- Chapter 1: Understanding the physical universe
- Chapter 2 Using mathematical tools in physics
- 2.1 Applying the scientific method
- 2.2 The use of variables to represent displacement and time
- 2.3 Representation of data
- 2.4 The use of differentiation in analysis: velocity and acceleration in linear motion
- 2.5 The use of integration in analysis
- 2.6 Maximum and minimum values of physical quantities: general linear motion
- 2.7 Angular motion: the radian
- 2.8 The role of mathematics in physics
- Chapter 2 Problems
- Chapter 3 The causes of motion: dynamics
- 3.1 The concept of force
- 3.2 The first law of dynamics (Newton's first law)
- 3.3 The fundamental dynamical principle (Newton's second law)
- 3.4 Systems of units: SI
- 3.5 Time dependent forces: oscillatory motion
- 3.6 Simple harmonic motion
- 3.7 Mechanical work and energy
- 3.8 Plots of potential energy functions
- 3.9 Power
- 3.10 Energy in simple harmonic motion
- 3.11 Dissipative forces: damped harmonic motion
- 3.12 Forced oscillations
- 3.13 Non-linear dynamics: chaos
- 3.14 Phase space representation of dynamical systems
- Chapter 3 Problems
- Chapter 4 Motion in two and three dimensions
- 4.1 Vector physical quantities
- 4.2 Vector algebra
- 4.3 Velocity and acceleration vectors
- 4.4 Force as a vector quantity: vector form of the laws of dynamics
- 4.5 Constraint forces
- 4.6 Friction
- 4.7 Motion in a circle: centripetal force
- 4.8 Motion in a circle at constant speed
- 4.9 Tangential and radial components of acceleration
- 4.10 Hybrid motion: the simple pendulum
- 4.11 Angular quantities as vectors: the cross product
- Chapter 4 Problems
- Chapter 5 Force fields
- 5.1 Newton's law of universal gravitation
- 5.2 Force fields
- 5.3 The concept of flux
- 5.4 Gauss' lew for gravitation
- 5.5 Applications of Gauss’ law
- 5.6 Motion in a constant uniform field: projectiles
- 5.7 Mechanical work and energy
- 5.8 Power
- 5.9 Energy in a constant uniform field
- 5.10 Energy in an inverse square law field
- 5.11 Moment of a force: angular momentum
- 5.12 Planetary motion: circular orbits
- 5.13 Planetary motion: elliptical orbits and Kepler's laws
- Chapter 5 Problems
- Chapter 6 Many-body interactions
- 6.1 Newton's third law
- 6.2 The principle of conservation of momentum
- 6.3 Mechanical energy of systems of particles
- 6.4 Particle decay
- 6.5 Particle collisions
- 6.6 The centre of mass of a system
- 6.7 The two-body problem: reduced mass
- 6.8 Angular momentum of systems of particles
- 6.9 Conservation principles in physics
- Chapter 6 Problems
- Chapter 7 Rigid-body dynamics
- 7.1 Rigid bodies
- 7.2 Rigid bodies in equilibrium: statics
- 7.3 Torque
- 7.4 Dynamics of rigid bodies
- 7.5 Measurement of torque: the torsion balance
- 7.6 Rotation of a rigid body about a fixed axis: moment of inertia
- 7.7 Calculation of moments of inertia: the parallel axis theorem
- 7.8 Conservation of angular momentum of rigid bodies
- 7.9 Conservation of mechanical energy in rigid body systems
- 7.10 Work done by a torque: torsional oscillations: rotational power
- 7.11 Gyroscopic motion
- 7.12 Summary - connection between rotational and translational motions
- Chapter 7 problems
- Chapter 8 Relative motion
- 8.1 Applicability of Newton's laws of motion: inertial reference frames
- 8.2 The Galilean transformation
- 8.3 The CM (centre-of-mass) reference frame
- 8.4 Example of a non-inertial frame: centrifugal force
- 8.5 Motion in a rotating frame: the Coriolis force
- 8.6 The Foucault pendulum
- 8.7 Practical criteria for inertial frames: the local view
- Chapter 8 problems
- Chapter 9 Special relativity
- 9.1 The velocity of light
- 9.2 The Principle of Relativity
- 9.3 Consequences of the Principle of Relativity
- 9.4 The Lorentz transformation
- 9.5 The Fitzgerald-Lorentz contraction
- 9.6 Time dilation
