Chapter 1. Physical Quantities and Units Flashcards
What are the seven base quantities?
- Time, t, s
- Length, l, m(metres)
- Mass, m, kg(kilograms)
- Temperature, T, K(kelvins)
- Electric current, I, A(ampere)
- Amount of substance, n, mol(moles)
7 luminous intensity, cd(candela)
Give prefixes and their multipliers
tera, T, 10¹²
giga, G, 109
mega, M, 106
kilo, k, 10³
deci, d, 10-1
centi, c, 10-2
milli, m, 10-3
micro, μ, 10-6
nano, n, 10-9
pico, p, 10-12
femto, f, 10-15
What is meant by homogeneity in equations?
For any correct equation there must be identical base units on both sides.
Why are homogenous equations not always correct
Because equations with different constants may also be homogenous but not exactly correct
Differentiate between a vector and a scalar quantity
A vector quantity is a physical quantity that has both a magnitude and a direction whilst a scalar quantity only has a magnitude and no direction
What are the three basic types of forces and explain?
- Electromagnetic force
-Holds electrons towards the nucleus.It is responsible when two objects are in contact. - Nuclear force
-Weak nuclear force as in radioactive isotopes and Strong nuclear force as in protons held together in the nucleus. - Gravitational force
-Force experienced by attraction of objects with mass.Every object has gravity just that the effects depend on the size of the object.
What are the other types of forces?
- Normal forces
-Experienced by two solids in contact at 90 degrees caused by electromagnetic forces - Tension
-Also caused by electromagnetic forces they come in pairs like normal forces - Frictional forces
-They depend on the type of surfaces in contact and how hard the surfaces are pressed against each other - Motive force
-These are forces that drive something forward.They are caused by friction. - Drag
-This is just friction in fluids.The greater the velocity the greater the drag.Drag also depends on viscosity.The higher the viscosity the higher the drag.The higher the temperature the lesser the viscosity in liquids but the higher in gases. - Lift
-Caused by the viscosity of air when a laminar flow is created following the downward curve of the aero foil.The greater the angle of attack the greater the lift but when there’s too great of an angle of attack it can cause the aircraft to stall in midair as the airflow becomes turbulent.
What are the two ways in which fluids flow?
- Streamline(laminar)
-Has streamlines.Steady movement.Particles at a point follow the same path at the same speed.Drag is proportional to velocity. - Turbulent flow
-Above a certain critical speed all streamline flow becomes turbulent. Drag is directly proportional to the square of the velocity.Hard shapes will create turbulent flow a lot sooner than smoother shapes.
What is Stoke’s law?
Sir George Stoke in 1851.Only for smooth spheres moving in a fluid.
viscous drag = 6 x viscosity of fluid(kgs-1m-1) x radius of sphere x velocity of sphere.
Stoke’s law assumes there’s laminar flow.
What is a free body force diagram?
A diagram that shows all the forces acting on just one object.
Basic estimates and their order of magnitude.
Height of bench
Length of room
Width of pencil lead
Thickness of aluminium foil 10-5m
Diameter of a hair 10-4m
Diameter of a carbon atom 1.3 x 10-10m
Diameter of a nucleus 10-14m
Wavelength of blue light 10-7 m
Volume of room
Volume of a small bean 0.5 m³
Volume of water in a cup 300 cm³
Volume of water in an Olympic sized swimming pool
Volume of water in a bath
Volume of a human head 4 x 10-3m³
Mass of person 70 kg
Mass of a nail
Mass of a cruise liner
Weight of an apple 1 N
Time between heartbeats
Period of a pendulum
Frequency of light 1015 Hz
Density of water 1000 kg m-3
Electric current in a cooker element
Energy converted by a 1kW heater in 1 hour 3.6 x 106J
Force on a tennis ball when served
Acceleration of a jet plane when taking off
Walking speed 1 m s-1
Speed of a car on a motorway 30 m s-1
Kinetic energy of a cricket ball 10 J
What is the usefulness of graphs?
(i) They give an immediate visual picture of how one variable quantity varies with another, e.g. to find resistance R = V/i,
a single set of readings does not indicate that 𝑖∝𝑉
(ii) They show up random errors and reduce them by drawing the line of best fit.
(iii) Systematic errors are shown, e.g. a zero error on an ammeter.
(iv) An anomalous or doubtful point is shown up which is difficult to detect in a table of results.
(v) The limitation of a relationship is shown up, e.g. when the elastic limit is reached in a Hooke’s law graph.
(vi) The constants of the equation which relates two quantities can be found from the gradient or intercept.
(vii) For the calibrating an instrument, e.g. the deflection of a galvanometer against temperature
What is a parralax error?
This is an error in reading an instrument due to the relative movement between the object and scale marking when the object is viewed from different positions against the scale markings.
What are the general rules for combining errors?
1.Addition or subtraction
-You just add the uncertainties
2.Multiplication and division
-Add the percentage errors
3. Powers
-Multiply the percentage uncertainties by the power.
Explain the two types of errors.
- Systematic
-Consistently shifted in one direction. Usually because apparatus has a fault or poor experimental techniques like parallax error or inherent errors like heat loss air resistance background count - Random
-Scatter of readings about a mean value usually because of irreproducible results or random fluctuations or lack of sensitivity of the instrument they can be reduced by repeating the measurements several times and averaging.
