DC Circuits: NEB Class 11 Physics Guide

Curriculum boundary

What a complete DC-circuit answer must connect

The current CDC Physics Grade 11 page is the official starting point for Nepal’s course context. Circuit analysis connects ideas from electric potential and potential difference, capacitors and energy conservation. The OpenStax treatment of Ohm’s law likewise describes a source supplying energy while a resistor transfers it, usually to thermal energy.

A circuit diagram is a topological map, not a picture of physical distance. Points joined by ideal wire form one node and have the same potential. A branch is a path between nodes. A loop is any closed path. Mark these before writing equations; many apparent algebra errors begin as a wrong node decision.

Charge flow

Current is a rate, not a substance consumed by a resistor

Current I=ΔQ/Δt measures charge crossing a section per unit time; 1 ampere is 1 coulomb per second. Conventional current points in the direction positive charge would move, from higher potential toward lower potential in an external resistive circuit. In metals, mobile electrons drift oppositely. Their average drift is slow, while the electric field establishing circuit behaviour propagates through the connected system much faster.

At a junction, charge cannot accumulate indefinitely in steady operation. This gives Kirchhoff’s junction rule. Choose every branch current direction freely. A negative solution simply says the real direction is opposite to the arrow chosen.

Resistance and models

Use V=IR only with the intended element and conditions

An ohmic conductor has an approximately linear V–I relation at constant physical conditions. Resistance R=V/I has unit ohm (Ω). For a uniform wire, R=ρL/A, where resistivity ρ describes material behaviour, L is length and A is cross-sectional area. Doubling L doubles R; doubling diameter makes area four times larger and ideally reduces R to one quarter.

Filament lamps and semiconductor devices may be non-ohmic because temperature or carrier behaviour changes. On a V-versus-I graph the slope is resistance; on an I-versus-V graph the slope is conductance 1/R. Always read the axes before taking a gradient.

Equivalent resistance

Identify shared current or shared voltage before combining

The checks are as important as the calculation: series voltage drops must sum to the source, parallel branch currents must sum to the total, and equivalent resistance must satisfy its bound.

Real sources

Separate emf from terminal voltage

Electromotive force ε is energy supplied per unit charge by a source; despite its name, it is measured in volts. A real cell can be modelled as ideal emf in series with internal resistance r. While delivering current, terminal voltage is V=ε−Ir. When a charging current enters the positive terminal, the terminal voltage can be written V=ε+Ir under a consistent sign convention.

Never estimate safe current by ignoring source limits. Use only teacher-approved low-voltage supplies in practical work. Do not open batteries or probe household mains.

Conservation laws

Write junction and loop equations with a sign table

The junction rule expresses charge conservation: total current entering a node equals total current leaving it. The loop rule expresses energy conservation: algebraic potential changes around a closed path sum to zero. The OpenStax Kirchhoff section shows how source rises and IR drops balance.

1. Label nodes and branch currents.
2. Choose current arrows; do not guess until all are “positive.”
3. Write independent junction equations.
4. Choose loop directions.
5. Across a resistor, moving with current gives −IR and against current gives +IR.
6. Across a source from negative to positive terminal gives +ε; positive to negative gives −ε.
7. Solve simultaneously and interpret negative currents.
8. Substitute back into every conservation equation.

Measurement

Connect meters according to the quantity measured

An ideal ammeter has negligible resistance and goes in series so branch current passes through it. An ideal voltmeter has extremely large resistance and connects in parallel across two points. OpenStax DC instrument guidance explains these placements. Putting an ammeter directly across a source can create a dangerously large current; using the wrong range or port can damage a real meter.

In a school investigation, begin with supply off, have the teacher check connections, choose the highest suitable range, energise briefly, record units and switch off before changing the circuit. Compare measured values with uncertainty and component tolerance; do not call a small difference a failed law automatically.

The PhET DC virtual lab can rehearse node identification, meter placement and predictions before physical equipment is used.

Energy transfer

Audit current, voltage and power together

Electrical power is P=IV. For an ohmic resistor, substitute V=IR to obtain P=I²R or P=V²/R. Choose the form whose current or voltage belongs to that element, not automatically the total circuit. Electrical energy transferred over steady time t is E=Pt.

In Nepal-relevant household reasoning, appliance power and operating time determine billed energy; voltage alone does not. Household mains is outside an unsupervised student experiment. Use labelled low-voltage circuits and proper protection only.

Exam readiness

Common mistakes, study method and practice tasks

• Do not say current is “lost” across a resistor.
• Mark nodes before declaring series or parallel.
• Read V–I graph axes before finding resistance.
• Keep ε, terminal voltage and resistor drop separate.
• Use one sign convention throughout a Kirchhoff solution.
• Connect ammeters in series and voltmeters in parallel.
• Apply power formulas to the named component.
• Use equivalent-resistance and conservation checks.

Study in three passes: first classify diagrams without calculation; next solve with symbolic equations; finally change one source polarity or branch resistance and predict the new behaviour before recalculating. Maintain an error log with codes for node, topology, sign, unit, meter, power and physical interpretation.

1. Reduce a three-resistor mixed network and recover every branch value.
2. Solve one two-loop circuit using junction and loop equations.
3. Infer resistance from each orientation of an I–V graph.
4. Find terminal voltage and internal power loss for a loaded cell.
5. Design safe meter connections for a resistor test.
6. Check total source power against element power.

For online or physical NEB tuition, call 9846662070. Continue with the DC Circuits Study Guide and DC Circuits Practice Set.

References and next steps

Sources and related study guides

• CDC Nepal: Physics Grade 11
• CDC Nepal: Secondary Education Curriculum
• OpenStax: Ohm’s Law and Simple Circuits
• OpenStax: Resistors in Series and Parallel
• OpenStax: Kirchhoff’s Rules
• PhET: Circuit Construction Kit DC Virtual Lab

Revise with the study guide, then test transfer using the practice set. Curriculum scope and linked institutional sources were checked on 2 August 2026; follow current CDC, NEB, school and laboratory instructions if requirements change.