- 9.7 Paradoxes in special relativity
- 9.8 Relativistic transformation of velocity
- 9.9 Momentum in relativistic mechanics
- 9.10 Four-vectors: the energy-momentum 4-vector
- 9.11 Energy-momentum transformations: relativistic energy conservation
- 9.12 Relativistic energy: mass-energy equivalence
- 9.13 Units in relativistic mechanics
- 9.14 Mass-energy equivalence in practice
- 9.15 General relativity
- Chapter 9 problems
- Chapter 10 Continuum mechanics: mechanical properties of materials: microscopic models of matter
- 10.1 Dynamics of continuous media
- 10.2 Elastic properties of solids
- 10.3 Fluids at rest
- 10.4 Elastic properties of fluids
- 10.5 Pressure in gases
- 10.6 Archimedes' principle
- 10.7 Fluid dynamics; the Bernoulli equation
- 10.8 Viscosity
- 10.9 Surface properties of liquids
- 10.10 Boyle's law (Mariotte's law)
- 10.11 A microscopic theory of gases
- 10.12 The SI unit of amount of matter; the mole
- 10.13 Interatomic forces: modifications to the kinetic theory of gases
- 10.14 Microscopic models of condensed matter systems
- Chapter 10 problems
- Chapter 11 Thermal physics
- 11.1 Friction and heating
- 11.2 The SI unit of thermodynamic temperature; the kelvin
- 11.3 Heat capacities of thermal systems
- 11.4 Comparison of specific heat capacities: calorimetry
- 11.5 Thermal conductivity
- 11.6 Convection
- 11.7 Thermal radiation
- 11.8 Thermal expansion
- 11.9 The first law of thermodynamics
- 11.10 Change of phase: latent heat
- 11.11 The equation of state of an ideal gas
- 11.12 Isothermal, isobaric and adiabatic processes: free expansion
- 11.13 The Carnot cycle
- 11.14 Entropy and the second law of thermodynamics
- 11.15 The Helmholtz and Gibbs functions
- Chapter 11 problems
- Chapter 12 Microscopic models of thermal systems: kinetic theory of matter
- 12.1 Microscopic interpretation of temperature
- 12.2 Polyatomic molecules: principle of equipartition of energy
- 12.3 Ideal gas in a gravitational field: the ‘law of atmospheres’
- 12.4 Ensemble averages and distribution functions
- 12.5 The distribution of molecular velocities in an ideal gas
- 12.6 Distribution of molecular speeds
- 12.7 Distribution of molecular energies; Maxwell-Boltzmann statistics
- 12.8 Microscopic interpretation of temperature and heat capacity in solids
- Chapter 12 problems
- Chapter 13 Wave motion
- 13.1 Characteristics of wave motion
- 13.2 Representation of a wave which is travelling in one dimension
- 13.3 Energy and power in a wave motion
- 13.4 Plane and spherical waves
- 13.5 Huygens’ principle: the laws of reflection and refraction
- 13.6 Interference between waves:
- 13.7 Interference of waves passing through openings: diffraction
- 13.8 Standing waves
- 13.9 The Doppler effect
- 13.10 The wave equation
- 13.11 Waves along a string
- 13.12 Waves in elastic media: longitudinal waves in a solid rod
- 13.13 Waves in elastic media: sound waves in gases
- 13.14 Superposition of two waves of slightly different frequencies: wave and group velocities
- 13.15 Other waveforms: Fourier analysis
- Chapter 13 problems
- Chapter 14 Introduction to quantum mechanics
- 14.1 Physics at the beginning of the twentieth century
- 14.2 The blackbody radiation problem; Planck’s quantum hypothesis
- 14.3 The specific heat capacity of gases
- 14.4 The specific heat capacity of solids
- 14.5 The photoelectric effect
- 14.6 The X-ray continuum
- 14.7 The Compton effect: the photon model
- 14.8 The de Broglie hypothesis: wave-particle duality
- 14.9 Interpretation of wave-particle duality
- 14.10 The Heisenberg uncertainty principle
- 14.11 The wavefunction: expectation values
- 14.12 The Schrödinger (wave mechanical) method
- 14.13 The free particle
- 14.14 The time-independent Schrödinger equation: eigenfunctions and eigenvalues
- 14.15 The infinite square potential well
- 14.16 Potential steps
