Variable Resistance

Wires and Resistance

The resistance of a wire depends on the wire’s length and cross-sectional area of the circular end of the wire. Resistance (in ohms) is proportional to (α) the length (in metres) divided by the cross-sectional area (in metres squared).

Size of a wire

Increasing the length of a wire, increases the resistance of the wire.
Decreasing the cross-sectional area of the wire increases its resistance.
So short, wide wires have less resistance than long, narrow wires.

Wire thickness

The cross-sectional area (A) of a circular wire is equal to π times its radius (r) squared.
Therefore, doubling the thickness of a wire quadruples its cross-sectional area.

1Motion, Forces & Energy

1.1Physical Quantities & Measurement Techniques

1.1.1Measuring Length

1.1.2Measuring Volume

1.1.3Measuring Time

1.1.4Coming Soon - Scalar & Vector Quantities

1.1.5Adding Vectors

1.2Motion

1.2.1Average Speed

1.2.2Calculating Average Speed

1.2.3Velocity

1.2.4Acceleration

1.2.5Distance-Time Graphs

1.2.6Speed-Time Graphs

1.2.7More Speed-Time Graphs

1.2.8Calculating Uniform Acceleration

1.2.9Gravity

1.2.10Free Fall - Distance-Time & Speed-Time Graphs

1.2.11Air Resistance

1.2.12Air Resistance - Graphs

1.2.13Optional: Calculating Acceleration

1.3Mass & Weight

1.3.1Mass

1.3.2Weight

1.3.3Gravity Calculations

1.4Density

1.4.1Density

1.4.2Density - Equation

1.4.3Density - Experiments

1.4.4Density - Calculations

1.5Forces: Effects of Forces

1.5.1Spring Constant

1.5.2Spring Constant Equation

1.5.3Force & Acceleration

1.5.4Friction

1.6Forces: Turning Effects of Forces

1.6.1Circular Motion

1.6.2Turning Effect

1.6.3Moments

1.6.4Calculating Moments

1.7Forces: Centre of Gravity

1.7.1Equilibrium

1.7.2Centre of Gravity

1.7.3Centre of Gravity Experiment

1.8Momentum

1.8.1Momentum

1.8.2Coming Soon - Impulses

1.9Energy, Work, & Power: Energy

1.9.1Energy Changes

1.9.2Energy Conversion

1.9.3Kinetic Energy Stored

1.9.4Calculating Changes in Kinetic Energy

1.9.5Gravitational Potential Energy Stored

1.9.6Calculating Changes in GPE

1.9.7Elastic Potential Energy Stored

1.9.8Calculating Changes in EPE

1.10Energy, Work & Power: Work

1.10.1Mechanical Work

1.10.2Calculating Mechanical Work

1.10.3Calculating Electrical Work Done

1.11Energy, Work & Power: Energy Resources

1.11.1Energy Resources

1.11.2Fossil Fuels

1.11.3Geothermal Energy

1.11.4Wind Energy

1.11.5Water Energy

1.11.6Tidal Energy

1.11.7Nuclear Energy

1.11.8Solar Energy

1.11.9Original Source of Energy

1.11.10Coming Soon - Fusion

1.11.11Efficiency

1.11.12Calculating Efficiency

1.11.13Energy Wastage

1.12Energy, Work & Power: Power

1.12.1Power

1.13Pressure

1.13.1Pressure

1.13.2Atmospheric Pressure

1.13.3Calculating Pressure

1.13.4Liquid Pressure

1.13.5Manometer

1.13.6Calculating Liquid Pressure

2Thermal Physics

3Waves

3.1General Properties of Waves

3.1.1Wave Basics

3.1.2Transverse Wave

3.1.3Longitudinal Wave

3.1.4Ripple Tank

3.1.5Wave Speed

3.1.6Wave Speed Formula

3.1.7Calculating Wave Speed

3.1.8Wave Frequency

3.1.9Calculating Wave Frequency

3.1.10Wavelength

3.1.11Diffraction

3.2Light: Reflection & Refraction

3.2.1Reflection

3.2.2Internal Reflection

3.2.3Refraction

3.2.4More Refraction

3.2.5Refractive Index Equation

3.2.6Refractive Index

3.2.7Critical Angle

3.2.8Optical Fibres

3.2.9Optical Fibres in Communications

3.2.10Medical Applications of Optical Fibres

3.3Light: Thin Lenses

3.3.1Thin Converging Lenses 1

3.3.2Thin Converging Lens 2

3.3.3Thin Converging Lens 3

3.3.4Thin Converging Lens 4

3.3.5Calculating Lenses

3.4Light: Dispersion of Light

3.4.1Dispersion of Light

3.4.2Triangular Prisms 1

3.4.3Triangular Prisms 2

3.5Electromagnetic Spectrum

3.5.1Properties of Electromagnetic Waves

3.5.2Gamma Rays

3.5.3X-Rays

3.5.4UV Light

3.5.5Visible Light

3.5.6Infrared Radiation

3.5.7Microwaves

3.5.8Radio Waves

3.5.9Other Uses of EM Waves

3.5.10Coming Soon - Analogue & Digital Signals

3.6Sound

3.6.1Sound Waves

3.6.2Ears

3.6.3Speed of Sound

3.6.4Measuring Sound Waves

3.6.5Oscilloscope Traces

3.6.6Sound Waves Experiment

4Electricity & Magnetism

4.1Simple Phenomena of Magnetism

