Chapter 1: Physical Quantities and Units Flashcards
What is a physical quantity?
All physical quantities consist of a numerical magnitude and a unit (in which it was measured).
Mark scheme:
Something that can be measured.
Represented by an alphabetical letter.
Contains a numerical value and a unit.
Estimate the size of the diameter of an atom.
10^-10m
Estimative the wavelength of UV radiation.
10nm
Estimate the height of an adult human.
2m
Estimate the distance between the Earth and the Sun (1AU).
1.5 * 10^11m
Estimate the mass of a hydrogen atom.
10^-27kg
Estimative the mass of an adult human.
70kg
Estimative the mass of a car.
1000kg
Estimate the number of seconds in a day.
90000s
Estimate the number of seconds in a year.
3*10^7s
Estimate the speed of sound in air.
300 m/s
Estimate the power of a light bulb.
60W
Estimate atmospheric pressure.
1*10^5Pa
Estimate the height of a staircase.
3m
Recall the equation for gain in gravitational potential energy.
Ep = mgh
State the 7 SI base units (only first 5 needed for exam).
Mass - kilogram
Length - metres
Time - seconds
Current - amperes
Temperature - kelvin
Amount of substance - mole
Luminosity - candela
Force (newton) formula
Force = mass * acceleration
Kinetic energy formula
Energy = 1/2 * mass * velocity^2
Pressure (pascal) formula
Pressure = force / area
What is a homogeneous physical equations?
When the units on both sides of an equation are equal.
Estimate the width of a galaxy.
10^21m
State the prefixes of powers of ten.
Tera - 10^12
Giga - 10^9
Mega - 10^6
Kilo - 10^3
Deci - 10^-1
Centi - 10^-2
Milli - 10^-3
Micro - 10^-6
Nano - 10^-9
Pico - 10^-12
Explain what is meant by uncertainty.
The uncertainty is an estimate of the difference between a measurement reading and the true value.
Explain the two types of measurement errors which lead to uncertainty.
Random errors:
Cause unpredictable fluctuations in an instruments readings as a result of uncontrollable factors, such as environmental conditions.
Affects precision.
To reduce random errors: repeat measurements several times and calculate an average from them.
Systematic errors:
Arise from the use of faulty instruments or from flaws in the experimental method.
Affects accuracy of all readings obtained.
To reduce systematic errors: instruments should be recalibrated or the technique being used should be corrected or adjusted.
State what is meant by zero error.
Zero error is a type of systematic errors which occurs when an instruments readings as gives a non-zero reading when the true reading is actually zero. This introduces a fixed error into readings which must be accounted for when the results are recorded.
State the difference between precision and accuracy.
The precision of a measurement is how close the measured values are to each other; if a measurement is repeated several times, then it can be described as precise when the values are very similar to, or the same as, each other.
The accuracy of a measurement is how close a measured value is to the true value; the accuracy can be increased by repeating measurements and finding a mean average.
Precision vs accuracy or resolution
A single reading cannot be precise - if something is measured to a high number of decimal points, it is a measurement with high resolution.
State the 3 ways uncertainties can be represented and explain each.
Absolute Uncertainty: where uncertainty is given as a fixed quantity (as above).
Fractional Uncertainty: where uncertainty is given as a fraction of the measurement.
Percentage Uncertainty: where uncertainty is given as a percentage of the measurement.
State the formula used to calculate percentage uncertainty.
(Absolute uncertainty/measured value) * 100
Explain how to calculate uncertainties in different situations.
To find uncertainties in different situations:
The uncertainty in a reading (e.g. from a voltmeter):± half the smallest division
The uncertainty in a measurement (e.g. from a ruler): at least ±1 smallest division
The uncertainty in repeated data: half the range i.e. ± ½ (largest - smallest value)
The uncertainty in digital readings: ± the last significant digit unless otherwise quoted
How to combine uncertainties?
When adding or subtracting two values with uncertainties, just add the absolute uncertainties.
