SI Units, Conversions & Measurement – Lecture Review

42 question-and-answer flashcards covering revised SI base-unit definitions, common derived units, unit conversions, and practical measurement facts useful for UPSC/general-science exam preparation.

What physical quantity does the revised 2019 definition of the ampere fix through the elementary charge e?

The ampere is defined so that the elementary charge e is exactly 1.602 176 634 × 10⁻¹⁹ A·s; 1 A corresponds to 1 / (1.602 176 634 × 10⁻¹⁹) elementary charges flowing per second.

According to the 2019 SI revision, which constant defines the kelvin and what is its fixed value?

The kelvin is defined by fixing the Boltzmann constant k at exactly 1.380 649 × 10⁻²³ kg·m²·s⁻²·K⁻¹.

State the modern definition of one candela.

One candela is the luminous intensity of a source emitting monochromatic radiation of frequency 540 × 10¹² Hz with a radiant intensity of 1 / 683 W·sr⁻¹ in that direction.

How is one mole defined in the revised SI?

One mole contains exactly 6.022 140 76 × 10²³ specified elementary entities, fixing the Avogadro constant NA to that value in mol⁻¹.

What distinguishes a derived SI unit from a base SI unit?

Derived units are obtained from the seven base units through algebraic relationships (products, quotients, powers) and may have special names; base units are the fundamental, independent quantities.

Which SI derived unit measures force and what is its symbol?

The newton (N) measures force.

Which SI derived unit measures pressure or stress and what is its relation to base units?

The pascal (Pa) measures pressure; 1 Pa = 1 kg·m⁻¹·s⁻² (or N m⁻²).

State the SI unit of energy and its base-unit expression.

Energy is measured in joules (J); 1 J = 1 kg·m²·s⁻².

What is the SI unit of power and its relation to energy and time?

Power is measured in watts (W); 1 W = 1 J s⁻¹.

Which SI unit corresponds to electrical charge?

The coulomb (C).

Give the SI unit for electric potential difference (voltage).

The volt (V).

Which unit measures electrical capacitance and how is it expressed in base units?

The farad (F); 1 F = 1 kg⁻¹·m⁻²·s⁴·A² (or C V⁻¹).

Name the SI unit of electrical resistance and its relationship to volts and amperes.

The ohm (Ω); 1 Ω = 1 V A⁻¹.

What unit expresses electrical conductance?

The siemens (S); 1 S = 1 A V⁻¹.

Which SI unit is used for magnetic flux?

The weber (Wb).

Identify the SI unit of frequency and its definition.

Hertz (Hz); 1 Hz = 1 cycle s⁻¹.

What is the SI unit of plane angle?

The radian (rad).

What is the SI unit of solid angle?

The steradian (sr).

Define electrical power in words and give its formula.

Electrical power is the rate at which electrical energy is transferred; P = W / t (work per unit time).

One horsepower equals how many watts?

Approximately 746 W.

What is the SI unit of electrical resistivity?

Ohm-metre (Ω·m).

Explain the relationship between resistivity (ρ), resistance (R), length (ℓ) and cross-sectional area (A).

ρ = R A / ℓ.

What is the SI unit of work and how else can it be expressed?

The joule; it can also be expressed as a newton-metre (N·m).

Why are astronomical distances often given in light-years?

Because the speed of light in vacuum is constant and precisely known, making it a convenient, universally applicable basis for large distance measurements.

How long is one light-year in metres?

Approximately 9.461 × 10¹⁵ m.

Define a parsec and give its equivalent in light-years.

A parsec is the distance at which 1 AU subtends 1 arc-second; 1 pc ≈ 3.26 light-years.

What unit is used for ship speed and what does it represent?

The knot (kt); 1 knot = 1 nautical mile per hour ≈ 1.852 km h⁻¹.

State the length of one nautical mile.

1 nautical mile = 1.852 kilometres.

How many inches are in one centimetre?

1 cm ≈ 0.39 inch.

Convert 6 feet to nanometres (approx.).

≈ 1.83 × 10⁸ nm.

What is the size of one nanometre in metres and centimetres?

1 nm = 1 × 10⁻⁹ m = 1 × 10⁻⁷ cm.

Define a micron (micrometre) in metres.

1 µm = 1 × 10⁻⁶ m.

Express one micron in millimetres.

1 µm = 1 / 1000 mm (0.001 mm).

What distance does an angstrom represent?

1 Å = 1 × 10⁻¹⁰ m (0.1 nm).

Which unit measures sound intensity?

The decibel (dB).

What is the SI unit of radioactivity and whom is it named after?

The becquerel (Bq), named after Henri Becquerel.

State the definition of a watt in base SI units.

1 W = 1 kg·m²·s⁻³.

Which constant rate makes the ampere equal to about 6.25 × 10¹⁸ electrons per second?

1 A equals a charge flow rate of 1 C s⁻¹, i.e., ~6.25 × 10¹⁸ electrons s⁻¹.

What is the primary SI unit for temperature measurement?

The kelvin (K).

List four quantities matched correctly with their SI units: Force, Pressure, Power, Electric Current.

Force – newton; Pressure – pascal; Power – watt; Electric current – ampere.

What physical quantity does the revised 2019 definition of the ampere fix through the elementary charge e?

The ampere is defined so that the elementary charge e is exactly 1.602 176 634 × 10⁻¹⁹ A·s; 1 A corresponds to 1 / (1.602 176 634 × 10⁻¹⁹) elementary charges flowing per second.

