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Lecture

Current and Conservation of Charges: Moving Charges

In course

PHYS-201(d): General physics: electromagnetism

The topics covered by the course are concepts of electromagnetism, properties of waves, and fluid mechanics. ...

Description

This lecture covers the definition of currents from moving charges, the continuity equation of charge currents, the difference between perfect conductors and metals, Ohm's law, the Drude model with relaxation/scattering times and mobility, electromotive force, Kirchhoff's laws, and the analysis of circuits based on resistors and capacitors. It also explains the redistribution of electrons in a conductor when a charged rod is brought close, the concept of current density, stationary current, and charge conservation.

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Instructor

Dirk Grundler

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Related concepts (30)

Electromotive force

In electromagnetism and electronics, electromotive force (also electromotance, abbreviated emf, denoted or ) is an energy transfer to an electric circuit per unit of electric charge, measured in volts. Devices called electrical transducers provide an emf by converting other forms of energy into electrical energy. Other electrical equipment also produce an emf, such as batteries, which convert chemical energy, and generators, which convert mechanical energy. This energy conversion is achieved by physical forces applying physical work on electric charges.

Kirchhoff's circuit laws

Kirchhoff's circuit laws are two equalities that deal with the current and potential difference (commonly known as voltage) in the lumped element model of electrical circuits. They were first described in 1845 by German physicist Gustav Kirchhoff. This generalized the work of Georg Ohm and preceded the work of James Clerk Maxwell. Widely used in electrical engineering, they are also called Kirchhoff's rules or simply Kirchhoff's laws. These laws can be applied in time and frequency domains and form the basis for network analysis.

Ohm's law

Ohm's law states that the current through a conductor between two points is directly proportional to the voltage across the two points. Introducing the constant of proportionality, the resistance, one arrives at the three mathematical equations used to describe this relationship: where I is the current through the conductor, V is the voltage measured across the conductor and R is the resistance of the conductor. More specifically, Ohm's law states that the R in this relation is constant, independent of the current.

Magnetomotive force

In physics, the magnetomotive force (abbreviated mmf or MMF, symbol ) is a quantity appearing in the equation for the magnetic flux in a magnetic circuit, Hopkinson's law. It is the property of certain substances or phenomena that give rise to magnetic fields: where Φ is the magnetic flux and is the reluctance of the circuit. It can be seen that the magnetomotive force plays a role in this equation analogous to the voltage V in Ohm's law, V = IR, since it is the cause of magnetic flux in a magnetic circuit: where N is the number of turns in the coil and I is the electric current through the circuit.

Magnetic circuit

A magnetic circuit is made up of one or more closed loop paths containing a magnetic flux. The flux is usually generated by permanent magnets or electromagnets and confined to the path by magnetic cores consisting of ferromagnetic materials like iron, although there may be air gaps or other materials in the path. Magnetic circuits are employed to efficiently channel magnetic fields in many devices such as electric motors, generators, transformers, relays, lifting electromagnets, SQUIDs, galvanometers, and magnetic recording heads.

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