State Kirchhoff's second law.

The sum of the electromotive forces in any closed loop equals the sum of the potential differences across all components in that loop.

Explain why the total resistance of resistors in series equals the sum of the individual resistances.

In series, the same current flows through each resistor, and the total potential difference is the sum of the p.d.s across each. Applying V = IR to each resistor and summing gives R_total = R₁ + R₂ + …

Calculate the total resistance when resistors of 930 Ω and 40 Ω are connected in series.

Total resistance = 930 + 40 = 970 Ω.

OCR A-Level Physics A H556

Practical Skills in the Written Papers

Topic overview Planning an experiment Implementing and recording Processing and interpreting data Plotting and interpreting graphs Evaluating results and procedures Mathematical skills and notation

The Practical Endorsement

Topic overview Analogue and digital measuring instruments Improving accuracy with repeat timing, fiducial markers and plumb lines Timing, calipers and micrometers Constructing and testing circuits Signal generators and oscilloscopes Generating and measuring waves Data loggers, modelling software and ionising radiation

Physical Quantities and Units

Topic overview Physical quantities, estimation and unit conversions S.I. base and derived units Prefixes, homogeneity and labelling conventions

Making Measurements and Analysing Data

Topic overview Systematic and random errors Precision, accuracy and uncertainties

Nature of Quantities

Topic overview Scalars and vectors Adding and resolving vectors

Motion

Topic overview Displacement, speed, velocity and acceleration Motion graphs and the equations of motion Measuring g and investigating motion experimentally Free fall, reaction time and stopping distance Projectile motion

Forces in Action

Topic overview Newton's second law and weight Drag and the factors affecting it Terminal velocity Moments, couples and equilibrium Density, pressure and upthrust

Work, Energy and Power

Topic overview Work done and conservation of energy Kinetic and gravitational potential energy Power and efficiency

Materials

Topic overview Hooke's law and springs Force-extension graphs and work done Elastic potential energy, stress and strain The Young modulus Stress-strain graphs and material behaviour

Newton’s Laws of Motion and Momentum

Topic overview Newton's three laws and linear momentum Force as rate of change of momentum Impulse and force-time graphs Conservation of momentum and collisions

Charge and Current

Topic overview Electric current and the coulomb Elementary charge, charge carriers and Kirchhoff's first law Mean drift velocity

Energy, Power and Resistance

Topic overview Circuit symbols and diagrams Potential difference and e.m.f.Energy transfer in circuits Ohm's law and I-V characteristics Filament lamps, diodes, thermistors and LDRs Resistivity and how it is measured How resistivity varies with temperature Electrical power and the kilowatt-hour

Electrical Circuits

Topic overview Kirchhoff's laws and combining resistances E.m.f., internal resistance and lost volts Potential dividers

Waves

Topic overview Progressive waves and their properties The wave equation Reflection, refraction and diffraction Wave graphs and intensity The electromagnetic spectrum Polarisation Refraction and refractive index Critical angle and total internal reflection The principle of superposition Interference, coherence and path difference Two-source interference, Young's double-slit experiment and the diffraction grating Forming stationary waves Stationary waves on strings and in air columns Measuring the speed of sound Node spacing, amplitude and phase in stationary waves Harmonics and the fundamental mode

Quantum Physics

Topic overview Photons, electronvolts and the Planck constant from LEDs Demonstrating the photoelectric effect Work function and Einstein's equation Electron diffraction The de Broglie equation

Thermal Physics

Topic overview Thermal equilibrium and absolute temperature The kinetic model of solids, liquids and gases Brownian motion and internal energy Specific heat capacity Measuring specific heat capacity Specific latent heat Measuring specific latent heat Moles, the Avogadro constant and kinetic theory Gas laws, the ideal gas equation and r.m.s. speed Maxwell-Boltzmann distribution, Boltzmann constant and internal energy

Circular Motion

Topic overview Radians, angular velocity and period Centripetal force and acceleration

Oscillations

Topic overview Describing simple harmonic motion The defining equation of simple harmonic motion and timing oscillations Velocity, period and graphical relationships Energy in a simple harmonic oscillator Damping, forced oscillations and resonance

Gravitational Fields

Topic overview Gravitational fields and field strength Newton's law of gravitation Field strength of a point mass and the uniform field near Earth Kepler's laws and orbital motion Orbital period and geostationary orbits Gravitational potential Gravitational potential energy and escape velocity

Astrophysics and Cosmology

Topic overview The solar system and the birth of a star The death of a low-mass star Supernovae, neutron stars and black holes The Hertzsprung-Russell diagram Energy levels and emission spectra Absorption spectra and spectral lines Photon energy and the diffraction grating Wien's law and Stefan's law Astronomical distances, parallax and the cosmological principle Doppler shift and Hubble's law The expanding universe and the Big Bang Evidence for the Big Bang and dark energy

