About General Science
General science at this level is three lists: the SI unit of each quantity, what each measuring instrument measures, and who is credited with each discovery. The questions are factual, the answers are settled, and the value of the topic is that the same facts recur in almost every competitive paper.
What you need to understand
- The seven base SI units are the metre for length, the kilogram for mass, the second for time, the ampere for electric current, the kelvin for thermodynamic temperature, the mole for amount of substance and the candela for luminous intensity.
- Derived units are built from the base ones. Velocity is metre per second, acceleration is metre per second squared, density is kilogram per cubic metre, and momentum is kilogram metre per second.
- Units named after people are worth learning with the person: newton for force, joule for energy, watt for power, pascal for pressure, volt for potential difference, ohm for resistance, farad for capacitance and henry for inductance.
- The units connected with electricity are the ampere, the coulomb, the volt, the ohm, the farad, the siemens and the henry, and they are best learned as a family.
- The units connected with radiation are the becquerel for activity, the gray for absorbed dose and the sievert for equivalent dose. The last two are the ones most often confused.
- The units connected with light are the candela for intensity, the lumen for flux and the lux for illuminance.
- Two units that look as though they should be the same are not: the plane angle is measured in radians and the solid angle in steradians.
- Measuring instruments are best learned by what they physically act on. A barometer measures the pressure of the atmosphere, a manometer the pressure of a gas, and an altimeter height - which is a pressure measurement in disguise.
- Instruments that are easily confused: the hygrometer measures humidity and the hydrometer the density of a liquid; the ammeter measures current and the voltmeter potential difference.
- Medical instruments are a small family - the stethoscope for sounds in the body, the sphygmomanometer for blood pressure, the electrocardiograph for the electrical activity of the heart, the audiometer for hearing and the lactometer for the purity of milk.
- Discoveries are best grouped by century. The seventeenth gives Newton and Harvey, the eighteenth Priestley and Lavoisier, the nineteenth Dalton, Mendeleev, Pasteur, Koch and Mendel, and the twentieth Fleming, Curie, Rutherford, Bohr and Watson and Crick.
- Where credit is shared, the standard reference works give the pair, and the questions follow the reference works rather than the laboratory notebooks.
- Nothing in this topic changes. There are no current records and no recent prizes, so a question answered correctly today stays correct.
How to work through these questions
- Learn the units in families - mechanics, electricity, light, radiation - rather than in one alphabetical list.
- For a question asking what an instrument measures, start from what the instrument physically does. A barometer balances the weight of the atmosphere, and that is what it measures.
- Where two units are close, such as the gray and the sievert, remember which is the physical dose and which is the dose weighted for its effect.
- For discoveries, learn the person with the single thing they are known for, and put them in a century. Ordering by century resolves most of the confusion.
- Watch the direction of the question. One asks for the unit of a quantity and another asks what a unit measures, and the same pair of facts answers both.
- Where a question asks about a unit named after a person, the person is often a clue: the volt goes with Volta, the joule with Joule, the watt with Watt.
Mistakes that cost marks
- Confusing the gray, the unit of absorbed dose, with the sievert, the unit of equivalent dose.
- Confusing the becquerel, which measures activity, with the curie, which is not an SI unit.
- Mixing up the hygrometer, which measures humidity, with the hydrometer, which measures the density of a liquid.
- Answering that a barometer measures temperature, which is the thermometer's work.
- Giving the lumen for illuminance; the illum inance is measured in lux and the flux in lumen.
- Forgetting that the kelvin is the SI unit of temperature, the degree Celsius being a scale derived from it.
- Crediting the discovery of radioactivity to Marie Curie alone, when Becquerel discovered it and the Curies pursued it.
- Mixing up the discovery of the electron, which is Thomson's, with the discovery of the neutron, which is Chadwick's.
Worked example
Which of these is the SI unit of force?
- Force is a mechanical quantity, so the answer is one of the units of mechanics rather than of electricity or light.
- The joule is the unit of energy and the watt of power, both of which are mechanical but not force.
- The pascal is the unit of pressure, which is force per unit area and not force itself.
- What remains is the newton, which is defined as the force that gives a mass of one kilogram an acceleration of one metre per second squared.
