Energy

Traps in Energy

• Forgets to square a quantity
  Check dimensions: energy must be kg m² s⁻². If v is not squared the units are wrong.
• Omits the ½ in s = ut + ½at²
  Displacement under constant acceleration is the AREA under the v–t graph: a rectangle (ut) plus a triangle (½at²). Never drop the ½.
• Inverts a formula
  Write the defining equation in words first (density = mass per unit volume), then rearrange algebraically. Check units: kg/m³.
• Confuses mass and weight
  Weight is a force: W = mg (N). Mass (kg) is the amount of matter and does not change with g.
• Fails to convert units to SI
  Convert every quantity to SI (m, kg, s) BEFORE substituting. 1 km/h = 1000/3600 m/s; 1 g = 10⁻³ kg; 1 cm² = 10⁻⁴ m².
• Confuses energy, power and time
  Power = energy/time (W = J/s). Energy = power × time. kWh = kW × h.
• Writes a quantity in the wrong unit
  Carry units through every line. A percentage is a ratio × 100; joules and newtons cannot become % without dividing by a quantity of the same kind.
• Gives the wasted part instead of the useful part
  Efficiency = useful output ÷ total input × 100%. Wasted % = 100 − efficiency. Check which one the stem asks for.
• Picks a related law, device or effect with a different job
  Learn laws and devices as 'name → what it tells you' cards. Contrast neighbours: left-hand rule = motor force, right-hand rule = dynamo current, grip rule = field around a wire.
• Puts a quantity or item in the wrong class
  Classify by the defining test: vector = has direction; charged radiation bends in a field; renewable = replaced naturally within a human lifetime.
• Uses everyday intuition instead of physics
  Name the law that applies (Newton's first law, Archimedes' principle, kinetic theory) and reason from it. If the option only 'feels' right, test it against the law.
• Mixes up a pair of similar physics terms
  Make a two-column table for each pair with one defining feature and an example. Underline the defining word in the stem before you look at the options.
• Chooses a true but irrelevant statement
  Re-read the question word (why, which, what causes). A true fact that answers a different question is still wrong.