Physics Level 4 flashcards

33 cards. Read the term and the definition here. The deck at the top of this page is optional.

SI base units

The seven base units are kilogram, metre, second, ampere, kelvin, mole and candela. Every derived unit, such as the newton or watt, reduces to a combination of these.

Dimensional consistency

Both sides of a valid equation must reduce to identical base-unit combinations. Checking dimensions catches algebraic slips before numbers are substituted and can reveal missing factors.

Scalar versus vector

A scalar has magnitude only, such as mass or energy. A vector has magnitude and direction, such as force or velocity, and must be combined by resolving into perpendicular components.

Equations of motion

For constant acceleration: v = u + at, s = ut + at squared over two, and v squared = u squared + 2as. Select the one containing the known quantities and the required unknown.

Newton's second law

The resultant force on a body equals the rate of change of its momentum, F = ma for constant mass. Resolve forces along each axis and apply the law separately.

Conservation of momentum

In any collision or explosion with no external resultant force, total momentum before equals total momentum after. This holds whether the collision is elastic or inelastic.

Elastic collision test

Kinetic energy is conserved only in elastic collisions. After applying momentum conservation, compare total kinetic energy before and after to classify the collision.

Centripetal acceleration

A body moving in a circle at constant speed accelerates toward the centre with magnitude v squared over r, or omega squared times r. The force causing it is a real force such as tension.

Work-energy principle

The net work done on a body equals its change in kinetic energy. Work done against friction transfers energy to thermal stores and must be included in energy balances.

Hooke's law energy

For a spring obeying F = kx, the stored elastic potential energy is half k x squared. This energy is recovered only if the deformation remains within the elastic limit.

Wave equation

Wave speed equals frequency multiplied by wavelength, v = f lambda. Frequency is set by the source and is unchanged when a wave crosses into a different medium; speed and wavelength change.

Diffraction grating equation

Principal maxima occur where d sin(theta) = n lambda, with d the slit spacing and n the order. Larger wavelength or smaller spacing spreads the pattern more widely.

Ohm's law conditions

V = IR applies to an ohmic conductor at constant temperature. Filament lamps and diodes are non-ohmic, so their resistance changes with current or with the direction of flow.

Resistors in combination

Series resistances add directly. For parallel resistors the reciprocals add, so the combined resistance is always smaller than the smallest individual resistance in the network.

Electrical power formulas

Power dissipated is P = VI, equivalently I squared R or V squared over R. The form chosen depends on which quantities are fixed: current in series, voltage in parallel.

Significant figures rule

Report a calculated result to the same number of significant figures as the least precise input. Keep extra digits through intermediate steps and round only at the end.

Instrument resolution

Resolution is the smallest change an instrument can display. For an analogue scale the reading uncertainty is usually half a division; for a digital meter it is the last digit.

Percentage uncertainty

Divide the absolute uncertainty by the measured value and multiply by 100. It expresses how large the uncertainty is relative to the size of the quantity measured.

Combining uncertainties

For addition and subtraction, add absolute uncertainties. For multiplication and division, add percentage uncertainties. For a quantity raised to a power, multiply its percentage uncertainty by that power.

Uncertainty from repeats

Repeat a reading several times and take the spread. The uncertainty may be taken as half the range of results, or as the standard deviation of the mean for a larger data set.

Gradient uncertainty

Plot error bars on both axes, draw the best-fit line, then draw the steepest and shallowest lines that still fit the bars. Half their gradient difference is the uncertainty.

Linearising a graph

Rearrange the theoretical equation into the form y = mx + c so a straight-line graph tests the relationship. The gradient and intercept then yield the physical constants of interest.

Systematic error

A systematic error shifts every reading in the same direction, as with a zero error or mis-calibrated instrument. Repeating measurements does not reduce it; the apparatus must be corrected.

Random error

Random errors scatter readings unpredictably above and below the true value, caused by fluctuations in conditions or judgement. Averaging repeated readings reduces their effect on the mean.

Accuracy versus precision

Accuracy is closeness to the true value; precision is the closeness of repeated readings to each other. A tightly clustered set can still be inaccurate if a systematic error exists.

Parallax error

A reading error caused by viewing a scale or meniscus from an angle rather than straight on. Positioning the eye level with the mark, or using a mirror scale, avoids it.

Zero error

A reading offset present when the instrument should indicate zero, such as a micrometer screw gauge not closing at zero. It is a systematic error removed by subtracting the offset.

Control variables

Quantities deliberately held constant during an experiment so that only the independent variable affects the dependent variable. They must be identified and monitored, not merely assumed constant.

Valid conclusion

A conclusion states the pattern found, quantifies it with gradient or constant values including uncertainties, and compares it with the accepted value while noting sources of error.

Percentage difference

Compare an experimental value with an accepted value by dividing their difference by the accepted value and multiplying by 100. State whether the discrepancy lies within the uncertainty.

Anomalous reading

A data point that lies far outside the trend of the other readings, usually from a procedural slip. It should be identified, excluded from analysis, and the exclusion justified.

Calibration check

Verify an instrument against a known standard before use, for example a known mass on a balance or ice point on a thermometer. It detects and quantifies systematic offset.

Safety assessment

Identify hazards such as hot masses, falling weights, mains electricity near water, or laser light, and state a specific control for each before starting the practical activity.