sensitivity of the balance is determined by the sharpness of the knife edge and the length of the pointer. A longer pointer makes it easier to detect small deviations from balance. The glass case is not just for protection; it is a functional necessity to prevent air currents from disturbing the delicate balance.
Important Points
- - A physical balance measures mass by comparing it with standard weights.
- - It consists of a vertical pillar, a horizontal beam, two pans, and a pointer.
- - It is enclosed in a glass case to protect it from dust and air currents.
- - Leveling screws are used to set the balance to the zero position before use.
- - The mass of an object is found by balancing it against known standard weights.
Remember
- - Physical Balance = Measures Mass.
- - Principle: Comparison with standard weights.
- - Key Part: The horizontal beam resting on a knife edge.
- - Glass Case: Protects from dust and air currents.
- - Leveling Screws: Used to set the pointer to zero.
Common Mistakes
- - Measuring weight instead of mass: A physical balance measures mass (in kg or g). Weight (in newtons) is a force and is measured by a spring balance.
- - Forgetting to level the balance: Always check that the pointer is at the center of the scale when the pans are empty. If not, use the leveling screws to adjust it.
- - Not using the glass case: Leaving the glass case open can allow air currents to disturb the balance, leading to inaccurate readings.
Quick Revision
A physical balance is a highly sensitive instrument used to measure mass by comparing an object with standard weights. It consists of a horizontal beam balanced on a central knife edge, with two pans suspended from either end. It is enclosed in a glass case to protect it from air currents and dust. The mass of the object is found by adding standard weights to the other pan until the pointer indicates perfect balance.
60-Second Revision
A physical balance measures mass by comparing it with standard weights. It has a horizontal beam on a knife edge, two pans, and a pointer. It is kept in a glass case to avoid air currents. The object's mass equals the total mass of weights when the pointer is centered.
Frequently Asked Questions (FAQs)
Q1. What is a physical balance? Answer: A physical balance is a very sensitive instrument used in laboratories to measure the mass of an object. It works by comparing the unknown mass with known standard weights.
Q2. What does a physical balance measure: mass or weight? Answer: A physical balance measures mass. It compares the mass of an object with standard masses (weights). Weight is a force and is measured by a spring balance.
- Q3. Why is a physical balance kept in a glass case?
- Answer: The glass case protects the delicate balance from dust and air currents, which could disturb the pans and affect the accuracy of the measurement.
Q4. What are leveling screws used for in a physical balance? Answer: Leveling screws are used to level the balance so that the horizontal beam is perfectly horizontal and the pointer rests exactly at the center of the scale when the pans are empty.
Q5. How is the mass of an object determined using a physical balance? Answer: The object is placed in one pan, and standard weights are added to the other pan until the pointer is centered and the beam is horizontal. The mass of the object is equal to the total mass of the standard weights used.
Q6. What is the function of the pointer in a physical balance? Answer: The pointer is attached to the middle of the beam and moves along a scale. It indicates when the beam is perfectly balanced (horizontal). When the pointer is at the center of the scale, the masses on both pans are equal.
Multiple Choice Questions (MCQs)
- Q1. A physical balance is used to measure:
- A. Length
- B. Mass
- C. Time
- D. Volume
- Correct Answer: B
- Explanation: The text describes the physical balance as a mass measuring instrument.
- Q2. The physical balance is enclosed in a glass case to protect it from:
- A. Water
- B. Light
- C. Dust and wind
- D. Sound
- Correct Answer: C
- Explanation: The text states that the glass case protects the balance so that even dust and wind cannot affect its accuracy.
- Q3. Leveling screws are used to:
- A. Measure the mass
- B. Level the balance and set the pointer at the center
- C. Open the glass case
- D. Store the standard weights
Correct Answer: B Explanation: The text states that leveling screws are used to level the physical balance, while the pointer is set at the center of the scale by adjusting the screws.
- Q4. The mass of a body is found by placing the body in one pan and:
- A. Measuring its volume in the other pan
- B. Placing some standard weights in the other pan
- C. Using a thermometer
- D. Using a metre rule
- Correct Answer: B
- Explanation: The text says the mass is found by placing the body in one pan and placing some standard weights in the other.
- Q5. What component rests on a knife edge in a physical balance?
- A. The pans
- B. The pointer
- C. The horizontal beam
- D. The glass case
- Correct Answer: C
- Explanation: The text describes the balance as consisting of a vertical pillar having a horizontal beam resting on a knife edge with two pans.
- Q6. The physical balance can measure masses in the order of:
- A. Kilograms
- B. Grams
- C. Milligrams
- D. Tonnes
- Correct Answer: C
- Explanation: The text states that a physical balance is a very sensitive common balance which can measure weights in milligram order.
- Q7. What comes with a physical balance to help measure mass?
- A. A set of leveling screws
- B. A weight box containing standard weights
- C. A magnifying glass
- D. A thermometer
- Correct Answer: B
- Explanation: The text states that "A weight box containing standard weights comes with the balance."
- Q8. The pointer is attached to the:
- A. Left pan
- B. Right pan
- C. Vertical pillar
- D. Middle of the beam
- Correct Answer: D
- Explanation: The text states that "A long pointer is attached to the middle of the beam."