- 14.17 Other potential wells and barriers
- 14.18 The simple harmonic oscillator
- 14.19 Further implications of quantum mechanics
- Chapter 14 problems
- Chapter 15 Electric currents
- 15.1 Electric currents
- 15.2 The electric current model; electric charge
- 15.3 The unit of electric current; the ampere
- 15.4 Heating effect revisited: electrical resistance
- 15.5 Strength of a power supply: emf
- 15.6 Resistance of a circuit
- 15.7 Potential difference
- 15.8 Effect of internal resistance
- 15.9 Comparison of emfs: the potentiometer
- 15.10 Multiloop circuits
- 15.11 Kirchhoff's rules
- 15.12 Comparison of resistances: the Wheatstone bridge
- 15.13 Power supplies connected in parallel
- 15.14 Resistivity and conductivity
- 15.15 Variation of resistance with temperature
- Chapter 15 problems
- Chapter 16 Electric fields
- 16.1 Electric charge at rest
- 16.2 Electric fields: electric field strength
- 16.3 Force between point charges: Coulomb's law
- 16.4 Electric flux and electric flux density
- 16.5 Electric fields due to systems of charges
- 16.6 The electric dipole
- 16.7 Gauss’ law for electrostatics
- 16.8 Applications of Gauss’s law
- 16.9 Potential difference in electric fields
- 16.10 Electric potential
- 16.11 Equipotential surfaces
- 16.12 Determination of electric field strength from electric potential
- 16.13 Acceleration of charged particles
- 16.14 The laws of electrostatics in differential form
- Chapter 16 problems
- Chapter 17 Electric fields in materials
- 17.1 Conductors in electric fields
- 17.2 Insulators in electric fields; polarisation
- 17.3 Electric susceptibility
- 17.4 Boundaries between dielectric media
- 17.5 Ferroelectricity and paraelectricity; permanently polarised materials
- 17.6 Uniformly polarised rod; the ‘bar electret’
- 17.7 Microscopic models of electric polarisation
- 17.8 Capacitors
- 17.9 Examples of capacitors with simple geometry
- 17.10 Energy stored in an electric field
- 17.11 Capacitors in series and in parallel
- 17.12 Charge and discharge of a capacitor through a resistance
- 17.13 Measurement of permittivity
- Chapter 17 problems
- Chapter 18 Magnetic fields
- 18.1 Magnetism
- 18.2 The work of Ampère, Biot and Savart
- 18.3 Magnetic pole strength
- 18.4 Magnetic field strength
- 18.5 Ampère's law
- 18.6 The Biot-Savart law
- 18.7 Applications of the Biot-Savart law
- 18.8 Magnetic flux and magnetic flux density
- 18.9 Magnetic fields of permanent magnets; magnetic dipoles
- 18.10 Forces between magnets; Gauss’ law for magnetism
- 18.11 The laws of magnetostatics in differential form
- Chapter 18 problems
- Chapter 19 Electric currents and moving charges in magnetic fields
- 19.1 Forces between currents magnets
- 19.2 The force between two long parallel wires
- 19.3 Current loop in a magnetic field
- 19.4 Magnetic fields due to moving charges
- 19.5 Force on a moving electric charge in a magnetic field
- 19.6 Applications of moving charges in uniform magnetic fields; the classical Hall effect
- 19.7 Charge in a combined electric and magnetic field; the Lorentz force
- 19.8 Magnetic dipole moments of charged particles in closed orbits
- 19.9 Polarisation of magnetic materials; magnetisation, magnetic susceptibility
- 19.10 Paramagnetism and diamagnetism
- 19.11 Boundaries between magnetic media
- 19.12 Ferromagnetism; the magnetic needle revisited
- 19.13 Moving coil meters and electric motors
- 19.14 Electric and magnetic fields in moving reference frames
- Chapter 19 problems
- Chapter 20 Electromagnetic induction: time-varying emfs
- 20.1 The principle of electromagnetic induction
- 20.2 Simple applications of electromagnetic induction
- 20.3 Self-inductance
- 20.4 The series L-R circuit
- 20.5 Discharge of a capacitor through an inductor and resistor
- 20.6 Time-varying emfs: mutual inductance: transformers
- 20.7 Alternating current (a.c.)
