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Use of SI units and their prefixes
This topic underpins calculation, practical and data-analysis work across A-level Physics.
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90 min
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AqaA LevelPhysicsMeasurements and their errors
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SI units, prefixes and equation consistency4 objectives
- Convert between common SI prefixes and standard form.
- Use base and derived SI units correctly in physical quantities.
- Check whether equations are homogeneous by comparing units.
- Present calculated values with appropriate units and powers of ten.
Key terms
SI PrefixStandard FormHomogeneous EquationSI unitderived unithomogeneous equationhomogeneous equationsSI unitsunit comparisonstandard form
Exam tips
- Converting Kilometers to Meters: To convert kilometers to meters, use the conversion factor: 1 km = 1000 m. Multiply the number of kilometers by 1000.
- Using Standard Form for Large Numbers: When expressing large numbers, convert them to standard form. For example, 3000 can be written as 3.0 x 10^3.
Common mistakes
- Incorrect Conversion of Prefixes: To fix this, remember that 1 Mg = 10^6 g. Therefore, when converting, use the formula: 1 Mg = 1 x 10^6 g. Substituting gives: 1 Mg = 1,000,000 g.
- Misunderstanding Standard Form: To correct this, recall that standard form requires a number between 1 and 10 multiplied by a power of ten. For 0.0005 m, rewrite it as 5 x 10^-4 m, ensuring the decimal is moved correctly.
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