Fields

Traps in Fields

• 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.
• Field and charge misconceptions
  Like charges/poles repel, unlike attract. Field lines go from N to S (magnetic) and + to − (electric). Fleming's LEFT hand: motors (force); RIGHT hand: generators (induced current).
• 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.
• Transformer errors
  Vs/Vp = Ns/Np. Step-up: Ns > Np. Works only with a.c. (changing flux). Ideal: VpIp = VsIs.
• Leaves a quantity out of the formula
  Write the full formula and tick off each symbol as you substitute. Every symbol needs a value.
• 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.
• 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.
• Friction and viscosity misconceptions
  Limiting friction ∝ normal reaction (F = μR), independent of contact area. Static friction > kinetic. Friction can be useful (walking, brakes). Viscosity is fluid friction; terminal velocity reached when drag + upthrust = weight.