Current electricity

Traps in Current electricity

• Rearranges a formula wrongly
  Use the triangle or do the algebra: from V = IR, divide both sides by R → I = V/R. Check with units (V/Ω = A).
• Calculates a different quantity
  Name the quantity asked for and its unit before calculating. If your number has the unit of a different quantity, you used the wrong formula.
• Inverts a formula
  Write the defining equation in words first (density = mass per unit volume), then rearrange algebraically. Check units: kg/m³.
• Forgets to invert the reciprocal at the end
  With 1/R = 1/R₁ + 1/R₂ (or 1/v), the sum gives 1/R. Finish with R = 1 ÷ (sum). Check: parallel R must be smaller than the smallest resistor.
• Averages values that do not combine by averaging
  Parallel: 1/R = 1/R₁ + 1/R₂, always less than the smallest. Series: add. Averaging is never the rule.
• Confuses series and parallel combinations
  Series: R = R₁ + R₂ (total is larger). Parallel: 1/R = 1/R₁ + 1/R₂ (total is SMALLER than the smallest). Use this to sanity-check.
• Confuses energy, power and time
  Power = energy/time (W = J/s). Energy = power × time. kWh = kW × h.
• 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.
• Uses the wrong arithmetic operation
  Write the formula symbolically and substitute only at the end. Check units: if the units of your answer are wrong, so is the operation.
• Takes part of the condition as the whole answer
  Check whether the option is enough on its own. Equilibrium needs zero resultant force AND zero total moment; a complete explanation names every factor in the stem.
• Leaves a quantity out of the formula
  Write the full formula and tick off each symbol as you substitute. Every symbol needs a value.
• 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.
• 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.
• 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.
• Reverses a relationship or direction
  Write the relationship with an arrow or proportionality sign first (E ∝ f; R ∝ L; cooling a metal → R falls). Then check the option follows the same direction.