--- title: "Resistance in Series and Parallel" description: "Resistances in series add. Adding a working parallel branch lowers total resistance and raises supply current when supply voltage stays the same. Predictions can be checked with measurements in a supe" canonical: https://lightmysky.com/learn/science/resistance-in-series-and-parallel-mt_B29m3h5B75 source: https://lightmysky.com/learn/science/resistance-in-series-and-parallel-mt_B29m3h5B75.md retrieved: 2026-11-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Resistance in Series and Parallel Resistances in series add. Adding a working parallel branch lowers total resistance and raises supply current when supply voltage stays the same. Predictions can be checked with measurements in a supervised low-voltage circuit. Subject: Science · Area: Energy · Ages 14 to 16 Page: https://lightmysky.com/learn/science/resistance-in-series-and-parallel-mt_B29m3h5B75 ## Ready when they can - Calculates the total resistance of resistors in series and predicts the current - Explains why two identical resistors in parallel halve the total resistance - Predicts the supply-current change when a working lamp is added in parallel at unchanged supply voltage, and checks it in a supervised low-voltage circuit ## Lesson: One path or many paths In series, charge follows one unbranched path through every resistor. The same current flows through each. Add all resistor values to find total resistance. The supply voltage is shared in proportion to resistance. Treat connecting wires as having negligible resistance and fixed resistor values as unchanged by heating. **Example.** To predict current through series resistors, first add their resistance values. Then use I = V / R total: current in amperes (A) = supply voltage in volts (V) divided by total resistance in ohms. For example, 8 ohms and 16 ohms give 24 ohms. At 6 V, I = 6 / 24 = 0.25 A through both resistors. Parallel branches join the same two points. When connected across the supply, each has its full voltage. Two identical resistors draw equal currents; together they draw twice one resistor's current. Total resistance is supply voltage divided by supply current, so doubling current at the same voltage halves resistance. Two 18 ohm resistors give 9 ohms in parallel; in series, add them to get 36 ohms. *(drawing: Resistance in ohms: one resistor is 18; two identical resistors in parallel have a total of 9. The bars compare resistance, not circuit paths.)* Supply current is the sum of the parallel branch currents. For example, 0.10 A + 0.15 A = 0.25 A. Adding a working lamp at unchanged supply voltage adds its current; the other lamps keep their settled currents. Supply current rises, so total resistance falls. Check voltage and supply current before and after adding the branch. A real supply may lose voltage or limit current if overloaded. An ammeter measures current and goes in series in the measured path. Its low resistance means connecting it across a supply can cause a short circuit and damage. A voltmeter goes across two points to measure their voltage difference. It has high resistance so it draws very little current and changes the circuit less. Use supervised low-voltage circuits with suitable components and meter ranges. Switch off before rewiring. Never experiment with household mains. If a bulb fails by breaking the only current path in a series string, every bulb goes out. Some strings have bypasses, so bulb failure does not always break the path. Household appliances use parallel connections to receive supply voltage and switch separately, within the supply's limits. **Recap.** Series resistances add. Parallel branch currents add. At unchanged supply voltage, adding a working parallel branch raises supply current and lowers total resistance. *Lesson, questions, worked solutions and flashcards written for this dataset.* ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Current-Voltage Graphs for Components](https://lightmysky.com/learn/science/current-voltage-graphs-for-components-mt_n_CiOa-05J) ## Opens up - [Electrical Power and the Cost of Running Appliances](https://lightmysky.com/learn/science/electrical-power-and-the-cost-of-running-appliances-mt_099G0drIbk) - [Discharging a Capacitor and the Time Constant](https://lightmysky.com/learn/science/discharging-a-capacitor-and-the-time-constant-mt_5sjDPudyad) - [Resistivity and What Sets a Wire's Resistance](https://lightmysky.com/learn/science/resistivity-and-what-sets-a-wires-resistance-mt_A4f8THP-xm) - [Thermistors, Light-Dependent Resistors and Sensing Circuits](https://lightmysky.com/learn/science/thermistors-light-dependent-resistors-and-sensing-circuits-mt_d92v1CiqIG)