Physical Quantities and Units | OCR A-Level Physics A (H556)
Physical Quantities and Units
- 38 questions
- 3 subtopics
- Module 2: Foundations of physics
- Paper 1, Paper 2 and Paper 3
How physics measures things: quantities and their units, the S.I.
base and derived units, prefixes, and the check that an equation balances dimensionally.
Sample questions from Physical Quantities and Units
Answer each one closed book first, then open the answer.
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Physical quantities, estimation and unit conversions
In the expression '9.81 m s⁻²', which part is the unit?
Show the answer
m s⁻² -
Physical quantities, estimation and unit conversions
When converting cm³ to m³, why must the conversion factor be cubed?
Show the answer
Because each linear dimension is converted (1 cm = 10⁻² m), and volume involves three dimensions, so the factor becomes (10⁻²)³ = 10⁻⁶. -
S.I. base and derived units
What is the S.I. base unit for electric current?
Show the answer
Ampere (A). -
S.I. base and derived units
Give an example of how a derived unit is constructed from base units.
Show the answer
Velocity has the derived unit m s⁻¹, combining the base units metre and second. -
Prefixes, homogeneity and labelling conventions
What power of ten does the prefix milli (m) represent?
Show the answer
10⁻³ -
Prefixes, homogeneity and labelling conventions
Why is the convention 'quantity / unit' used for labelling axes and columns?
Show the answer
So that the numbers in the table or on the axis are pure numerical values, with the unit already accounted for by the heading.
The 3 subtopics
One subtopic is one session. Work down the list.
| Subtopic | What it covers | Questions |
|---|---|---|
| Physical quantities, estimation and unit conversions | Recall questions on the numerical value and unit of a physical quantity, order-of-magnitude estimates and their assumptions, and converting squared and cubed units with prefixes. | 8 |
| S.I. base and derived units | Recall questions on the base quantities and their units, and on how derived units are built from them. | 10 |
| Prefixes, homogeneity and labelling conventions | Recall questions on the prefixes from pico to tera, checking whether an equation is homogeneous and what that does not prove, and labelling graph axes and table columns as 'quantity / unit'. | 20 |
How the guide is worked
Answering a question from memory stores it far better than reading the answer again. The guide runs that as a fixed procedure on one subtopic at a time, about twenty minutes a session.
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Step 1 · Closed book
Cover the answers. Work through one subtopic and write down what you can. Leave blanks where you have nothing.
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Step 2 · Open book
Go back to the top. Read each printed answer and write it out in full, including the ones you had right.
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Step 3 · Closed book again
Same questions, same order, from memory. The gap between pass one and pass three is the session result.
Read the full method, the return schedule and the research behind it.
Nearby topics
OCR A-Level Physics A Active Recall Guide
Every topic, not just this one. 1,862 questions with their answers.