physcial quantaties and units 1 Flashcards
What is a physical quantity
A physical quantity consists of a numerical value and a unit. For example the length of an object, l, has a magnitude of 4 and a unit, metres (m).
What is estimation
Estimation is a skill physicists must use in order to approximate the values of physical quantities, in order to make comparisons, or to check if a value they’ve calculated is reasonable.
What are SI units
SI units are thefundamental units which are used alongside the base SI quantities.
what are SI units made up of:
Quantity - Unit Mass: Kilogram (kg) Length: Metre (m) Time: Second (s) Current: Ampere (A) Temperature: Kelvin (K)
how can the SI units of quantities be derived
The SI units of quantities can be derived using their equation, e.g. F = ma.
For example, to find the SI units of force (F) multiply the SI units of mass and acceleration: kg x ms^-2 =kgms^-2Which is the SI unit of force, also known as N.
What is a homogenous equation
A homogeneous equation is one where the units on either side are the same. All equations in physics (which are valid) are homogeneous, which is why you can derive a quantity’s SI units as above. You can assess whether an equation is homogeneous by checking whether the units on either side are equal.
what are prefixes
Prefixes can be added to SI units and they will act as a multiplier, which is a different power of 10 depending on the prefix.
Prefix name symbol and multipliers
Name Symbol Multiplier Tera T 10^12 Giga G 109^9 Mega M 106^6 Kilo k 10^3 Deci d 10^−1 Centi c 10^−2 Milli m 10^−3 Micro μ 10^−6 Nano n 10^−9 Pico p 10^−12
what do random errors affect
Random errors affect precision, meaning they cause differences in measurements which causes a spread about the mean. You cannot get rid of all random errors. An example of random error is electronic noise in the circuit of an electrical instrument.
To reduce random errors:
take at least 3 repeats and calculate a mean, this method also allows anomalies to be identified
Use computers/data loggers/cameras to reduce human error and enable smaller intervals
Use appropriate equipment, e.g a micrometer has higher resolution (0.1 mm) than a ruler(1 mm)
What are systematic errors
Systematic errors affect accuracy and occur due to the apparatus or faults in the experimental method. Systematic errors cause all results to be too high or too low by the same amounteach time. An example is a balance that isn’t zeroed correctly (zero error) or reading a scale at a different angle (this is a parallax error)``
To reduce systematic error:
`Calibrate apparatus by measuring a known value (e.g. weigh 1 kg on a mass balance), if the reading is inaccurate then the systematic error is easily identified
In radiation experiments correct for background radiation by measuring it beforehand and subtracting it from the final results
Read the meniscus (the central curve on the surface of a liquid) at eye level (to reduce parallax error) and use controls in experiments
Precision vs accuracy
Precision: Precise measurements are consistent, fluctuate slightly about a mean value, how close results are to each other - doesn’t indicate the value is accurate.
Accuracy: A measurement close to the true value is accurate
What is the uncertainty of a measurment
The uncertainty of a measurement is the bounds in which the accurate value can be expected to lie e.g. 20°C ± 2°C , the true value could be within 18-22°C.
Absolute Uncertainty:
Fractional Uncertainty:
Percentage Uncertainty:
Absolute Uncertainty: uncertainty given as a fixed quantity e.g. 7.0 ±0.6 V
Fractional Uncertainty: uncertainty as a fraction of the measurement e.g. 7.0 ±335 V
Percentage Uncertainty: uncertainty as a percentage of the measurement e.g. 7.0 ±8.6% V
