Circuits
Learn H1 Physics circuit diagrams, resistance and I–V characteristics, then solve series, parallel, internal-resistance and potential-divider problems.
Offline pages
Saved on this device only. Connect at least once every 30 days to confirm Premium. Sign-out or a change of account removes saved copies.
Before you begin
Real circuits are built from components whose resistance may change with temperature or light, powered by sources that have resistance of their own. The lessons start with circuit symbols and where to connect meters. They then define resistance and resistivity, compare the current–voltage characteristics of four components and explain them with charge carriers, and add internal resistance. They finish by combining resistors in series and parallel and using sensors in potential dividers.
Be comfortable with: current and drift velocity, because I = nAvq explains how temperature changes resistance, and potential difference and e.m.f..
Learning goals
- Recall circuit symbols and draw or interpret circuit diagrams.
- Draw circuit diagrams containing sources, switches, resistors, meters, lamps, thermistors, light-dependent resistors and diodes.
- Apply resistance and resistivity, interpret I–V characteristics and explain temperature effects.
- Analyse e.m.f., terminal potential difference and internal resistance in real sources.
- Analyse series, parallel and potential-divider resistor networks.
Syllabus statements covered
- recall and use appropriate circuit symbols
- draw and interpret circuit diagrams containing sources, switches, resistors (fixed and variable), ammeters, voltmeters, lamps, thermistors, light-dependent resistors, diodes and any other type of component referred to in the syllabus
- define the resistance of a circuit component as the ratio of the potential difference across the component to the current in it, and solve problems using the equation V = IR
- recall and solve problems using the equation relating resistance to resistivity, length and cross-sectional area, R = ρl/A
- sketch and interpret the I-V characteristics of various electrical components in a d.c. circuit, such as an ohmic resistor, a semiconductor diode, a filament lamp and a negative temperature coefficient (NTC) thermistor
- explain the temperature dependence of the resistivity of typical metals (e.g. in a filament lamp) and semiconductors (e.g. in an NTC thermistor) in terms of the drift velocity and number density of charge carriers respectively
- show an understanding of the effects of the internal resistance of a source of e.m.f. on the terminal potential difference and output power
- solve problems using the formula for the combined resistance of two or more resistors in series
- solve problems using the formula for the combined resistance of two or more resistors in parallel
- solve problems involving series and parallel arrangements of resistors for one source of e.m.f., including potential divider circuits which may involve NTC thermistors and light-dependent resistors
Lessons
Work through them in order.
- Circuit symbols and diagramsStandard circuit symbols, reading circuit diagrams and where to connect ammeters and voltmeters.
- Resistance, resistivity and component behaviourResistance and resistivity, the current–voltage characteristics of four components, and internal resistance.
- Series, parallel and potential-divider networksResistors in series and in parallel, and potential dividers with thermistors and light-dependent resistors.
Practise and check
Or choose
Topic reference
Quick reference
| Idea | Relationship | Note |
|---|---|---|
| resistance | R = V/I | at any point; constant only for an ohmic conductor |
| resistivity | R = (ρ l)/A | ρ in Ω m; A in m² |
| internal resistance | ε = V + Ir, so V = ε-Ir | V equals ε only when I = 0 |
| output power | P = VI = ε I-I²r | I²r is wasted inside the source |
| series | R = R₁ + R₂ + … | same current; p.d.s add |
| parallel | 1/R = 1/R₁ + 1/R₂ + … | same p.d.; currents add |
| potential divider | Vₒᵤₜ = Vᵢₙ R₂/(R₁ + R₂) | output across R₂, with no load |
Core knowledge to remember
- The resistance of a circuit component is the ratio of the potential difference across the component to the current in it.
- An ohmic resistor has a straight-line I–V characteristic through the origin. A filament lamp’s resistance rises as it heats up; an NTC thermistor’s resistance falls; a semiconductor diode conducts in one direction only.
- The resistivity of a metal increases with temperature because the drift velocity of the electrons falls. The resistivity of a semiconductor, such as an NTC thermistor, decreases because the number density of charge carriers increases.
- The internal resistance of a source of e.m.f. makes its terminal p.d. fall as the current increases, and reduces the output power.
- An ammeter is connected in series and a voltmeter in parallel.
- The resistance of an LDR falls as the light intensity increases; the resistance of an NTC thermistor falls as the temperature increases.
Common mistakes
- Connecting a voltmeter in series or an ammeter in parallel.
- Finding the resistance of a lamp or thermistor from the gradient of its I–V curve instead of V/I.
- Forgetting to convert mm² to m² in R = ρ l/A.
- Explaining a thermistor’s falling resistance in the same way as a metal’s rising resistance.
- Using the e.m.f. instead of the terminal p.d. to find the output power.
- Using the unloaded potential divider equation after a load has been connected.