Topic 1 of 6
Read connections and place meters
The connections determine the circuit. Moving a component on the page changes nothing if both of its terminals still connect to the same nodes.
A node is a set of points joined by ideal wire without an intervening component. They share one potential. A junction dot shows a connection; an unconnected crossing needs a clear gap or bridge. Do not decide whether two components are in series or parallel from their visual alignment alone.
Recognise the component and its terminals
Cell
The longer plate is the positive terminal.
Battery
Connected cells provide the indicated overall polarity.
D.c. supply
The two terminals have the labelled fixed polarity.
Switch
The open gap breaks this conducting path.
Fixed resistor
Resistance belongs to the component between its two terminals.
Variable resistor
The diagonal adjustment arrow denotes an adjustable two-terminal resistance.
Ammeter
Insert it in the current path being measured.
Voltmeter
Connect its terminals to the two points whose p.d. is required.
Filament lamp
Its settled resistance changes as the filament warms.
NTC thermistor
The marking identifies a thermistor; this NTC model has lower resistance when warmer.
LDR
The two arrows represent incoming light, not current.
Semiconductor diode
The bar identifies the cathode K; the other terminal is the anode.
Potentiometer
Two track ends and a separate wiper make three terminals.
A cell's long plate is positive and its short plate negative; a battery contains multiple cells. An open switch breaks its path. A semiconductor diode's bar marks the cathode, while the other terminal is the anode. Its symbol does not mean that current always flows: the voltage polarity and component behaviour matter.
A two-terminal variable-resistor connection changes the resistance in its path. A three-terminal potentiometer has two track ends and a sliding wiper; using both ends and the wiper gives a selectable potential-divider output. A thermistor changes resistance with temperature, while an LDR responds to illumination. Their symbols do not specify one universal resistance or switching threshold.
Redraw the same network
Let R1 join A to B. Let R2 and R3 each join B to C. The two latter resistors are parallel because they share both B and C. R1 carries the total current before it divides.
Read the terminal pairs
A node can be drawn in several places. Follow the uninterrupted wire and the named endpoints.
The unfamiliar drawing is equivalent
Changing the shape or position of a branch changes nothing if its terminal pair is preserved.
To check an unfamiliar drawing, trace each continuous wire and label its node. List the terminal pair of each component. A parallel pair needs equal p.d. because it shares both endpoints; a series pair needs an unbranched path between its components so they carry the same current.
Adding a wire directly from B to C would be a new connection, not a harmless redrawing. It would bypass both resistors and make VB - VC zero in the ideal-wire model.
Match the meter to the quantity
An ammeter goes in the selected current path. A voltmeter connects across the selected endpoints. To find a component's resistance, pair its own current with its own p.d. To find the equivalent resistance of the complete external network, use total entering current and VA - VC.
Ideal calculations assume negligible ammeter resistance and negligible voltmeter current. A real voltmeter can draw current and change a high-resistance circuit; a real ammeter can introduce a voltage drop. Check instrument range, resolution, polarity and loading against the intended measurement.
A source model can include an ideal e.m.f. between C and a private internal node D, followed by internal resistance from D to external terminal A. Its terminal voltmeter belongs across A/C. D/C spans only the ideal e.m.f. inside that model. The source calculation explains the difference.