When multiplying or dividing measurements with uncertainties, just add the percentage uncertainties.
When the measurement is raised to a power, multiply the fractional or percentage uncertainty by the power.
Scalar vs Vector quantities difference.
Scalar quantities have magnitude but not direction, and vector quantities have both magnitude and direction.
State what is meant by displacement.
Displacement is a measure of how far it is between two points in space, including the direction. It is the length and direction of a straight line drawn from the starting point to the finishing point. It is a vector quantity.
State common examples of scalar and vector quantities.
Scalar: distance, speed, mass
Extra Scalar: volume, energy, temperature, time, electric charge, density (VETTED)
Vector: displacement, velocity, weight
Extra Vector: force, acceleration, momentum (FAM)
Explain 2 methods of combining vectors.
Triangle method:
Step 1: link the vectors head-to-tail.
Step 2: the resultant vector is formed by connecting the tail of the first vector to the head of the second vector
To subtract vectors, change the direction of the vector from positive to negative and add them in the same way.
Parallelogram method
Step 1: link the vectors tail-to-tail
Step 2: complete the resulting parallelogram
Step 3: the resultant vector is the diagonal of the parallelogram
State what is meant by coplanar forces.
Forces which act in the same plane.
State the condition for equilibrium.
Closed vector triangles.
Horizontal and Vertical vector components.
For the horizontal component, Fx = Fcosθ
For the vertical component, Fy = Fsinθ
Range of visible light
400 - 700 nm
Estimate the young modulus of a metal
50 - 400 GPa
Estimate the diameter of a hydrogen nucleus (1fm).
1*10^-15m
State two possible physical quantities represented by the Greek letter rho.
Density and resistivity
State three possible physical quantities represented by the capital letter P or the lower case letter p.
Pressure, power, and momentum
Estimate the work done by the average human in lifting a normal dining room chair off the ground, so the forearms of the person lifting are parallel with the floor at the end of the lift.
Average height if human: 2m
Average lift height of a human to bent elbows: 2/3* height
Average mass of a dining room chair: 20kg
Work=forcedistance=1020(2/3)2=266,667 Joules
What is the formula for specific heat capacity?
Q =mcΔT
c=4200 J^-1K^-1 of water
Joules in SI base units
kg m^2 s^-2
State the Stephan-Boltzmann constant
5.67*10^-8 W m^-2 K^-4
Watts in SI base units
Energy/time
J s^-1 = kg m^2 s^-3
How many metres is 1 light year?
1 light year = c * 1 year = 3*10^8 * 60 * 60 * 24 * 365=9,461×10¹⁵
Define the term physical quantity.
A physical property that can be measured and quantified.
State wave speed equation.
c=f λ
Latent heat formula
Energy/mass
J/kg
Pa are units for what? (3 things)
Pressure, stress and young modulus
Wavelength of green light
550nm
State the definition of electron volt.
The definition of an electron volt is the energy that an electron gains when it travels through a potential of 1 volt.
1 eV is equal to 1.6*10^-19 J
State potential difference in its SI base units.
P.D. = work done/charge = J/C^-1
J = kg m^2s^-2
Q=It = As
P.D= kg m^2 s^-3 A^-1
Estimate the density of air.
1.3 kg/m^3
Convert the following measurement: 42km^3 into mm^3
4.2*10^19 mm^3
Convert the following measurement:
25 light years into nm
2.3652*10^26
Convert 0.05 kWh to PeV
1.1*10^9 PeV
State the Boltzmann constant
1.38*10^-23 J K^-1
State the mass of a proton
1.67*10^-27 kg
State the energy-mass equation
Δ E = Δ mc^2
State the equation for young modulus.
Force/area * length/extension
= (loadlength)/(pi/4diameter^2*extension)
State the minimum and maximum length vernier callipers and micro meters can read.
Micrometer: 0.01mm to 2.5 cm
Vernier calliper: 0.1mm to 15cm
State what is meant by resolution of an instrument.
The smallest observable change in reading on the instrument.