- Q9. Which of the following is NOT a part of a physical balance?
- A. Horizontal beam
- B. Pointer
- C. Measuring cylinder
- D. Leveling screws
- Correct Answer: C
- Explanation: A measuring cylinder is a separate instrument for measuring volume, not a part of the physical balance.
- Q10. What indicates that the masses on the two pans of a physical balance are equal?
- A. The pans are empty
- B. The pointer is at the center of the scale
- C. The glass case is closed
- D. The weights are removed
Correct Answer: B Explanation: The pointer is set at the center of the scale by adjusting screws, and when the mass is measured, the pointer comes to rest at the center, indicating balance.
Frequently Asked Questions (FAQs)
Q1. What is the difference between mass and weight? Answer: Mass is a measure of the amount of matter in an object and is measured in kilograms (kg). Weight is the force of gravity acting on an object and is measured in newtons (N). A physical balance measures mass, while a spring balance measures weight.
Q2. Why must the pans be empty when leveling a physical balance? Answer: The balance must be leveled with empty pans to set the zero point. This ensures that any reading taken afterward reflects only the mass of the object and not a built-in imbalance.
Q3. How does the glass case affect the accuracy of a physical balance? Answer: The glass case isolates the balance from drafts and dust. Even a tiny air current can disturb the delicate balance, so the case is essential for accurate readings.
Q4. What are the standard weights made of? Answer: Standard weights used with a physical balance are typically made of non-corrosive metals like brass or stainless steel, precisely machined to specific masses.
Q5. Can a physical balance measure the mass of a liquid? Answer: Yes, you can measure the mass of a liquid by placing it in a container, measuring the combined mass, and then subtracting the mass of the empty container.
Q6. Why is the physical balance considered a highly sensitive instrument? Answer: The physical balance uses a knife edge (a very sharp pivot) which provides extremely low friction. This allows the beam to respond to even very small differences in mass, making it capable of measuring to the milligram level.
Multiple Choice Questions (MCQs)
- Q1. A physical balance is used to measure:
- A. Length
- B. Mass
- C. Time
- D. Volume
- Correct Answer: B
- Explanation: The text describes the physical balance as a mass measuring instrument.
- Q2. The physical balance is enclosed in a glass case to protect it from:
- A. Water
- B. Light
- C. Dust and wind
- D. Sound
- Correct Answer: C
- Explanation: The text states that the glass case protects the balance so that even dust and wind cannot affect its accuracy.
- Q3. Leveling screws are used to:
- A. Measure the mass
- B. Level the balance and set the pointer at the center
- C. Open the glass case
- D. Store the standard weights
Correct Answer: B Explanation: The text states that leveling screws are used to level the physical balance, while the pointer is set at the center of the scale by adjusting the screws.
- Q4. The mass of a body is found by placing the body in one pan and:
- A. Measuring its volume in the other pan
- B. Placing some standard weights in the other pan
- C. Using a thermometer
- D. Using a metre rule
- Correct Answer: B
- Explanation: The text says the mass is found by placing the body in one pan and placing some standard weights in the other.
- Q5. What component rests on a knife edge in a physical balance?
- A. The pans
- B. The pointer
- C. The horizontal beam
- D. The glass case
- Correct Answer: C
- Explanation: The text describes the balance as consisting of a vertical pillar having a horizontal beam resting on a knife edge with two pans.
- Q6. The physical balance can measure masses in the order of:
- A. Kilograms
- B. Grams
- C. Milligrams
- D. Tonnes
- Correct Answer: C
- Explanation: The text states that a physical balance is a very sensitive common balance which can measure weights in milligram order.
- Q7. What comes with a physical balance to help measure mass?
- A. A set of leveling screws
- B. A weight box containing standard weights
- C. A magnifying glass
- D. A thermometer
- Correct Answer: B
- Explanation: The text states that "A weight box containing standard weights comes with the balance."
- Q8. The pointer is attached to the:
- A. Left pan
- B. Right pan
- C. Vertical pillar
- D. Middle of the beam
- Correct Answer: D
- Explanation: The text states that "A long pointer is attached to the middle of the beam."
- Q9. Which of the following is NOT a part of a physical balance?
- A. Horizontal beam
- B. Pointer
- C. Measuring cylinder
- D. Leveling screws
- Correct Answer: C
- Explanation: A measuring cylinder is a separate instrument for measuring volume, not a part of the physical balance.
- Q10. What indicates that the masses on the two pans of a physical balance are equal?
- A. The pans are empty
- B. The pointer is at the center of the scale
- C. The glass case is closed
- D. The weights are removed
Correct Answer: B Explanation: The pointer is set at the center of the scale by adjusting screws, and when the mass is measured, the pointer comes to rest at the center, indicating balance.
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Quick revision
Revision Zone
Review the key facts, formulas and checks before you move on.1.7.4 Physical Balance
Sticky Notes · Quick Revision
Key Point 1
- Physical Balance: Measures mass.
Key Point 2
- Parts: Vertical pillar, horizontal beam, two pans, pointer, leveling screws.
Key Point 3
- Protection: Kept in a glass case.
Key Point 4
- Weights: Comes with a weight box of standard masses.
Key Point 5
- Accuracy: Can measure weights in the milligram order.
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