4.1.1Magnets

4.1.2Magnetic Forces

4.1.3Magnetic Materials

4.1.4Induced Magnetism

4.1.5Temporary & Permanent Magnets

4.1.6Electromagnets

4.1.7Magnetic Field Lines

4.2Electrical Quantities: Electric Charge

4.2.1Electrical Charge

4.2.2Charged Objects

4.2.3Static Electricity Experiments

4.2.4Charging by Induction

4.2.5Electric Fields

4.2.6Electric Field Patterns

4.2.7Conductors

4.2.8Electrical Conductors - Electrons

4.3Electrical Quantities: Electric Current

4.3.1Current

4.3.2Conventional Current

4.3.3Current Equation

4.3.4Calculating Current

4.3.5AC/DC

4.4Electromotive Force & Potential Difference

4.4.1Electromotive Force

4.4.2Potential Difference

4.4.3Potential Difference Equation

4.4.4Calculating Voltage

4.5Electrical Quantities: Resistance

4.5.1Circuit Measurements

4.5.2Resistance

4.5.3Equation for Resistance

4.5.4Measuring Resistance

4.5.5Calculating Resistance & Ohm's Law

4.5.6Variable Resistance

4.5.7Resistance Graph

4.6Electrical Energy & Electrical Power

4.6.1Electrical Work

4.6.2Electrical Power

4.6.3Calculating Electrical Work

4.7Electric Circuits: Circuit Diagrams & Components

4.7.1Circuit Diagrams

4.7.2Circuit Symbols

4.7.3More Circuit Symbols

4.7.4Diodes

4.7.5LDRs & Thermistors

4.8Electric Circuits: Series & Parallel Circuits

4.8.1Series Circuits

4.8.2Series Circuits - Resistors

4.8.3Series Circuits - Cells

4.8.4Series Circuits - Potential Difference

4.8.5Calculating Series Circuits

4.8.6Parallel Circuits

4.8.7Parallel Circuits - Current

4.8.8Parallel Circuits - Resistance

4.8.9Parallel Circuits in Practice

4.9Electric Circuits: Action & Use

4.9.1Potentiometer

4.9.2Potential Divider Equation

4.10Electrical Safety

4.10.1Dangers of Electricity

4.10.2Live & Neutral Wires

4.10.3Earthing Metal Cases

4.10.4Fuses & Trip Switches

4.10.5Fuse Ratings

4.11Electromagnetic Effects: Electromagnetic Induction

4.11.1Electromagnetic Induction

4.11.2Size of e.m.f

4.11.3Forces Between Electricity & Magnets

4.11.4Rotation of Coil & Induced e.m.f

4.12Electromagnetic Effects: The A.C. Generator

4.12.1The Rotating-Coil Generator

4.12.2Features of a Rotating-Coil Generator

4.13Magnetic Effect of a Current

4.13.1Magnetic Effect of a Current

4.13.2Solenoid Field

4.13.3Relays

4.14Force on a Current-Carrying Conductor

4.14.1Magnetic Field Strength

4.14.2Uses of Magnets & Currents

4.14.3Catapult Experiment

4.14.4Beam Force Experiment

4.15Electromagnetic Effects: The D.C. Motor

4.15.1Motor Effect

4.15.2Direction of Motor Effect

4.15.3DC Motors

4.15.4Coil of Wire in a Magnetic Field

4.16Electromagnetic Effects: The Transformer

4.16.1Transformers

4.16.2Transformers Equation

4.16.3Types of Transformer

4.16.4Principles of Transformer Operation

4.16.5Energy in Transformers

5Nuclear Physics

5.1The Nuclear Model of the Atom

5.1.1Atomic Model

5.1.2Scattering of Alpha Particles

5.1.3Subatomic Particles

5.1.4Proton & Nucleon Numbers

5.1.5Calculating Number of Neutrons

5.1.6Isotopes

5.1.7More Isotopes

5.1.8Carbon Nuclides

5.1.9Nuclear Fusion

5.1.10Nuclear Fission

5.1.11More Nuclear Fission

5.2Radioactivity: Detection of Radioactivity

5.2.1Radioactivity

5.2.2Types of Radiation

5.2.3Detection of Radioactivity

5.2.4Background Radiation

5.3Radioactivity: The Three Types of Nuclear Emission

5.3.1Three Types of Emission

5.3.2Relative Ionising Effects

5.3.3Electric Fields

5.3.4Magnetic Fields Radiation

5.4Radioactive Decay

5.4.1Radioactive Decay

5.4.2Decay Equations

5.4.3Half Life

5.4.4Measuring Half-Life

5.4.5Practical Applications of Radiation

5.4.6Ionising Radiation

5.4.7Safety Precautions

6Space Physics

6.1The Earth & the Solar System

6.1.1The Earth

6.1.2The Moon

6.1.3The Solar System & The Accretion Model

6.1.4Gravitational Field & Speed of Light

6.1.5The Sun's Gravitational Field

6.2Stars & the Universe

6.2.1The Sun as a Star

6.2.2Galaxies

6.2.3Life Cycle of a Star like the Sun

6.2.4Life Cycle of a Star Much Bigger than the Sun

6.2.5Creation of Elements

6.2.6Red-shift

6.2.7The Big Bang

6.2.8Cosmic Microwave Background Radiation

6.2.9The Hubble Constant

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