uncertainly

practical physics

how are headings for a table written

quantity/unit, or in special cases:

log(quantity/unit)

leftmost column for independent variable, each following column for each following dependent variable

SI units

kg, m, s, mol, K, A

pico

x10⁻¹²

femto

x10⁻⁻¹⁵

kWh to joules

1kWh = 3.6MJ

reducing random errors

Take at least 3 repeats and calculate a mean

Use computers/data loggers/cameras to reduce human error and enable smaller intervals

Use appropriate equipment, e.g a micrometer (0.1 mm resolution) vs a ruler (1 mm)

what do random errors affect

precision, spread about the mean

how to avoid systematic errors

Calibrate apparatus by measuring a known value (e.g. weigh 1 kg on a mass balance)

in radiation experiments correct for background radiation by measuring it beforehand and excluding it from 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

what do systematic errors affect

accuracy, the closeness of the measurement to the real value

define precision

the closeness to the mean value

doesnt indicate accuracy

how to reduce uncertainty

absolute uncertainty can never be changed

for percentage and fractional uncertainty measure larger quantities

reading vs measurement

reading is one value, measurement is difference the between two

uncertainty in a reading

± half smallest division

uncertainty in a measurement

± smallest division

uncertainty in repeated data

mean ± half range

Combining uncertainties

adding/subtracting data - ADD ABSOLUTE UNCERTAINTIES

Multiplying/dividing data- ADD PERCENTAGE UNCERTAINTIES

Raising to a power - MULTIPLY PERCENTAGE UNCERTAINTY BY POWER

percentage uncertainty

uncertainty/value x 100

percentage uncertainty in gradient or y intercept

|best - worst|/best x 100%

Variables

quantities that have potential to change

Independent variable

the variable changed in an experiment

Dependent variable

variable that is measured in an experiment

Control variables

variables that are kept constant in an experiment