According to the 2019 SI revision, which constant defines the kelvin and what is its fixed value?

The kelvin is defined by fixing the Boltzmann constant k at exactly 1.380 649 × 10⁻²³ kg·m²·s⁻²·K⁻¹.

State the modern definition of one candela.

One candela is the luminous intensity of a source emitting monochromatic radiation of frequency 540 × 10¹² Hz with a radiant intensity of 1 / 683 W·sr⁻¹ in that direction.

How is one mole defined in the revised SI?

One mole contains exactly 6.022 140 76 × 10²³ specified elementary entities, fixing the Avogadro constant NA to that value in mol⁻¹.

What distinguishes a derived SI unit from a base SI unit?

Derived units are obtained from the seven base units through algebraic relationships (products, quotients, powers) and may have special names; base units are the fundamental, independent quantities.

Which SI derived unit measures force and what is its symbol?

The newton (N) measures force.

Which SI derived unit measures pressure or stress and what is its relation to base units?

The pascal (Pa) measures pressure; 1 Pa = 1 kg·m⁻¹·s⁻² (or N m⁻²).

State the SI unit of energy and its base-unit expression.

Energy is measured in joules (J); 1 J = 1 kg·m²·s⁻².

What is the SI unit of power and its relation to energy and time?

Power is measured in watts (W); 1 W = 1 J s⁻¹.

Which SI unit corresponds to electrical charge?

The coulomb (C).

Give the SI unit for electric potential difference (voltage).

The volt (V).

Which unit measures electrical capacitance and how is it expressed in base units?

The farad (F); 1 F = 1 kg⁻¹·m⁻²·s⁴·A² (or C V⁻¹).

Name the SI unit of electrical resistance and its relationship to volts and amperes.

The ohm (Ω); 1 Ω = 1 V A⁻¹.

What unit expresses electrical conductance?

The siemens (S); 1 S = 1 A V⁻¹.

Which SI unit is used for magnetic flux?

The weber (Wb).

Identify the SI unit of frequency and its definition.

Hertz (Hz); 1 Hz = 1 cycle s⁻¹.

What is the SI unit of plane angle?

The radian (rad).

What is the SI unit of solid angle?

The steradian (sr).

Define electrical power in words and give its formula.

Electrical power is the rate at which electrical energy is transferred; P = W / t (work per unit time).

One horsepower equals how many watts?

Approximately 746 W.

What is the SI unit of electrical resistivity?

Ohm-metre (Ω·m).

Explain the relationship between resistivity (ρ), resistance (R), length (ℓ) and cross-sectional area (A).

ρ = R A / ℓ.

What is the SI unit of work and how else can it be expressed?

The joule; it can also be expressed as a newton-metre (N·m).

Why are astronomical distances often given in light-years?

Because the speed of light in vacuum is constant and precisely known, making it a convenient, universally applicable basis for large distance measurements.

How long is one light-year in metres?

Approximately 9.461 × 10¹⁵ m.

Define a parsec and give its equivalent in light-years.

A parsec is the distance at which 1 AU subtends 1 arc-second; 1 pc ≈ 3.26 light-years.

What unit is used for ship speed and what does it represent?

The knot (kt); 1 knot = 1 nautical mile per hour ≈ 1.852 km h⁻¹.

State the length of one nautical mile.

1 nautical mile = 1.852 kilometres.

How many inches are in one centimetre?

1 cm ≈ 0.39 inch.

Convert 6 feet to nanometres (approx.).

≈ 1.83 × 10⁸ nm.

What is the size of one nanometre in metres and centimetres?

1 nm = 1 × 10⁻⁹ m = 1 × 10⁻⁷ cm.

Define a micron (micrometre) in metres.

1 µm = 1 × 10⁻⁶ m.

Express one micron in millimetres.

1 µm = 1 / 1000 mm (0.001 mm).

What distance does an angstrom represent?

1 Å = 1 × 10⁻¹⁰ m (0.1 nm).

Which unit measures sound intensity?

The decibel (dB).

What is the SI unit of radioactivity and whom is it named after?

The becquerel (Bq), named after Henri Becquerel.

State the definition of a watt in base SI units.

1 W = 1 kg·m²·s⁻³.

Which constant rate makes the ampere equal to about 6.25 × 10¹⁸ electrons per second?

1 A equals a charge flow rate of 1 C s⁻¹, i.e., ~6.25 × 10¹⁸ electrons s⁻¹.

What is the primary SI unit for temperature measurement?

The kelvin (K).

List four quantities matched correctly with their SI units: Force, Pressure, Power, Electric Current.

Force – newton; Pressure – pascal; Power – watt; Electric current – ampere.

How is the metre defined in the SI system?

The metre (m) is defined by fixing the speed of light in vacuum c at exactly 299,792,458 m s⁻¹.

How is the kilogram defined in the revised SI (post-2019)?

The kilogram (kg) is defined by fixing the Planck constant h at exactly 6.626 070 15 × 10⁻³⁴ J·s (kg·m²·s⁻¹).

How is the second defined in the SI system?

The second (s) is defined by fixing the unperturbed ground-state hyperfine transition frequency of the caesium-133 atom, \Delta \nu_{Cs} , at exactly 9,192,631,770 Hz.

What is the SI unit of magnetic

The tesla (T).