A Level Physics 1.2 SI Units MCQs – 30 Practice Questions with Answers (Cambridge 9702)

Free practice MCQs for Cambridge International AS & A Level Physics (9702), Section 1.2 SI Units. Practise the seven SI base units, expressing derived units in base units, and prefixes and conversions – with a worked explanation for every answer.

Formula recap: Every derived unit can be broken down into SI base units using defining equations such as F = ma, W = Fs, P = E/t, p = F/A and V = W/Q. Keep the full set of equations and units at hand in our A Level Physics Formula Sheet.

The SI Base Units

Q1. How many base quantities are there in the SI system?

  • (A) 7
  • (B) 5
  • (C) 6
  • (D) 8
Show Answer

Answer: A – There are seven SI base quantities: length, mass, time, electric current, thermodynamic temperature, amount of substance and luminous intensity. Every other quantity is derived from these seven.

Q2. What is the SI base unit of amount of substance?

  • (A) gram
  • (B) mole
  • (C) molecule
  • (D) kilogram
Show Answer

Answer: B – The base unit of amount of substance is the mole, symbol mol. The gram and kilogram measure mass, and a molecule is a particle, not a unit.

Q3. What is the SI base unit of luminous intensity?

  • (A) lumen
  • (B) lux
  • (C) watt
  • (D) candela
Show Answer

Answer: D – The base unit of luminous intensity is the candela, symbol cd. The lumen and lux are derived units, and the watt is the unit of power.

Q4. Which of the following is the correctly written symbol for the SI base unit of mass?

  • (A) kg
  • (B) Kg
  • (C) KG
  • (D) kgs
Show Answer

Answer: A – The kilogram is written kg: lower-case k for the prefix kilo and lower-case g for gram. Unit symbols never take a capital K here, capitals throughout, or a plural “s”.

Q5. Which of the following is an SI base unit?

  • (A) newton
  • (B) joule
  • (C) kelvin
  • (D) pascal
Show Answer

Answer: C – The kelvin (K), the unit of thermodynamic temperature, is a base unit. The newton, joule and pascal are all derived units built from base units.

Q6. Which is the correct symbol for the SI base unit of time?

  • (A) sec
  • (B) S
  • (C) secs
  • (D) s
Show Answer

Answer: D – The second has the symbol s, a single lower-case letter. “sec” and “secs” are informal abbreviations, and capital S is the symbol for the siemens, the unit of conductance.

Q7. Some units are named after scientists. Which pair shows the unit of force written correctly as a name and as a symbol?

  • (A) Newton, n
  • (B) newton, N
  • (C) Newton, N
  • (D) newton, NT
Show Answer

Answer: B – When written out in full, unit names are lower case (newton, joule, watt), even though they honour a person. The symbol, however, is a capital letter: N for newton, J for joule, W for watt.

Q8. What is the SI base unit of electric current?

  • (A) ampere
  • (B) coulomb
  • (C) volt
  • (D) ohm
Show Answer

Answer: A – Electric current is a base quantity and its unit is the ampere, symbol A. The coulomb, volt and ohm are derived units.

Q9. The unit with symbol K is the base unit of which quantity?

  • (A) mass
  • (B) luminous intensity
  • (C) thermodynamic temperature
  • (D) amount of substance
Show Answer

Answer: C – K is the kelvin, the base unit of thermodynamic temperature. Mass uses kg, luminous intensity uses cd, and amount of substance uses mol.

Q10. Which set contains only SI base units?

  • (A) m, kg, N
  • (B) m, s, mol
  • (C) s, A, J
  • (D) kg, K, Pa
Show Answer

Answer: B – The metre, second and mole are all base units. Each other set contains a derived unit: N (newton) in A, J (joule) in C and Pa (pascal) in D.

Derived Units Expressed in Base Units

Q11. The newton, the unit of force, expressed in SI base units is

  • (A) kg m s−1
  • (B) kg m s−2
  • (C) kg m2 s−2
  • (D) kg s−2
Show Answer

Answer: B – From F = ma, force = mass × acceleration, so 1 N = 1 kg × 1 m s−2 = kg m s−2.