- 20.8 Alternating current transformers
- 20.9 Resistance, capacitance and inductance in a.c. circuits
- 20.10 The series L-C-R circuit: phasor diagrams
- 20.11 Power in an a.c. circuit
- Chapter 20 problems
- Chapter 21 Maxwell’s equations; electromagnetic radiation
- 21.1 Reconsideration of the laws of electromagnetism: Maxwell's equations
- 21.2 Plane electromagnetic waves
- 21.3 Experimental observation of electromagnetic radiation
- 21.4 The electromagnetic spectrum
- 21.5 Polarization of electromagnetic waves
- 21.6 Energy, momentum and angular momentum in electromagnetic waves
- 21.7 The photon model revisited
- 21.8 Reflection of electromagnetic waves at an interface between non-conducting media
- 21.9 Electromagnetic waves in a conducting medium
- 21.10 Invariance of electromagnetism under the Lorentz transformation
- 21.11 Maxwell’s equations in differential form
- Chapter 21 problems
- Chapter 22 Wave optics
- 22.1 Electromagnetic nature of light
- 22.2 Coherence: the laser
- 22.3 Diffraction at a single slit
- 22.4 Two slit interference and diffraction: Young's double slit experiment
- 22.5 Multiple slit interference: the diffraction grating
- 22.6 Diffraction of X-rays: Bragg scattering
- 22.7 The SI unit of luminous intensity, the candela
- Chapter 22 problems
- Chapter 23 Geometrical optics
- 23.1 The ray model: geometric optics
- 23.2 Reflection of light
- 23.3 Image formation by spherical mirrors
- 23.4 Refraction of light
- 23.5 Refraction at successive plane interfaces
- 23.6 Image formation by spherical lenses
- 23.7 Image formation of extended objects: magnification
- 23.8 Dispersion of light
- Chapter 23 problems
- Chapter 24 Atomic Physics
- 24.1 Atomic models
- 24.2 The spectrum of hydrogen: the Rydberg formula
- 24.3 The Bohr postulates
- 24.4 The Bohr theory of the hydrogen atom
- 24.5 The quantum mechanical (Schrödinger) solution of the one-electron atom
- 24.6 Interpretation of the one-electron atom eigenfunctions
- 24.7 Intensities of spectral lines: selection rules
- 24.8 Quantisation of angular momentum
- 24.9 Magnetic effects in one-electron atoms: the Zeeman effect
- 24.10 The Stern-Gerlach experiment: electron spin
- 24.11 The spin-orbit interaction
- 24.12 Identical particles in quantum mechanics: the Pauli exclusion principle
- 24.13 The periodic table: multielectron atoms
- 24.14 The theory of multielectron atoms
- 24.15 Further uses of the solutions of the one-electron atom
- Chapter 24 problems
- Chapter 25 Electrons in solids: quantum statistics
- 25.1 Bonding in molecules and solids
- 25.2 The classical free electron model of solids
- 25.3 The quantum mechanical free electron model of solids: Fermi energy
- 25.4 The electron energy distribution at zero K
- 25.5 Electron energy distributions at T > zero K
- 25.6 Specific heat capacity and conductivity in the quantum free electron model
- 25.7 Quantum statistics: systems of bosons
- 25.8 Superconductivity
- Chapter 25 problems
- Chapter 26 Semiconductors
- 26.1 The band theory of solids
- 26.2 Conductors, insulators and semiconductors
- 26.3 Intrinsic and extrinsic (doped) semiconductors
- 26.4 Junctions in conductors
- 26.5 Junction in semiconductors; the p-n junction
- 26.6 Biased p-n junctions; the semiconductor diode
- 26.7 Photodiodes, particle detectors and solar cells
- 26.8 Light emitting diodes; semiconductor lasers
- 26. 9 The tunnel diode
- 26.10 Transistors
- Chapter 26 problems
- Chapter 27 Nuclear and particle physics
- 27.1 Properties of atomic nuclei
- 27.2 Nuclear binding energies
- 27.3 Nuclear models
- 27.4 Radioactivity
- 27.5 alpha-, beta- and gamma-decay
- 27.6 Detection of radiation: units of radioactivity
- 27.7 Nuclear reactions
- 27.8 Nuclear fission and nuclear fusion
- 27.9 Fission reactors
- 27.10 Thermonuclear fusion
- 27.11 Sub-nuclear particles
- 27.12 The quark model
- Chapter 27 problems
- Errata
15.2 The electric current model; electric charge
The fluid (or hydraulic) model of an electric current is discussed in more detail in the DC Circuit Water Analogy page and the pages that follow on the Hyperphysics website. Many of the cancepts to be addressed and quantities to be defined in this chapter, such as voltage, potential difference and resistance, are introduced here. For a nice interactive illustration see by Molecular Expressions on the MagLab website.
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The convention for direction of flow of an electric current is illustrated in the interactive tutorial by Molecular Expressions on the MagLab website. In the simulation, the direction of the current is shown by the black arrow. Ignore the small yellow blobs moving in the opposite direction to the current.*
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* NOTE
The question of what is actually going on at a microscopic level when an electric current flows is not considered in Understanding Physics at this stage — it will not be addressed until Chapter 25. It is sufficient to say at this point that, when these issues are discussed later, it will turn out the such microscopic 'charge carriers' can be positive (moving in the same direction as the current) or negative (moving in the opposite direction to the current) or both. It so happens that in metals these 'carriers' are negatiuve.