CBSE Class 11 Physics Unit and Measurement Worksheet Set A

Short Answer

Question. What do you mean by cesium atomic clock? 
Answer. The cesium atomic clock is used at the National Physical Laboratory, New Delhi to maintain the Indian standard of the time. The cesium atomic clocks are very accurate.The cesium atoms regulate the rate of the cesium atomic clock just as the vibrations of a balanced wheel regulate an ordinary wristwatch.

Question. What do you mean by unit? 
Answer. A standard amount of a physical quantity, such as length, mass, energy, etc., specified multiples of which are used to express magnitudes of that physical quantity the second is a unit of time.

Question. What are the base units for length, mass and time in the international system of units? 
Answer. The base units for length, mass and tie in these systems are:
1. In CGS system they were centimetre, gram and second.
2. In FPS system they were foot, pound and second.
3. N MKS system they were metre, kilogram and second.

Question. Distinguish between accuracy and precision? 
Answer. 1. Accuracy indicates how close a measurement is to the correct or accepted value whereas precession indicates the closeness of two or more measurements to each other.
2. Accuracy is not dependent on the precession whereas precession is not dependent on accuracy.

Question. What do you mean by the parallax method? 
Answer. Parallax is one of the most important distance measurement methods used by astronomers. It can only be used for nearby stars but it is very accurate. The method works by measuring at the apparent movement of a nearby object against the background of a distant object.

Long Answer

Question. Explain the dimensional analysis and its application?
Answer. 
Dimensional Analysis is a problem-solving method that uses the fact that any number or expression can be multiplied by one without changing its value. It is a useful technique. It is also known as factor label method or unit analysis. It is used to convert from one set of units to another. Dimensional analysis is a simple qualitative method for determining the dependence of physical quantities important for the description of physical process by using dimensions of quantities. Dimensional analysis applications are:
1. Consistency of dimensional equation: A dimensionally consistent equation naturally takes the same form in all possible systems of units, since the same conversion factors are applied to both sides of the equation when transforming from one system to another. If an equation fails the consistency test then it is proved wrong but if it passes, it is not proved right. So, the dimensionally correct equation needs not to be actually a correct equation but a dimensionally wrong or inconsistent equation must be wrong.
2. Deducing relations among the physical quantities: The method of dimensions can sometimes be used to deduce relation among the physical quantities. Dimensional analysis is very useful in deducing relations among the interdependent physical quantities. The method of dimensions can only test the dimensional validity, but not the exact relationship between physical quantities in any equation. It does not distinguish between the physical quantities having same dimensions.

Question. What are the errors in the measurement explain? 
Answer. 
Measurement error is the difference between a measured quantity and its true value. It includes random error naturally occurring errors that are to be expected with any experiment and systematic error caused by a miss-calibrated instrument that affects all measurements. It is also known as observational error. There are three types of errors:
1. Systematic error: Systematic error or systematic bias is consistent, repeatable error associated with faulty equipment or a flawed experiment design. These errors are usually caused by measuring instruments that are incorrectly calibrated or are used incorrectly. Systematic errors primarily influence a measurement's accuracy, For example: Forgetting to tare or zero a balance produces mass measurements that are always off by the same amount. There are three types of systematic errors: instrumental errors, personal errors and imperfection in experimental technique or procedure.
2. Random error: Random errors are errors in measurement that lead to measurable values being inconsistent when repeated measurements of a constant attribute or quantity are taken, for example: electronic noise in the circuit of an electrical instrument, irregular changes in the heat loss rate from a solar collector due to changes in the wind.
3. Least count errors: The smallest value that can be measured by the measuring instrument is called its least count. Measured values are good only up to this value. The least count error is the error associated with the resolution of the instrument, for example: a vernier callipers has the least count as 0.01 cm, a spherometer may have a least count of 0.001 cm.

Question. Write short note on measurement of mass? 
Answer. Mass is the quantity of matter contained in body. Mass is a basic characteristic of matter. It is independent of temperature, pressure or position of an object in space.
Mass is expressed in different measures, the mass of an object is measured in kilogram.
The mass of an object is measured by a beam balance. The light objects are measured in gram and milligram whereas heavy objects are measured in quintal and ton. The mass of an object can be measured relative to a standard mass. The common way to measure the mass of an object is to compare its weight with the weight of a standard or known object on a scale. Dealing with the atoms and molecules the kilogram is an inconvenient unit. There is an important standard unit of mass i.e. called unified atomic mass unit. The masses of the objects come across in the universe vary over a very wide range. The unified atomic mass is a standard unit for measuring mass on the atomic and sub-atomic level. It is denoted by the symbol (u) It is also known as an atomic mass unit or Amu. Unified Atomic Mass is defined as the one-twelfth mass of Carbon-12 isotope.
1 unified atomic mass unit = 1 amu = 1/12 × mass of C-12 carbon isotope
= 1/12 × 1.992646547 × 10 ⁻²³ gram
= 1.660539040 × 10 ⁻²⁴ gram
= 1.66 × 10^-27 kilograms

Question. Explain the rules for the used for analysing significant figures? 
Answer. Significant figures refer to the number of important single digits (0 through 9 inclusive) in the coefficient of an expression in scientific notation. The number of significant figures in an expression indicates the confidence or precision with which an engineer or scientist states a quantity. The rules for analysing significant figures are:
1. A choice of change of different units does not change the number of significant digits or figures in a measurement.
2. All the non-zero digits are significant and all the zeros between two non-zero digits are significant no matter where the decimal point is.
3. If the number is less than 1 the zero on the right of the decimal point but to the left of the first non-zero digit are not significant.
4. The terminal or trailing zero in a number without a decimal point is not significant.
5. The trailing zero in a number with a decimal point is significant.
6. There can be some confusion regarding the trailing zero.
7. To remove such ambiguities in determining the number of the significant figures.
8. The scientific notation is ideal for the reporting measurement.
9. The digit zero is conventionally put on the left of a decimal for a number less than 1.
10. The multiplying od dividing factors which are neither rounded numbers nor numbers representing measured values are exact and have the infinite number of significant digits

Question. State the difference between systematic error and random error?
Answer. 1. Systematic error is the one that deviates from the true value of measurement any a fixed amount whereas random error is the one that varies and which is likely to be positive or negative.
2. Systematic error caused by some flaw in the experimental apparatus or a flawed experimental design whereas random error caused by unpredictable variations in the readings of the measurement device.
3. Systematic error can be eliminated by using the proper technique, calibrating equipment and employing standards whereas random errors can be reduced by taking average of a large number of observations.
4. Systematic error remains constant or changes in a regular fashion in repeated measurements of the same quantity whereas random error is inconsistent and does not repeat in the same magnitude or direction except by chance.
5. Systematic error discovered experimentally by comparing a given result with a measurement of the same quantity performed using a different method whereas random errors discovered by performing measurements of the same quantity number of times under the same conditions.