Physical quantities and SI units

Question 1

What are the 7 SI base units?

Answer 1

Mnemonic = SMMACKK (seconds, metres, moles, amperes, candela, kelvin, kilograms)

Length - metre (m)
The distance travelled by light in a vacuum in a fixed time period

Time - second (s)
Based on the frequency of radiation emitted from Caesium-133

Mass - kilogram (kg)
Defined by linking the definitions of matre and second with Plank’s constant - i.e. now a derived unit

Amount of a substance - mole (mol)
One mole of a substance contains the same number of elementary particles as there are carbon atoms in 12g of carbon-12 (Avogadro’s number = 6.02x10²³)

Temperature = kelvin (K)
1/273.16 of the thermodynamic temperature of the triple point of water

Electrical current = ampere (A)
Based on the elementary charge e, and the definition of time

Luminous intensity = candela (Cd)
One candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 × 1012 Hz and that has a radiant intensity in that direction of 1/683 watt per steradian

Question 2

What is the unit of force?

Answer 2

Newton (N)
Force = mass x acceleration
N = kg.m.s⁻²

Question 3

What is the unit of pressure?

Answer 3

Pacal (Pa)
Pressure = force / area
Pa = N.m⁻²

kPA more relevant in clinical practice

Question 4

What is the unit of energy?

Answer 4

Joule (J)
Energy = force x distance
J = N.m

Question 5

What is the unit of power?

Answer 5

Watt (W)
Power = energy per unit time
W = J.s⁻¹

Question 6

What is the unit of charge?

Answer 6

Coulomb (C)
Charge = current x time
C = A.s

Question 7

What is the unit of voltage?

Answer 7

Volt (V)
The potential difference across a segment of wire necessary to allow the flow of one ampere of current while there is dissipation of one watt of power
Voltage = power / current
V = W.A⁻¹

Question 8

What values of various pressure measurements are equal to atmospheric pressure (1atm)?

Answer 8

1.013 bar
101.3 kPa
14.69 PSI
760 mmHg
1020 cmH2O
101325 Pascal

Question 9

What is the conversion between degrees centigrade and kelvin?

Answer 9

Temperature in degrees centigrade = temperature in kelvin - 273

Question 10

1 kPa =

Answer 10

• 1000N/m2
• 0.01 Bar
• 0.01 atm
• 7.5 mmHg
• 10.2 cmH20