Trade Theory
Trade coverage
Trade Theory follows the trade you applied for-electrician, fitter, welder, carpenter, mason, painter, mechanic and similar. Expect tools and their uses, materials, measurement, basic calculations, safety rules and simple fault finding.
Core rules
- Measurement: know least count. A vernier calliper commonly reads to 0.02 mm, a micrometer to 0.01 mm, and a steel rule to 1 mm.
- Electrician: Ohm's law V = IR; power P = VI = I²R; series current is common, parallel voltage is common; resistor colour code-black 0, brown 1, red 2, orange 3, yellow 4, green 5, blue 6, violet 7, grey 8, white 9.
- Fitter: marking, filing, drilling and tapping; drill size for a tap is approximately major diameter minus pitch.
- Welder: electrode angle, arc length and polarity; strike the arc on scrap, not on finished work.
- Carpenter, mason and painter: material properties, mixing ratios, curing and surface preparation.
Worked example
A 12 V battery is connected across a 6 Ω resistor. Find the current and power. I = V/R = 12/6 = 2 A. P = VI = 12 × 2 = 24 W. Check with P = I²R = 4 × 6 = 24 W. If the resistor were 3 Ω, current would double to 4 A and power would rise to 48 W-showing why a low-resistance fault draws high current.
Traps that cost marks
- Using wire diameter instead of radius in area calculations.
- Confusing series and parallel voltage and current rules.
- Forgetting to apply the least-count zero error.
- Treating a term from one trade as universal across all trades.
- Mixing resistor colour-code order after green.