2 - Foundations of Physics

Question 1

What are SI units?

Answer 1

Fundamental (base) units of physical quantities.

Question 2

What is the SI unit of mass?

Answer 2

Kilogram - kg.

Question 3

What physical quantity is measured in mol?

Answer 3

Amount of substance.

Question 4

What is the SI unit of current?

Answer 4

Amperes (A)

Question 5

Is the SI unit for temperature ºC or K ?

Answer 5

K (kelvin) as this is the absolute scale

Question 6

What is the SI unit of length?

Answer 6

Metres - m.

Question 7

What quantity is measured in seconds?

Answer 7

Time

Question 8

Are Newtons (N) an SI unit?

Answer 8

No, Newtons are not fundamental, the SI units for force are kg m s⁻² (since F = ma).

Question 9

Derive the SI units of energy.

Answer 9

Kinetic energy = ½ x mass x velocity²
Units = kg x (m/s)² = kgm²s⁻²

Question 10

Derive the SI units of force.

Answer 10

Force = mass x acceleration
Units = kg x ms⁻²
= kg ms⁻²

Question 11

Express 60TΩ in standard form.

Answer 11

6 x 10¹³

(T is tera and the multiplier is 1012)

Question 12

Write 0.000003m with a suitable prefix.

Answer 12

3µm (3 x 10⁻⁶m)

Question 13

What is the actual value of 8MΩ?

Answer 13

8,000,000Ω or 8 x 10⁶Ω

Question 14

What is 6000pF in nF?

Answer 14

6nF (since 1 nF = 1000 pF)

Question 15

What multiplier is associated with the prefix kilo (k)?

Answer 15

1000 (10³)

Question 16

What multiplier is associated with the prefix femto (f)?

Answer 16

10⁻¹⁵

Question 17

Express 7GΩ in standard form.

Answer 17

7 x 10⁹ Ω

Question 18

What is accuracy?

Answer 18

● 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.

Question 19

What is validity?

Answer 19

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.

Question 20

What is a random error?

Answer 20

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.

Question 21

What is an anomaly?

Answer 21

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.

Question 22

What is a systematic error?

Answer 22

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

Question 23

State three ways of reducing random errors.

Answer 23

● Take at least three repeats and calculate a mean (this increases the likelihood of identifying anomalies).
● Use computers/data loggers.
● Use higher resolution equipment.

Question 24

A mass balance reads 1004 g when a 1kg mass is placed on it. Is this a random or systematic error?

Answer 24

Systematic, as the reading is too high by 4g each time.

Question 25

How can systematic error be reduced?

Answer 25

Calibrate apparatus before using e.g. zero the balance when it is empty.

Question 26

Is electronic noise in the circuit of an ammeter random error or systematic error?

Answer 26

Random error, as it will cause fluctuations in readings that affect precision and it cannot be removed.

Question 27

Why should you measure background radiation before measuring the radioactivity of a source?

Answer 27

So that only the source’s radioactivity is measured - by accounting for background radiation, systematic error can be reduced.

Question 28

What is precision?

Answer 28

How consistent/close together repeat readings are - the closer they are, the more precise they are.

Question 29

What makes an experiment repeatable?

Answer 29

If the original experimenter can redo the experiment with the same equipment and method then get the same results, the experiment is repeatable.

Question 30

What makes an experiment reproducible?

Answer 30

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.

Question 31

What is meant by resolution?

Answer 31

The smallest change in the quantity being measured that gives a recognisable change in reading (e.g. on a ruler, the resolution is 1mm).

Question 32

What is absolute uncertainty?

Answer 32

Uncertainty given as a fixed quantity.
E.g. 7 ± 0.6 V

Question 33

What is the percentage uncertainty in 17 ± 3 A ?

Answer 33

3/17 x 100 = 17.647 %
≈ 18 % (2 s.f.)

Question 34

What is the fractional uncertainty in 8 ± 0.5 m?

Answer 34

0.5/8 = 1/16

Question 35

How can percentage and fractional uncertainty be reduced?

Answer 35

Measure larger quantities. For example, a longer rope will have a smaller percentage uncertainty than a shorter one.

Question 36

The time for 10 swings of a pendulum is 13 ± 0.3 s. What is the time and uncertainty for one swing?

Answer 36

1 swing = 13/10 = 1.3s
Uncertainty = 0.3/10 = 0.03 s
Time = 1.3 ± 0.03 s

Question 37

What is the difference between a reading and a measurement?

Answer 37

Readings are when one value is found, measurements are when the difference between two readings is found.

Question 38

What is the uncertainty of a thermometer whose smallest division is 5ºC?

Answer 38

The uncertainty in a reading is ± half the smallest division, so the uncertainty is ± 5/2 or ±2.5 ºC.

Question 39

What is the percentage uncertainty in the measurement of a 2cm line?

Answer 39

● 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)

Question 40

What is the uncertainty in the charge of an electron (1.6 x 10-19C)?

Answer 40

The uncertainty in a given value is ± the last significant digit
= 1.6 x 10-19 ± 0.1 x 10-19 C

Question 41

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.

Answer 41

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

Question 42

What is wrong with writing 7 ± 0.673 V?

Answer 42

The uncertainty should be the same number of significant figures as the data (ie. 7 ± 0.7 V)

Question 43

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?

Answer 43

298 - 273 = 25K 0.5 + 0.5 = 1K
(when adding or subtracting data, add the absolute uncertainties)
Difference = 25 ± 1 K

Question 44

A force of 91 ± 3 N is applied to a mass of 7 ± 0.2 kg. What is the acceleration of the mass?

Answer 44

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%

Question 45

What is an order of magnitude?

Answer 45

Powers of ten which describe the size of an object, and which can also be used to compare the sizes of objects.

Question 46

What is the order of magnitude for the diameter of a nucleus?

Answer 46

10⁻¹⁵ (femtometres)

Question 47

What is 9.71x10⁻²¹ to the nearest order of magnitude?

Answer 47

It is 1x10⁻²⁰ to 1s.f.
So the nearest order of magnitude is 10⁻²⁰

Question 48

What is meant by a scalar quantity?

Answer 48

A quantity that has only magnitude.

Question 49

What is a vector quantity?

Answer 49

A quantity that has magnitude as well as direction.

Question 50

Is acceleration a vector or scalar quantity?

Answer 50

Vector

Question 51

Is mass a scalar or vector quantity?

Answer 51

Scalar

Question 52

Calculate the magnitude of a force F that is 30º to the horizontal and has a horizontal component of 7.0N.

Answer 52

Fhorizontal = Fcosθ
7.0 = Fcos30º
F = 7.0 / cos30º = 8.1N