# Engineering Physics Questions and Answers – Dimensions of a Physical Quantity

This set of Engineering Physics Multiple Choice Questions & Answers (MCQs) focuses on “Dimensions of a Physical Quantity”.

1. Which of the following is used for measuring long time intervals?
a) Electrical oscillators
b) Atomic clocks
c) Decay of elementary particles

Explanation: Radioactive dating is used to measure long time intervals by finding the ratio of the number of radioactive atoms that have undergone decay to the number of atoms left undecayed. Carbon dating is used to estimate the age of fossils, uranium dating is used to estimate the age of rocks.

2. The average life of an Indian is 56 years. Find the number of times the human heart beats in the life of an Indian, if the heat beats once in 0.8 s.
a) 20.4×109 times
b) 2.5×109 times
c) 2.2×109 times
d) 6.1×109 times

Explanation:
Average life of an Indian = 56 years = 56×365.25×24×60×60 s
Period of heart beat = 0.8 s
Total number of heart beats in 56 years = (56×365.25×24×60×60)/0.8 = 2.2×109 times.

3. How many dimensions of the world are there?
a) 7
b) 3
c) 2
d) 4

Explanation: All the derived quantities can be expressed in terms of some combination of the seven fundamental or base quantities. We call these seven fundamental quantities as the seven dimensions of the world.

4. What is the dimensional formula for Gravitational constant?
a) ML2 T(-3)
b) ML(-1) T(-2)
c) ML(-1) T(-1)
d) M(-1) L3 T(-2)

Explanation: F = G(m1 m2)/r2
G = ([F][r2])/([m1 ][m2]) = (MLT(-2) L2)/MM = M(-1) L3 T(-2).

5. Which of the following is a dimensional constant?
a) e
b) Area
c) Specific gravity
d) Gravitational constant

Explanation: The physical quantities which posses dimensions and have constant values are called dimensional constants. Hence Gravitational constant is a dimensional constant.
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6. If the unit of force is 1kN, unit of length 1km and the unit of time is 100s, what will be the unit of mass?
a) 1000 kg
b) 104 kg
c) 100 kg
d) 105 kg

Explanation: M = (MLT(-2) T2)/L = (FT2)/L
M = (1000 N ×104 s2)/(1000 m) = 104 kg.

7. Which of the following is a systematic error?
a) Constant error
b) Least count error
c) Gross error
d) Personal error

Explanation: The errors which occur in one direction, either positive or negative, are called systematic error. Personal errors arise due to improper setting of instruments. Hence it comes under systematic error.

8. All physical quantities have dimensions.
a) True
b) False

Explanation: All physical quantities do not have dimensions. Like angle, strain and relative density are examples for dimension less quantity.

9. An instrument cannot be precise without being accurate.
a) False
b) True

Explanation: An instrument can be precise without being accurate but the instrument cannot be accurate without being precise.

10. Can the diameter of a thread be measure by using a scale?
a) Yes
b) No

Explanation: The thread is wound on a meter scale such that its turns are close together. Thickness of the thread coil is measure and the number of turns made by the thread is counted. Diameter is then given by dividing the thickness by a number of turns.

11. Calculate the de-Broglie wavelength of an electron which has been accelerated from rest on application of potential of 400volts.
a) 0.1653 Amstrong
b) 0.5125 Amstrong
c) 0.6135 Amstrong
d) 0.2514 Amstrong

Explanation: de-Broglie wavelength = h/√(2×m×e×V)
De-Broglie wavelength = (6.625×10(-14))/√(2×9.11×10(-31)×1.6×10(-19)×400)
Wavelength = 0.6135 Amstrong.

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