Q12. The joule, the unit of energy, expressed in SI base units is

  • (A) kg m s−2
  • (B) kg m−1 s−2
  • (C) kg m2 s−2
  • (D) kg m2 s−3
Show Answer

Answer: C – Work done = force × distance, so 1 J = 1 N × 1 m = (kg m s−2)(m) = kg m2 s−2.

Q13. The watt, the unit of power, expressed in SI base units is

  • (A) kg m2 s−3
  • (B) kg m2 s−2
  • (C) kg m s−3
  • (D) kg s−3
Show Answer

Answer: A – Power = energy / time, so 1 W = 1 J s−1 = kg m2 s−2 × s−1 = kg m2 s−3.

Q14. The pascal, the unit of pressure, expressed in SI base units is

  • (A) kg m s−2
  • (B) kg m−2 s−2
  • (C) kg m−1 s−1
  • (D) kg m−1 s−2
Show Answer

Answer: D – Pressure = force / area, so 1 Pa = 1 N m−2 = (kg m s−2)(m−2) = kg m−1 s−2.

Q15. The volt, the unit of potential difference, expressed in SI base units is

  • (A) kg m2 s−2 A−1
  • (B) kg m2 s−3 A−1
  • (C) kg m2 s−3 A−2
  • (D) kg m s−3 A−1
Show Answer

Answer: B – Voltage = power / current = watt / ampere, so 1 V = (kg m2 s−3) × A−1 = kg m2 s−3 A−1. This also follows from V = W/Q with the coulomb equal to A s.

Q16. The combination kg m2 s−3 A−2 is the base-unit form of which derived unit?

  • (A) volt
  • (B) tesla
  • (C) farad
  • (D) ohm
Show Answer

Answer: D – Resistance = voltage / current, so the ohm = volt × A−1 = (kg m2 s−3 A−1)(A−1) = kg m2 s−3 A−2.

Q17. The coulomb, the unit of electric charge, expressed in SI base units is

  • (A) A s
  • (B) A s−1
  • (C) A m
  • (D) s A−1
Show Answer

Answer: A – Charge = current × time (Q = It), so 1 C = 1 A × 1 s = A s. Note that charge itself is not a base quantity; only its unit combination is asked for.

Q18. The tesla, the unit of magnetic flux density, expressed in SI base units is

  • (A) kg s−1 A−1
  • (B) kg m s−2 A−1
  • (C) kg s−2 A−1
  • (D) kg s−2 A
Show Answer

Answer: C – From F = BIL, B = F / (IL). So 1 T = (kg m s−2) / (A × m) = kg s−2 A−1: the metres cancel.

Q19. The farad is defined as one coulomb per volt (C V−1). Expressed in SI base units, the farad is

  • (A) kg m2 s−4 A−2
  • (B) kg−1 m−2 s4 A2
  • (C) kg−1 m−2 s2 A2
  • (D) kg m−2 s4 A2
Show Answer

Answer: B – F = C / V = (A s) / (kg m2 s−3 A−1). Dividing adds the negative indices the other way: A1−(−1) = A2, s1−(−3) = s4, giving kg−1 m−2 s4 A2.

Q20. The hertz, the unit of frequency, expressed in SI base units is

  • (A) s
  • (B) m s−1
  • (C) kg s−2
  • (D) s−1
Show Answer

Answer: D – Frequency = 1 / period, so 1 Hz = 1 s−1. The hertz is simply the reciprocal second; it contains no other base unit.


Prefixes & Unit Conversions

Q21. A volume of 5.0 cm3 expressed in m3 is

  • (A) 5.0 × 10−2 m3
  • (B) 5.0 × 10−4 m3
  • (C) 5.0 × 10−6 m3
  • (D) 5.0 m3
Show Answer

Answer: C – 1 cm = 10−2 m, and the conversion factor must be cubed for a volume: 1 cm3 = (10−2)3 m3 = 10−6 m3. So 5.0 cm3 = 5.0 × 10−6 m3.

