Electron Theory
Electron theory questions in Module 3 test your grasp of atomic structure, charge, and how charge movement defines current. You need to distinguish electron flow from conventional current direction, apply the coulomb, and recognise conductors, semiconductors, and insulators by their valence-electron behaviour.
Core rules
- The atom has a nucleus (protons, neutrons) and orbiting electrons; protons are positive, electrons negative, neutrons neutral.
- The coulomb (C) is the unit of charge; one electron carries about 1.602 × 10^-19 C.
- Current is the rate of charge flow: I = Q/t, with I in amperes, Q in coulombs, t in seconds.
- Conventional current runs positive to negative; electron drift is negative to positive.
- Conductors have loosely bound valence electrons; insulators hold them tightly; semiconductors sit between and are sensitive to doping, heat, and light.
- EMF is the energy per unit charge supplied by a source, measured in volts; potential difference is energy per unit charge between two points.
Worked example
A steady current of 2.0 A flows for 30 s. Find the charge transferred and the number of electrons. Q = I × t = 2.0 × 30 = 60 C. Electrons = Q/e = 60 / (1.602 × 10^-19) = 3.74 × 10^20 electrons. Reasoning: the ampere is one coulomb per second, so multiply current by time; then divide by the charge per electron.
Traps
- Reversing conventional current and electron flow.
- Treating the coulomb as a rate; it is a quantity of charge.
- Forgetting that current direction in circuit diagrams follows conventional current.
- Mixing EMF with potential difference: EMF is the source's energy per unit charge, not the voltage drop across a load.