Units & Equations Page

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.

UNITS

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.

 Physical Quantity Unit used Symbol 1. Force newton N 2. Weight newton N 3. Mass kilogram kg 4. Moment of a force newton metre N m 5. Average speed metre per second m / s 6. Acceleration metre per second squared m / s2 7. Velocity metre per second (in a given direction) m / s + direction 8. Momentum kilogram metre per second kg. m / s 9. Work joule J 10. Power watt W 11. Energy (all forms) joule J 12. Efficiency no units - 13. Temperature kelvin K 14. Specific heat capacity joule per kilogram kelvin J / (kg.K) 15. Density kilogram per metre cubed kg / m3 16. Pressure newton per square metre or Pascal N / m2 or Pa 17. Electric Current Ampere A 18. Electric Charge Coulomb C 19. Voltage or potential difference Volt V 20. Capacitance * farad F 21. Resistance ohm W 22. Commercial unit of electrical energy kilowatt hour kWh 23. Refractive index no units -

EQUATIONS

 Physical Quantities involved Equation (or relationship) Notes 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 = m . a (N) = (kg) . ( m / s 2 ) F = unbalanced force m = mass a = acceleration 3. Weight of a body, expressed in newtons W = m . g ( N ) = (kg) . (m / s2) 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. D-t graphs The gradient (or slope) of a D-t graph represents the speed or velocity - 8. V-t graphs i) The slope or gradient of a V-t graph represents the acceleration (ii) The area under a V-t 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 momentum = mass x velcoity ( kg.m / s ) = (kg) (m/s) 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 . t2 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/s2) . m Note: h MUST BE IN METRES 14. Kinetic energy K.E. = ½ m . v2 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 vice-versa (temp.) in oC + 273 = temp. in Kelvin temp. in Kelvin - 273 = temp. in Celsius e.g. 37oC = 37 + 273 = 310 K 21. Specific heat capacity and quantity of heat energy absorbed or given out Q = m . c. Dq where Q = quantity of heat given out or absorbed, m = mass c = sp. Heat capacity, D q =temperature difference 22. Density (r) density = mass / volume 23. Pressure (p) pressure = Force / Area force acts at right angles to the area over which it acts If force is in N and area in m2, then pressure is in Pascals 24. Pressure in a fluid p = h. r . g h = vertical height 25. Pressure Law p1 / T 1 = p2 / T 2 London only T must be in kelvin volume constant 26. Charles' Law V 1 / T 1 = V 2 / T 2 London only pressure constant 27. Boyle's Law p1 . V 1 = p 2 . V 2 London only temperature constant 28. Electric charge (Q) Q = I . t 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) C = Q / C or Q = C . V Matsec only 32. Ohm's Law V = I . R I = current V = p.d. or voltage R = resistance 33. Adding resistors in series R T = R 1 + R 2 + R 3 + ………. 33. Adding resistors in parallel General Equation (for any number of resistors) 1 / RT = 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 sub-divides in a parallel pair of resistors Let current through R 1 be i 1 and let that through R2 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 Y-plates/ time-base frequency (across X-plates) 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 c or v = f . l 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

Go back to my homepage by clicking here

Go back to my helppage by clicking here

Go to geocities homepage by clicking here