What you’ll learn15 learning objectivesChoose one objective for a focused lesson, or study the complete topic.—B.5.1—Cells provide a source of emf• Cells provide a source of emf.Syllabus objective—B.5.2—Electrical energy sources• Use chemical cells and solar cells as electrical energy sources.Syllabus objective—B.5.3—Resistance• Resistance: R=V/I.Syllabus objective—B.5.4—Ohmic behaviour• Ohm’s law for ohmic devices: V ∝ I at constant temperature.Syllabus objective—B.5.5—Electrical power• Electrical power: P=IV=I^2R=V^2/R.Syllabus objective—B.5.6—Electrical energy transfer• Electrical energy transfer: E=IVt.Syllabus objective—B.5.7—Electric cells• Cells convert chemical energy to electrical energy.Syllabus objective—B.5.8—Conventional current• Conventional current is flow of positive charge.Syllabus objective—B.5.9—Current and charge flow• Current relates to charge flow: I=Δq/Δt.Syllabus objective—B.5.10—DC and AC• Direct current flows one way; alternating current periodically reverses direction.Syllabus objective—B.5.11—Kirchhoff’s laws• Kirchhoff junction rule conserves charge.• Kirchhoff loop rule conserves energy.Syllabus objective—B.5.12—Resistance and resistivity• Resistance depends on resistivity and dimensions: R=ρL/A.Syllabus objective—B.5.13—Series and parallel circuits• Use series/parallel rules for current, voltage and equivalent resistance.• Series: same current; voltages and resistances add.• Parallel: same voltage; currents add; reciprocal resistance adds.Syllabus objective—B.5.14—Emf and internal resistance• Cell emf with internal resistance: ε = I(R+r).Syllabus objective—B.5.15—Variable resistance• Resistors can have variable resistance.Syllabus objective