Circuit diagrams and circuit components
Section: Electricity & Magnetism | Syllabus: Cambridge AS Level Physics 9702
Circuit Symbols Circuit diagrams use standard symbols to represent components. You must be able to draw and interpret these symbols. Component Symbol Description Function Cell Long line (+) and short thick line (−) Provides e.m.f.
(single unit) Battery Two or more cells in series Provides larger e.m.f. Power supply (d.c.) Circle with + and − terminals Variable d.c. source Generator Circle with G inside Produces a.c. or d.c. Switch (open) Break in line with contacts Controls circuit on/off Fixed resistor Rectangle Limits current (constant R) Variable resistor Rectangle with arrow through Adjustable resistance Potential divider Rectangle with arrow touching Splits voltage Lamp Circle with cross inside Converts electrical to light energy Heater Rectangle with diagonal lines Converts electrical to heat energy Motor Circle with M inside Converts electrical to kinetic energy Bell Semicircle shape Produces sound Ammeter Circle with A inside Measures current (in series) Voltmeter Circle with V inside Measures p.d.
(in parallel) Fuse Rectangle with line through Melts to break circuit if current too high Thermistor (NTC) Rectangle with diagonal line through Resistance decreases as temperature increases LDR Rectangle with arrows pointing at it Resistance decreases as light intensity increases Magnetising coil Coiled line Produces magnetic field when current flows Relay Coil with switch contacts Electromagnetic switch Transformer Two coils with lines between Changes a.c.
voltage Diodes and LEDs Component Symbol Description Behaviour Diode Triangle pointing at line (bar) Allows current in one direction only (arrow direction) LED Diode symbol with arrows pointing outward Emits light when forward biased; one-way current Diode Behaviour Forward bias: Current flows easily (low resistance) Reverse bias: No current flows (very high resistance) Diodes have a threshold voltage (~0.6 V for silicon) Special Components Thermistor (NTC) A Negative Temperature Coefficient (NTC) thermistor has resistance that decreases as temperature increases .
Cold → high resistance → low current Hot → low resistance → high current Used in temperature sensors, thermostats Light-Dependent Resistor (LDR) An LDR has resistance that decreases as light intensity increases .
Dark → high resistance → low current Bright → low resistance → high current Used in automatic lights, light meters
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