Q22. A car travels at 72 km h−1. This speed in m s−1 is

  • (A) 20 m s−1
  • (B) 2.0 m s−1
  • (C) 200 m s−1
  • (D) 259 m s−1
Show Answer

Answer: A – 72 km h−1 = 72 × 1000 m / 3600 s = 72 000 / 3600 = 20 m s−1. (Shortcut: divide km h−1 by 3.6.)

Q23. A liquid has density 0.80 g cm−3. This density in kg m−3 is

  • (A) 0.80 kg m−3
  • (B) 8.0 kg m−3
  • (C) 80 kg m−3
  • (D) 800 kg m−3
Show Answer

Answer: D – 1 g cm−3 = (10−3 kg) / (10−6 m3) = 103 kg m−3. So 0.80 g cm−3 = 0.80 × 1000 = 800 kg m−3.

Q24. A current of 250 mA expressed in amperes is

  • (A) 0.25 A
  • (B) 2.5 A
  • (C) 0.025 A
  • (D) 25 A
Show Answer

Answer: A – The prefix milli means 10−3, so 250 mA = 250 × 10−3 A = 0.25 A.

Q25. The prefix pico, symbol p, represents a factor of

  • (A) 10−9
  • (B) 10−12
  • (C) 10−6
  • (D) 10−15
Show Answer

Answer: B – pico = 10−12. For comparison: nano = 10−9, micro = 10−6 and femto = 10−15.

Q26. A radio station broadcasts at a frequency of 3.6 MHz. This frequency in hertz is

  • (A) 3.6 × 103 Hz
  • (B) 3.6 × 105 Hz
  • (C) 3.6 × 106 Hz
  • (D) 3.6 × 109 Hz
Show Answer

Answer: C – The prefix mega means 106, so 3.6 MHz = 3.6 × 106 Hz. (kHz would be 103 and GHz would be 109.)

Q27. One kilowatt-hour (1 kWh), the unit used on electricity bills, is equal to how many joules?

  • (A) 3.6 × 103 J
  • (B) 6.0 × 104 J
  • (C) 3.6 × 105 J
  • (D) 3.6 × 106 J
Show Answer

Answer: D – 1 kWh = 1000 W × 3600 s = 3 600 000 J = 3.6 × 106 J = 3.6 MJ. Energy = power × time, with both in SI units.

Q28. An area of 12 cm2 expressed in m2 is

  • (A) 1.2 × 10−3 m2
  • (B) 0.12 m2
  • (C) 1.2 × 10−1 m2
  • (D) 1.2 × 10−5 m2
Show Answer

Answer: A – The conversion factor must be squared for an area: 1 cm2 = (10−2 m)2 = 10−4 m2. So 12 cm2 = 12 × 10−4 = 1.2 × 10−3 m2. Simply dividing by 100 is the classic trap.

Q29. A current of 45 µA expressed in amperes is

  • (A) 4.5 × 10−3 A
  • (B) 4.5 × 10−6 A
  • (C) 4.5 × 10−5 A
  • (D) 4.5 × 10−8 A
Show Answer

Answer: C – The prefix micro means 10−6, so 45 µA = 45 × 10−6 A = 4.5 × 10−5 A.

Q30. A volume of 2.0 mm3 expressed in m3 is

  • (A) 2.0 × 10−3 m3
  • (B) 2.0 × 10−6 m3
  • (C) 2.0 × 10−9 m3
  • (D) 2.0 × 10−12 m3
Show Answer

Answer: C – 1 mm = 10−3 m, and the factor must be cubed for a volume: 1 mm3 = (10−3)3 m3 = 10−9 m3. So 2.0 mm3 = 2.0 × 10−9 m3.

Keep practising: Work through more boards and topics on our Physics MCQs hub, and revise every key equation with the A Level Physics Formula Sheet.

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