2 - Foundations of Physics Flashcards
What are SI units?
Fundamental (base) units of physical quantities.
What is the SI unit of mass?
Kilogram - kg.
What physical quantity is measured in mol?
Amount of substance.
What is the SI unit of current?
Amperes (A)
Is the SI unit for temperature ºC or K ?
K (kelvin) as this is the absolute scale
What is the SI unit of length?
Metres - m.
What quantity is measured in seconds?
Time
Are Newtons (N) an SI unit?
No, Newtons are not fundamental, the SI units for force are kg m s⁻² (since F = ma).
Derive the SI units of energy.
Kinetic energy = ½ x mass x velocity²
Units = kg x (m/s)² = kgm²s⁻²
Derive the SI units of force.
Force = mass x acceleration
Units = kg x ms⁻²
= kg ms⁻²
Express 60TΩ in standard form.
6 x 10¹³
(T is tera and the multiplier is 1012)
Write 0.000003m with a suitable prefix.
3µm (3 x 10⁻⁶m)
What is the actual value of 8MΩ?
8,000,000Ω or 8 x 10⁶Ω
What is 6000pF in nF?
6nF (since 1 nF = 1000 pF)
What multiplier is associated with the prefix kilo (k)?
1000 (10³)
What multiplier is associated with the prefix femto (f)?
10⁻¹⁵
Express 7GΩ in standard form.
7 x 10⁹ Ω
What is accuracy?
● Accuracy is a measure of how close a measured value is to the true, accepted value.
● If a experiment result is accurate it is very close to the true value.
● In practice, the true value usually is not known.
What is validity?
A measurement is valid if it measures what it is supposed to be measuring. An experimental procedure is valid if the obtained values provide an answer to the question that is being asked.
What is a random error?
An error that occurs due to unexpected changes during an experiment. These cannot be predicted and they can cause repeated results to differ from one another.
What is an anomaly?
An anomaly (also known as an outlier) is a data point or a value in a set of results that does not fit the trend of the data and is therefore an unexpected result. These can happen as a result of random errors during an experiment.
What is a systematic error?
An error that occurs due to faults in equipment or experimental method. Systematic errors cause the result to differ by the same amount each time, making them predictable. They can occur due to not calibrating an instrument correctly
State three ways of reducing random errors.
● Take at least three repeats and calculate a mean (this increases the likelihood of identifying anomalies).
● Use computers/data loggers.
● Use higher resolution equipment.
A mass balance reads 1004 g when a 1kg mass is placed on it. Is this a random or systematic error?
Systematic, as the reading is too high by 4g each time.
How can systematic error be reduced?
Calibrate apparatus before using e.g. zero the balance when it is empty.
Is electronic noise in the circuit of an ammeter random error or systematic error?
Random error, as it will cause fluctuations in readings that affect precision and it cannot be removed.
Why should you measure background radiation before measuring the radioactivity of a source?
So that only the source’s radioactivity is measured - by accounting for background radiation, systematic error can be reduced.
What is precision?
How consistent/close together repeat readings are - the closer they are, the more precise they are.
What makes an experiment repeatable?
If the original experimenter can redo the experiment with the same equipment and method then get the same results, the experiment is repeatable.
What makes an experiment reproducible?
If the experiment can be redone by a different person or with different techniques and equipment and the same results are found, it is reproducible.
What is meant by resolution?
The smallest change in the quantity being measured that gives a recognisable change in reading (e.g. on a ruler, the resolution is 1mm).
What is absolute uncertainty?
Uncertainty given as a fixed quantity.
E.g. 7 ± 0.6 V
What is the percentage uncertainty in 17 ± 3 A ?
3/17 x 100 = 17.647 %
≈ 18 % (2 s.f.)
What is the fractional uncertainty in 8 ± 0.5 m?
0.5/8 = 1/16
How can percentage and fractional uncertainty be reduced?
Measure larger quantities. For example, a longer rope will have a smaller percentage uncertainty than a shorter one.
The time for 10 swings of a pendulum is 13 ± 0.3 s. What is the time and uncertainty for one swing?
1 swing = 13/10 = 1.3s
Uncertainty = 0.3/10 = 0.03 s
Time = 1.3 ± 0.03 s
What is the difference between a reading and a measurement?
Readings are when one value is found, measurements are when the difference between two readings is found.
What is the uncertainty of a thermometer whose smallest division is 5ºC?
The uncertainty in a reading is ± half the smallest division, so the uncertainty is ± 5/2 or ±2.5 ºC.
What is the percentage uncertainty in the measurement of a 2cm line?
● Each end has uncertainty ±0.5mm
● 0.5 + 0.5 = 1
● so the absolute uncertainty = ±1mm
● % uncertainty = 1/20 x 100 = 5%
● (2cm is 20mm)
What is the uncertainty in the charge of an electron (1.6 x 10-19C)?
The uncertainty in a given value is ± the last significant digit
= 1.6 x 10-19 ± 0.1 x 10-19 C
The times for a ball to drop are measured as 3.2s, 3.6s and 3.1s. Find the mean and absolute uncertainty of these times.
Mean : (3.2 + 3.6 + 3.1) / 3 = 3.3 s
Uncertainty = half the range (3.6 - 3.1) / 2 = 0.25 (≈ 0.3 to 1 s.f.)
3.3 ± 0.3s
What is wrong with writing 7 ± 0.673 V?
The uncertainty should be the same number of significant figures as the data (ie. 7 ± 0.7 V)
A thermometer with an uncertainty of 0.5 K shows the temperature of water falling from 298 ± 0.5 K to 273 ± 0.5K. What is the difference in temperature and the uncertainty in this difference?
298 - 273 = 25K 0.5 + 0.5 = 1K
(when adding or subtracting data, add the absolute uncertainties)
Difference = 25 ± 1 K
A force of 91 ± 3 N is applied to a mass of 7 ± 0.2 kg. What is the acceleration of the mass?
When multiplying/dividing data add percentage uncertainties
a = F / m = 91 / 7 = 13 m s-2
% uncertainty = ((0.2 / 7) x 100)+ ((3 / 91) x 100)= 6.2%
a = 13 ± 6.2%
What is an order of magnitude?
Powers of ten which describe the size of an object, and which can also be used to compare the sizes of objects.
What is the order of magnitude for the diameter of a nucleus?
10⁻¹⁵ (femtometres)
What is 9.71x10⁻²¹ to the nearest order of magnitude?
It is 1x10⁻²⁰ to 1s.f.
So the nearest order of magnitude is 10⁻²⁰
What is meant by a scalar quantity?
A quantity that has only magnitude.
What is a vector quantity?
A quantity that has magnitude as well as direction.
Is acceleration a vector or scalar quantity?
Vector
Is mass a scalar or vector quantity?
Scalar
Calculate the magnitude of a force F that is 30º to the horizontal and has a horizontal component of 7.0N.
Fhorizontal = Fcosθ
7.0 = Fcos30º
F = 7.0 / cos30º = 8.1N