Capacitors

Topic overview Capacitance and charging a capacitor Capacitors in series Capacitors in parallel and combined circuits Energy stored by a capacitor Charging and discharging through a resistor Investigating capacitor discharge The time constant Iterative modelling, exponential decay and the constant-ratio property

Electric Fields

Topic overview Electric fields and field lines Coulomb's law and electric field strength Uniform fields, parallel plates and permittivity Electric potential Electric potential energy and capacitance of a sphere

Electromagnetism

Topic overview Magnetic fields and field patterns Fleming's left-hand rule and the motor effect Magnetic flux density and the tesla Charged particles in magnetic fields Magnetic flux, flux linkage and Faraday's law Transformers and the a.c. generator

Nuclear and Particle Physics

Topic overview Alpha-particle scattering and the nuclear model Proton number, nucleon number and isotopes The strong nuclear force and nuclear radius Particles, antiparticles and hadrons Leptons and the quark model Quark composition and charges Beta decay in the quark model Radioactive decay and the three radiations Nuclear decay equations Activity, decay constant and half-life Exponential decay and radioactive dating Mass-energy equivalence Annihilation, mass defect and binding energy Binding energy per nucleon and induced fission The fission reactor and nuclear waste Nuclear fusion and balancing nuclear equations

Medical Imaging

Topic overview The X-ray tube X-ray attenuation and imaging CAT scanning Medical tracers and the gamma camera PET scanning Ultrasound and the piezoelectric transducer Acoustic impedance and reflection Doppler ultrasound

Electrical Circuits

3 subtopics
9 things to know
Papers 2 and 3

What Electrical Circuits covers

Analysing complete circuits: Kirchhoff's two laws, why a real source cannot deliver its full e.m.f., and the potential divider.

Where it is assessed

Papers 2 and 3

Electrons, waves and photons, examined on Exploring Physics and Unified Physics.

Check the OCR specification ↗

The 3 subtopics

Each one has its own page.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Numbers

What is meant by 'lost volts', and what is its formula?

Numbers

State the equations relating e.m.f. to current, external resistance and internal resistance, and to terminal potential difference and lost volts.

Numbers

Write the formula for the total resistance of resistors connected in series.

Numbers

State the potential divider equation for the output voltage across the second resistor.

Process

How does the resistance of a light-dependent resistor (LDR) change with light intensity?

Key term

What fundamental conservation principle underlies Kirchhoff's second law?

Key term

Kirchhoff's first law is a consequence of which conservation principle?

Key term

Define terminal potential difference.

Key term

How is the total potential difference shared in a series circuit?

Take Electrical Circuits further

The same content, four other ways.

Commonly confused

Pairs that lose marks when they are blurred together.

Kirchhoff's first law vs Kirchhoff's second law
Kirchhoff's first law: the total current into a junction equals the total current out of it. Kirchhoff's second law: the sum of the e.m.f.s around a loop equals the sum of the potential differences.
Electromotive force vs Potential difference
Electromotive force: the energy transferred to each coulomb by a source. Potential difference: the energy transferred from each coulomb by a component.
Internal resistance vs Terminal potential difference
Internal resistance: the resistance of the source itself, which causes the lost volts. Terminal potential difference: the potential difference actually available across the source's terminals.

Questions from this topic

Each one with the model answer underneath.

1
State Kirchhoff's second law.
Model answer
The sum of the electromotive forces in any closed loop equals the sum of the potential differences across all components in that loop.
2
What fundamental conservation principle underlies Kirchhoff's second law?
Model answer
Conservation of energy.
3
Kirchhoff's first law is a consequence of which conservation principle?
Model answer
Conservation of charge.
4
Write the formula for the total resistance of resistors connected in series.
Model answer
R = R₁ + R₂ + …
5
Explain why the total resistance of resistors in series equals the sum of the individual resistances.
Model answer
In series, the same current flows through each resistor, and the total potential difference is the sum of the p.d.s across each. Applying V = IR to each resistor and summing gives R_total = R₁ + R₂ + …
6
Calculate the total resistance when resistors of 930 Ω and 40 Ω are connected in series.
Model answer
Total resistance = 930 + 40 = 970 Ω.

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Electrical Circuits revision questions

What does Electrical Circuits cover in OCR A-Level Physics A?: 3 subtopics: kirchhoff's laws and combining resistances, e.m.f., internal resistance and lost volts and potential dividers. Each one has its own page with questions and model answers.
How is Electrical Circuits organised?: By the OCR H556 specification, subtopic by subtopic, so it matches the order you are taught it in.

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