Current of electricity
Everything you switch on runs on a handful of ideas, and this lesson builds them one careful step at a time. We define electric current as the rate of flow of charge, Q = It, and see that charge is quantised in lumps of the elementary charge, Q = ne. We go inside a wire to derive I = nAvq and meet the surprisingly slow drift velocity. We define potential difference and the volt (V = W/Q), separate e.m.f. from p.d., define resistance R = V/I and the ohm, state Ohm's law, and draw the three current–voltage characteristics the examiner loves — a metallic conductor, a filament lamp and a semiconductor diode. Finally we meet resistivity, ρ = RA/L, and the three faces of electrical power, P = VI = I²R = V²/R.
By the end you should be able to (NSSCAS Physics (AS) 3.2):
- Explain that electric current is a flow of charge carriers, and that the charge on carriers is quantised (Q = ne)
- Define the coulomb, and recall and use Q = It
- Derive and use, for a current-carrying conductor, the expression I = nAvq
- Define potential difference and the volt, and recall and use V = W/Q
- Distinguish between electromotive force (e.m.f.) and potential difference (p.d.)
- Define resistance and the ohm, and recall and use R = V/I (V = IR)
- State Ohm's law, and sketch and discuss the I–V characteristics of a metallic conductor, a filament lamp and a semiconductor diode
- Recall that an LDR's resistance falls as light intensity rises, and an NTC thermistor's resistance falls as temperature rises
- Define resistivity and recall and use ρ = RA/L
- Recall and use the electrical power relationships P = VI, P = I²R and P = V²/R
Miss Elizabeth and Mike talk through the whole topic — with the figure and working drawn live.