10.1 Practical circuits

Circuit diagrams use agreed symbols for cells, power supplies, switches, resistors, lamps, diodes, meters and sensors so a circuit can be read without a picture of the hardware.

Keep wires joined only where junctions are marked and show meter connections according to their measuring role.

A voltmeter symbol is drawn across a component, while an ammeter symbol belongs in series with the branch current.

A symbol is not just decoration: reversing a diode or omitting a junction changes the circuit being described.

A circuit diagram represents the topology of components, supply polarity and junctions using standard symbols and straight labelled connections.

Trace a complete path from one supply terminal to the other, place ammeters in series and voltmeters in parallel, and label relevant values.

To measure a lamp’s current and voltage, put the ammeter in the lamp branch and the voltmeter across the lamp.

A voltmeter placed in series can greatly alter or effectively break a circuit; an ammeter across a supply can cause a dangerous short.

The e.m.f. ε of a source is the energy supplied by the source per coulomb as charge moves through it: ε=W/Q.

E.m.f. describes the source’s energy rise, whereas terminal potential difference can be lower when internal resistance causes a drop.

A 12 V ideal source supplies 12 J per coulomb; a real source may show less terminal voltage while delivering current.

E.m.f. is not a force in newtons and is not always identical to the voltage measured across a working battery.

E.m.f. is the source energy rise per coulomb; potential difference is the energy transferred per coulomb across a component.

In a complete loop, source energy is shared among component transfers and internal losses. Use the direction of energy transfer to distinguish rises from drops.

A cell with ε=1.5 V supplies 1.5 J per coulomb; a lamp may receive less terminal p.d. when the cell is delivering current.

E.m.f. is not automatically the same as every measured voltage in a working circuit.

A real source can be modelled with internal resistance r. When current I flows, terminal voltage V=ε−Ir, so some source energy is dissipated internally.

The lost voltage Ir grows with current. At open circuit I=0, terminal voltage approaches ε.

For ε=6.0 V, r=0.50 Ω and I=2.0 A, terminal voltage is 5.0 V.

Internal resistance is not a separate external component that must be drawn outside the source; it represents loss inside the source model.