D.C. circuits
Circuit symbols, series and parallel rules, whole-circuit calculations, potentiometers, LDRs and NTC thermistors for G3 Pure and O-Level Physics.
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Before you begin
D.C. circuits is about working out the current and p.d. anywhere in a circuit. The lessons start with circuit symbols and how to connect meters, then use two simple rules for series and parallel connections to find effective resistances and solve whole circuits. They finish with potential dividers, which share a supply’s p.d. to give a smaller or adjustable output, and with LDRs and thermistors, which make that output respond to light and temperature.
Be comfortable with: current electricity, especially p.d., e.m.f. and R = V/I.
Learning goals
- Recognise and interpret circuit symbols for cells, batteries, switches, lamps, LEDs, resistors, fuses, ammeters and voltmeters
- Draw circuit diagrams with cells, batteries, switches, lamps, LEDs, fixed and variable resistors, fuses, ammeters and voltmeters
- Recognise and interpret circuit symbols for D.C. and A.C. supplies, potentiometers, bells, light-dependent resistors and thermistors
- Draw circuit diagrams with D.C. and A.C. supplies, potentiometers, bells, light-dependent resistors and thermistors
- Apply the same-current rule in series circuits
- Apply the potential-difference sum in series circuits
- Apply current conservation at parallel junctions
- Apply equal potential difference across parallel branches
- Calculate effective resistance in series
- Calculate effective resistance in parallel
- Solve whole-circuit problems using consistent quantities
- Describe variable-potential-divider action
- Describe NTC thermistor action as an input transducer
- Describe light-dependent resistor (LDR) action as an input transducer
- Solve simple NTC and LDR potential-divider problems
Syllabus statements covered
- draw circuit diagrams with power sources (cell, battery, d.c. supply or a.c. supply), switches, lamps, resistors (fixed and variable), variable potential divider (potentiometer), fuses, ammeters and voltmeters, bells, light-dependent resistors, thermistors and light-emitting diodes
- state that the current at every point in a series circuit is the same and apply the principle to new situations or to solve related problems
- state that the sum of the potential differences in a series circuit is equal to the potential difference across the whole circuit and apply the principle to new situations or to solve related problems
- state that the sum of the currents in the separate branches of a parallel circuit is equal to the current from the source and apply the principle to new situations or to solve related problems
- state that the potential difference across the separate branches of a parallel circuit is the same and apply the principle to new situations or to solve related problems
- recall and apply the formulae for the effective resistance of a number of resistors in series and in parallel to new situations or to solve related problems
- recall and apply the relevant relationships, including R = V / I and those for current, potential differences and resistors in series and in parallel circuits, in calculations involving a whole circuit
- describe the action of a variable potential divider (potentiometer)
- describe the action of negative temperature coefficient (NTC) thermistors and light-dependent resistors and explain their use as input transducers in potential dividers
- solve simple circuit problems involving NTC thermistors and light-dependent resistors.
Lessons
Work through them in order.
Circuit diagrams
Series and parallel rules
Resistance and whole-circuit calculations
Potential dividers and sensors
Supporting: direct and alternating current
Practise and check
Or choose
Topic reference
Quick reference
| Quantity | Series | Parallel |
|---|---|---|
| current | same through each component | branch currents add: I = I₁ + I₂ + … |
| p.d. | p.d.s add: V = V₁ + V₂ + … | same across each branch |
| effective resistance | R = R₁ + R₂ + … | 1/R = 1/R₁ + 1/R₂ + … |
| size of effective resistance | larger than each resistance | smaller than the smallest branch resistance |
| Device or circuit | Key relationship or behaviour |
|---|---|
| potential divider | Vₒᵤₜ = R₂/(R₁ + R₂) × E, output across R₂ |
| LDR | resistance decreases as light intensity increases |
| NTC thermistor | resistance decreases as temperature increases |
| ammeter | in series; very small resistance |
| voltmeter | in parallel; very large resistance |
Core knowledge to remember
- A current flows only round a complete loop. An ammeter goes in series with a component; a voltmeter goes in parallel across it.
- In series there is one path, so the current is the same everywhere and the p.d.s add up. In parallel each branch has the same p.d. and the branch currents add up.
- The effective resistance of a group of resistors is the single resistance that would draw the same current from the same supply. Use it with R = V/I to find the current from the supply.
- In a mixed circuit, replace one wholly series or wholly parallel group at a time, then work back to find each current and p.d.
- In a potential divider, the same current passes through both resistors, so each takes a share of the supply’s p.d. in proportion to its resistance. A potentiometer’s slider changes how the p.d. is shared, giving an output anywhere from zero to the full supply.
- An LDR or NTC thermistor in a potential divider acts as an input transducer: a change in light or temperature changes its resistance, and so the output p.d.
- D.C. flows in one direction only; A.C. reverses direction again and again.
Common mistakes
- Connecting an ammeter in parallel or a voltmeter in series.
- Saying the current is the same in parallel branches, or the p.d. is the same across series components.
- Adding parallel resistances, or forgetting to invert 1/R.
- Giving each parallel branch the full supply p.d. when another resistor is in series with the group.
- Using R = V/I with a p.d. and a current that belong to different parts of the circuit.
- Describing a change in a divider’s output without saying which component it is taken across.
- Saying an LDR’s or NTC thermistor’s resistance rises as light or temperature increases.