Answer: newton
Practice questions with answers
A few General Science questions with the full solution shown, so you can see
how the method is applied before you attempt the timed set.
Question 1
Which of these quantities is measured in henry?
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A
electrical resistance
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B
thermal conductivity
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C
absorbed dose of ionising radiation
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D
inductance
Answer: Option D — with explanation
The SI unit of inductance is the henry.
The units are worth learning in families - the ones named after Newton, Joule, Watt and Volta sit in mechanics and electricity, while the ones named after Becquerel, Gray and Sievert are all to do with radiation. Instruments are best learned by what they physically measure, and discoveries by the century in which they were made.
Common mistakes
- this is measured in another unit: electrical resistance
- this is measured in another unit: thermal conductivity
- this is measured in another unit: absorbed dose of ionising radiation
Question 2
Which of these quantities is measured in tesla?
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A
momentum
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B
magnetic flux density
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C
electric field strength
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D
surface tension
Answer: Option B — with explanation
The SI unit of magnetic flux density is the tesla.
The units are worth learning in families - the ones named after Newton, Joule, Watt and Volta sit in mechanics and electricity, while the ones named after Becquerel, Gray and Sievert are all to do with radiation. Instruments are best learned by what they physically measure, and discoveries by the century in which they were made.
Common mistakes
- this is measured in another unit: surface tension
- this is measured in another unit: electric field strength
- this is measured in another unit: momentum
Question 3
metre is the SI unit of which of the following quantities?
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A
radioactive activity
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B
electric current
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C
length
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D
equivalent dose of ionising radiation
Answer: Option C — with explanation
The SI unit of length is the metre.
The units are worth learning in families - the ones named after Newton, Joule, Watt and Volta sit in mechanics and electricity, while the ones named after Becquerel, Gray and Sievert are all to do with radiation. Instruments are best learned by what they physically measure, and discoveries by the century in which they were made.
Common mistakes
- this is measured in another unit: radioactive activity
- this is measured in another unit: equivalent dose of ionising radiation
- this is measured in another unit: electric current
Question 4
siemens is the SI unit of which of the following quantities?
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A
electric charge
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B
electrical conductance
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C
momentum
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D
solid angle
Answer: Option B — with explanation
The SI unit of electrical conductance is the siemens.
The units are worth learning in families - the ones named after Newton, Joule, Watt and Volta sit in mechanics and electricity, while the ones named after Becquerel, Gray and Sievert are all to do with radiation. Instruments are best learned by what they physically measure, and discoveries by the century in which they were made.
Common mistakes
- this is measured in another unit: momentum
- this is measured in another unit: electric charge
- this is measured in another unit: solid angle
Question 5
The anemometer is used to measure which of the following?
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A
speed of the wind
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B
humidity of the air
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C
very small electric currents
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D
earthquakes
Answer: Option A — with explanation
The anemometer is used to measure speed of the wind.
The units are worth learning in families - the ones named after Newton, Joule, Watt and Volta sit in mechanics and electricity, while the ones named after Becquerel, Gray and Sievert are all to do with radiation. Instruments are best learned by what they physically measure, and discoveries by the century in which they were made.
Common mistakes
- this is measured by another instrument: humidity of the air
- this is measured by another instrument: very small electric currents
- this is measured by another instrument: earthquakes
Frequently asked questions
How many units do I need to know?
About forty quantities and their units cover what competitive papers ask, and they fall into four families: mechanics, electricity, light and radiation. The base units of the seven quantities are worth knowing exactly.
Why are some units not SI units?
The curie, the calorie, the atmosphere and the millimetre of mercury are all still in use in particular fields, and some questions use them. Where a question asks for the SI unit, the others are the distractors.
Is it worth learning who discovered what?
Yes. The same few dozen names reappear, and they are usually asked as a straightforward pairing of a discovery with a person.
What is the difference between the gray and the sievert?
The gray measures the energy absorbed per unit mass. The sievert multiplies that by a factor for the kind of radiation, so it measures the biological effect. A dose in gray and a dose in sievert are the same only for X-rays and gamma rays.
Where does this topic appear in a paper?
In the general science sections of bank, SSC and railway papers, and in the general studies papers of state examinations. The units and the instruments are the parts that recur most.
