Here is a list of important S.I. units of Physical Quantities used in Physics for ordinary level examinations (first examinations)
A sound knowledge of these units is a first step towards success in examinations.
I advise Physics students to revise these units at least once
per week.
Note: Those units marked with an asterisk (*) are only required by the Matsec
Syllabus. All the rest are included both in the University of London and in the
Matsec syllabus.
Note:
Matsec stands for Matriculation Certificate
in Secondary Education which is set by the University of Malta
Examinations Board. The G.C.E. London is set by the University
of London (United Kingdom) Examinations and Assessment Council.
1. Force  
2. Weight  
3. Mass  
4. Moment of a force  
5. Average speed  
6. Acceleration  
7. Velocity 


8. Momentum  
9. Work  
10. Power  
11. Energy (all forms)  
12. Efficiency  
13. Temperature  
14. Specific heat capacity  
15. Density  
16. Pressure 


17. Electric Current  
18. Electric Charge  
19. Voltage
or potential difference  
20. Capacitance *  
21. Resistance  
22. Commercial unit of
electrical energy  
23. Refractive index 
 
1. Moment of a force  Moment of a force about a given point
= Force x perpendicular distance from line of action of force to point  
2. Unbalanced force (F) 
 F = unbalanced force
m = mass a = acceleration 
3. Weight of a body, expressed in newtons 
 W = weight
g = acceleration due to gravity 
4. Average speed  average speed = total distance moved / time  
5. Average velocity  average velocity = total displacement / time  displacement = distance + direction 
6. Acceleration  acceleration = change in velocity / time
= ( final velocity  initial velocity) / time  
7. Dt graphs  The gradient (or slope) of a Dt graph represents the speed or velocity  
8. Vt graphs  i) The slope or gradient of a Vt graph represents the acceleration
(ii) The area under a Vt graph ( i.e. the area enclosed between the graph and the time axis) represents the distance or displacement covered in a particular time.  
9. Momentum 
 N.B. direction has to be indicated if given 
10. Equations of motion with uniform acceleration  (i) v = u + a . t
(ii) s = ½ a . t^{2 }used when u = 0 (or object starts moving from rest) (iii) s = ( u + v) /2 . t  where:
v = final velocity 8 = initial velocity a = acceleration t = time s = distance moved Note: a is negative for retardation 
11. Work (or energy converted)  Work = Force x distance moved (in direction of force)  Work in joules if force in newtons and distance in metres 
12. Power (or rate of energy conversion)  Power = work done / time taken
= energy converted / time  power in watts if work or energy in joules and time in seconds 
13. Gravitational potential energy  G.P.E. = m . g . h
(J) = (kg) . (m/s^{2}) . m  Note: h MUST BE IN METRES 
14. Kinetic energy  K.E. = ½ m . v^{2}  K.E. in joules if m in kg and v in m/s 
15. Power  Power = ( force x distance) / time
or power = force x velocity  Not very common 
16. Efficiency of machines  (%) efficiency =
( useful work output / total work input) x 100 = work done on load / work done by effort x 100  In a practical machine the efficiency is less than 100% due to energy losses like heat in overcoming frictional forces 
17. Work output  work output = load x distance moved by load   
18. Work input  work input = effort x distance moved by effort   
19. Wasted work  work wasted = (work input) minus (work output)   
20. Conversion of temperature from Celsius scale to Kelvin Scale and viceversa 
 e.g.
37^{o}C = 37 + 273 = 310 K 
21. Specific heat capacity and quantity of heat energy absorbed or given out  where
Q = quantity of heat given out or absorbed, m = mass c = sp. Heat capacity, D q =temperature difference  
22. Density (r)  
23. Pressure (p)  force acts at right angles to the area over which it acts
If force is in N and area in m^{2}, then pressure is in Pascals  
24. Pressure in a fluid  h = vertical height  
25. Pressure Law 
 T must be in kelvin
volume constant 
26. Charles' Law 
 pressure constant 
27. Boyle's Law 
 temperature constant 
28. Electric charge (Q)  t must be in seconds, for Q to be in coulombs  
29. Electric energy  a) Energy = V . Q
b) Energy = V . I . t c) Energy = I ^{2 }. R. t  where
V = p.d. in volots I = current in amps R = resistance in ohms t = time in seconds energy = in joules 
30. Electric power  a) p = V . I
b) p = I ^{2} . R c) p = V ^{2} / R  
31. Capacitance (C)  Matsec only  
32. Ohm's Law  I = current
V = p.d. or voltage R = resistance  
33. Adding resistors in series  
33. Adding resistors in parallel  General Equation (for any number of resistors)
1 / R_{T} = 1 / R _{1} + 1 / R _{2} + 1 / R _{3} etc Special Case of 2 resistors in parallel R _{T} = (R _{1} x R _{2}) / (R_{ 1} + R _{2}) Note: In Matsec syllabus only 2 resistors will be given connected in parallel  where R _{T} represents the total or effective resistance 
34. How current subdivides in a parallel pair of resistors  Let current through R_{ 1} be i _{1} and let that through R_{2} be i _{2}
then, i_{ 1} = I ( R _{2} ) / ( R _{1} + R _{2}) and 1_{ 2} = I ( R _{1} ) / ( R _{1} + R _{2})  I = total current entering the junction of the 2 resistors in parallel 
35. Commercial unit of electrical energy  the kWh  no. of kwh = (no. of watts) / 100 x (no. of h)
or no. of kWh = (no. of kW) x no. of hours  
36. C. R. O  no. of complete cycles seen on screen
= input (signal) frequency across Yplates/ timebase frequency (across Xplates)  
37. Periodic Time (T) or period  T = 1 / f where f = frequency  
38. Induced e.m.f.
(Faraday's Law)  size of induced e.m.f. change in flux / time  
39. Transformer equations  a) Turns ratio equation:
n_{ p} / n _{s }= v _{p} / v _{s} b) Ideal transformer (= 100% efficient) V _{p }x I _{p} = V _{s} x I _{s} c) Efficiency of a transformer (%) efficiency = power output (in secondary) / power input (in primary) x 100  
40. Refractive index  a) Refractive index for a ray of light travelling from air into a medium
= sine angle of incidence in air / sine angle of refraction in the medium b) ref. Index = real depth / apparent depth c) ref. Index = 1 / sine c (critical angle) d) ref. Index = (velocity of light in air) / velocity of light in medium  In London exam. Only 
42. magnification (m)  m = height of image / height of object
or m = image distance / object distance from lens  
43. Wave equation  velocity = frequency x wavelength  
44. Factors affecting frequency of vibration of a stretched wire  a) f is directly proportional to 1 / l
b) f is directly proportional to root T c) f is directly proportional to 1 / root m  where l = length of wire
T = tension of wire m = mass per unit length or thickness 
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