IGCSE 0625 · ELECTRICITY & MAGNETISM · 4.3

Electric circuits

Circuit symbols, the rules for series and parallel circuits, and sensing circuits built from an LDR, a thermistor, and a potential divider. It builds on the electrical quantities and leads into safety and control.

TOPIC 4.3: ELECTRIC CIRCUITS CAMBRIDGE IGCSE PHYSICS 0625 · PATHWAYS TheLucidSTEM · thelucidstem.com BUILDS ON 4.2 Electrical quantities LEADS TO 4.4 Safety Sensors & control ELECTRIC CIRCUITS 1 · SYMBOLS & COMPONENTS Circuits are drawn with standard symbols. cell lamp resistor switch A ammeter V voltmeter Sensing parts have symbols too: the LDR, the thermistor, the diode, the fuse, and the relay. An ammeter goes in series; a voltmeter in parallel. 2 · SERIES CIRCUITS A series circuit is a single loop with one path. The current is the same at every point. The supply voltage is shared between parts. Resistances add up. R = R₁ + R₂ + ... same current all the way round 3 · PARALLEL CIRCUITS A parallel circuit has more than one path. Current splits at a junction and rejoins: what flows in equals what flows out. Each branch gets the full supply voltage. The combined resistance is less than the smallest. each lamp has the full voltage 4 · SENSORS & CONTROL EXTENDED Some resistances change with their surroundings. An LDR has a high resistance in the dark and a low resistance in bright light. A thermistor's resistance falls as it gets hotter. A potential divider uses two resistors to split the supply and give a changing output voltage. V out + supply 0 V R₁ (e.g. LDR) R₂
TOPIC 4.3: ELECTRIC CIRCUITS CAMBRIDGE IGCSE PHYSICS 0625 · PATHWAYS TheLucidSTEM · thelucidstem.com BUILDS ON 4.2 Electrical quantities LEADS TO 4.4 Safety Sensors & control ELECTRIC CIRCUITS 1 · SYMBOLS & COMPONENTS Circuits are drawn with standard symbols. cell lamp resistor switch A ammeter V voltmeter Sensing parts have symbols too: the LDR, the thermistor, the diode, the fuse, and the relay. An ammeter goes in series; a voltmeter in parallel. 2 · SERIES CIRCUITS A series circuit is a single loop with one path. The current is the same at every point. The supply voltage is shared between parts. Resistances add up. R = R₁ + R₂ + ... same current all the way round 3 · PARALLEL CIRCUITS A parallel circuit has more than one path. Current splits at a junction and rejoins: what flows in equals what flows out. Each branch gets the full supply voltage. The combined resistance is less than the smallest. each lamp has the full voltage 4 · SENSORS & CONTROL EXTENDED Some resistances change with their surroundings. An LDR has a high resistance in the dark and a low resistance in bright light. A thermistor's resistance falls as it gets hotter. A potential divider uses two resistors to split the supply and give a changing output voltage. V out + supply 0 V R₁ (e.g. LDR) R₂
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