A second resistor is added into a single loop that already contains one. What happens to the net resistance, and why?

It increases, because all the charge now has to pass through both resistors one after the other.

A second resistor is added as a new branch alongside one already there. What happens to the net resistance, and why?

It decreases, because the charge now has more than one path to take, so more current flows in total for the same potential difference.

Explain why adding an extra branch lowers the net resistance even though the added resistor has resistance of its own.

The new branch does not block the old one; it carries extra current alongside it, so the total current from the supply rises and the net resistance falls.

One lamp is joined to a cell, and then a second identical lamp is added into the same loop. What happens to the brightness, and why?

Both lamps are dimmer than the single lamp was, because the net resistance has risen so the current is smaller.

One lamp is joined to a cell, and then a second identical lamp is added on its own branch. What happens to the brightness of the first lamp and to the current from the cell?

The first lamp stays as bright as before, but the current from the cell doubles.

OCR GCSE Physics A J249

Electricity

Topic overview Charge, and producing static electricity Transfer of electrons, and electric fields Current as a rate of flow of charge Charge, current and time Series and parallel circuits, and circuit symbols Current, potential difference and resistance Resistors whose resistance changes, and investigating them Linear and non-linear circuit elements from graphs Net resistance in series and parallel Circuits for measurement, and electrical power

Energy

Topic overview Conservation of energy, and energy stores Energy changes on heating, and calculating them Energy of a moving body, a spring and a raised object Dissipation, and energy transfer in domestic devices Power ratings, efficiency, and increasing it Reducing unwanted energy transfer, and the rate of cooling

Forces

Topic overview Measuring distance and time, and calculating speed Converting units, and rates Vectors and scalars in motion Distance-time and velocity-time graphs Average speed, and the equations of uniform motion How objects interact, and forces as vectors Newton's first law Resolving forces, and free body diagrams Resultant forces, and balanced forces Newton's second law, and inertia Momentum, and force as a change in momentum Work done, and stored energy Power, and Newton's third law Circular motion and changing velocity Stretching, bending and compressing; elastic and plastic Force and extension, linear and non-linear The spring constant, and work done in stretching Gravitational fields, weight and free fall Calculations with weight and gravitational field strength Turning forces and the moment of a force Levers and gears Pressure in fluids, and the force on a surface

Global Challenges

Topic overview Typical speeds and everyday accelerations Reaction time, stopping distance and the dangers of large decelerations Estimating stopping distances and the forces in a collision Energy resources, and trends in their use The national grid, and step-up and step-down transformers Why the national grid is efficient, and the transformer equation Mains electricity: a.c., d.c. and the domestic supply Live, neutral and earth wires, and electrical danger Red-shift and the Big Bang The formation of the Sun, and radiation from bodies The solar system Circular orbits and stable orbits Radiation and the temperature of a body, and seismic waves and sonar

Magnetism and Magnetic Fields

Topic overview Permanent and induced magnets, and magnetic poles Magnetic fields, and the dipping compass The magnetic effect of a current, and solenoids The motor effect and Fleming's left-hand rule The force on a conductor, and how a motor turns Induced potential, alternators and dynamos Transformers and the turns ratio The microphone

Matter

Topic overview How the atomic model changed, and the structure of the atom The size of atoms, and defining density Density of the states of matter, and density calculations Conservation of mass in changes of state Heating a system, and specific heat capacity Calculations with specific heat capacity and latent heat Gas molecules, temperature and pressure Compressing a gas, and changing its volume Doing work on a gas Atmospheric pressure and why it varies with height Floating, sinking and pressure in a liquid

Radioactivity

Topic overview Protons, neutrons and isotopes Unstable nuclei and the emissions they give out Balancing nuclear equations Electron energy levels, and radiation from atoms Half-life and the randomness of decay Penetration of alpha, beta and gamma Contamination and irradiation, and half-life as a hazard Uses of nuclear radiation Nuclear fission and fusion

Waves in Matter

Topic overview Amplitude, wavelength, frequency and period The wave equation Transverse and longitudinal waves Sound crossing a boundary; reflection, transmission and absorption Converting sound to vibrations, and the range of hearing Modelling waves with ripples and sound Electromagnetic waves as transverse, and energy transfer Frequency and wavelength across the spectrum What our eyes detect, and light as an electromagnetic wave Uses of electromagnetic waves Hazards of ultraviolet, X-rays and gamma rays Exploring structures with waves, and producing radio waves Absorption, transmission, refraction and reflection Ray diagrams for reflection and refraction How colour arises

Electricity

Net resistance in series and parallel

3 things to know
Paper 3

Overview

How adding a resistor in the same loop or on a new branch changes net resistance, what that does to lamp brightness and current, and calculations of net resistance and potential difference.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Is the net resistance of two resistors on separate branches larger or smaller than the smaller of the two resistances?

Key term

Why does the current supplied by a cell increase when more branches are added across it?

Key term

Two 12 Ω resistors on separate branches across a 6 V supply take a total current of 1.0 A. What is their net resistance?

Questions from this subtopic

Each one with the model answer underneath.

1
A second resistor is added into a single loop that already contains one. What happens to the net resistance, and why?
Model answer
It increases, because all the charge now has to pass through both resistors one after the other.
2
A second resistor is added as a new branch alongside one already there. What happens to the net resistance, and why?
Model answer
It decreases, because the charge now has more than one path to take, so more current flows in total for the same potential difference.
3
Explain why adding an extra branch lowers the net resistance even though the added resistor has resistance of its own.
Model answer
The new branch does not block the old one; it carries extra current alongside it, so the total current from the supply rises and the net resistance falls.
4
Is the net resistance of two resistors on separate branches larger or smaller than the smaller of the two resistances?
Model answer
Smaller than either of them.
5
One lamp is joined to a cell, and then a second identical lamp is added into the same loop. What happens to the brightness, and why?
Model answer
Both lamps are dimmer than the single lamp was, because the net resistance has risen so the current is smaller.
6
One lamp is joined to a cell, and then a second identical lamp is added on its own branch. What happens to the brightness of the first lamp and to the current from the cell?
Model answer
The first lamp stays as bright as before, but the current from the cell doubles.

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