Describe a circuit that could be used to test whether the resistance of a component stays constant.

Join a cell, an ammeter, a variable resistor and the component in one loop with a voltmeter across the component, then take pairs of current and potential difference readings as the variable resistor is changed.

Why should an ammeter have as low a resistance as possible?

So that adding it to the circuit does not change the current it is there to measure.

Why should a voltmeter have as high a resistance as possible?

So that almost no current passes through the voltmeter itself and the circuit it is connected across is left almost unchanged.

Why is a variable resistor useful when a set of current and potential difference readings is being collected?

It lets the current be changed in steps so that several pairs of readings can be taken from one circuit.

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

Circuits for measurement, and electrical power

3 things to know
Paper 3

Overview

A circuit for testing whether resistance is constant, why ammeters need low and voltmeters high resistance, thermistor and diode test circuits, and calculations linking power, potential difference, current and energy.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Describe a circuit that could be used to monitor the temperature of a room.

Key term

How can a simple circuit be used to check which way round a diode has been connected?

Key term

State the relationship between the power transferred by a circuit device, the potential difference across it and the current through it.

Questions from this subtopic

Each one with the model answer underneath.

1
Describe a circuit that could be used to test whether the resistance of a component stays constant.
Model answer
Join a cell, an ammeter, a variable resistor and the component in one loop with a voltmeter across the component, then take pairs of current and potential difference readings as the variable resistor is changed.
2
Why should an ammeter have as low a resistance as possible?
Model answer
So that adding it to the circuit does not change the current it is there to measure.
3
Why should a voltmeter have as high a resistance as possible?
Model answer
So that almost no current passes through the voltmeter itself and the circuit it is connected across is left almost unchanged.
4
Describe a circuit that could be used to monitor the temperature of a room.
Model answer
Put a thermistor in a loop with a cell and a fixed resistor and connect a voltmeter across the thermistor; as the room warms the thermistor's resistance falls and the voltmeter reading changes.
5
How can a simple circuit be used to check which way round a diode has been connected?
Model answer
Include it in a loop with a cell, a protective resistor and an ammeter; a reading close to zero shows it is the way round that blocks the current.
6
Why is a variable resistor useful when a set of current and potential difference readings is being collected?
Model answer
It lets the current be changed in steps so that several pairs of readings can be taken from one circuit.

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