BBC Bitesize - GCSE Combined Science - Electric circuits - AQA - Revision 2
This formula is derived from Ohm's law. Where we have: V: voltage. I: current If rate of flow of charge is not constant then the current at any instant is given by the emf of the source is exactly equal to the potential difference between its ends. Electric power basic formulas calculator voltage current mathematical . For a temporally constant power, the relationship between the charge and current. Charge, Current & Potential Difference in circuits. Conventional current flows around a circuit from the positive (+) side of the potential difference equation.
The direction of current is taken as the direction in which positive charges move. In conduction although the current is only due to electrons, the current was earlier assumed to be due to positive charges flowing from the positive of the battery to the negative.
The direction of current therefore is taken as opposite to the flow of electrons. If current is constant: This law is based on the principle of conservation of charge and states that in an electrical circuit or network of wires the algebraic sum of currents meeting at a point is zero.
- Electric circuits - AQA
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The arrow head marked in circuit represents the direction of conventional current i. The algebraic sum of the product of the current and resistance in any closed loop of a circuit is equal to the algebraic sum of electromotive forces acting in that loop. The average flow of electrons in a conductor not connected to battery is zero i. Thus no current flows through the conductor until it is connected to the battery.
Drift velocity of free electrons in a metallic conductor In the absence of an electric field, the free electrons in a metal randomly in all directions and therefore their average velocity are zero. When an electric field is applied, they are accelerated opposite to the direction of the field and therefore they have a net drift in that direction.
However, due to frequent collisions with the atoms, their average velocity is very small. This average velocity with which the electrons move in a conductor under a potential difference is called the drift velocity.
It is characterized by the Meissner effectthe complete ejection of magnetic field lines from the interior of the superconductor as it transitions into the superconducting state. The occurrence of the Meissner effect indicates that superconductivity cannot be understood simply as the idealization of perfect conductivity in classical physics.
Semiconductor In a semiconductor it is sometimes useful to think of the current as due to the flow of positive " holes " the mobile positive charge carriers that are places where the semiconductor crystal is missing a valence electron. This is the case in a p-type semiconductor. A semiconductor has electrical conductivity intermediate in magnitude between that of a conductor and an insulator.
In the classic crystalline semiconductors, electrons can have energies only within certain bands i.
Energetically, these bands are located between the energy of the ground state, the state in which electrons are tightly bound to the atomic nuclei of the material, and the free electron energy, the latter describing the energy required for an electron to escape entirely from the material. The energy bands each correspond to a large number of discrete quantum states of the electrons, and most of the states with low energy closer to the nucleus are occupied, up to a particular band called the valence band.
Electric current - Wikipedia
Semiconductors and insulators are distinguished from metals because the valence band in any given metal is nearly filled with electrons under usual operating conditions, while very few semiconductor or virtually none insulator of them are available in the conduction band, the band immediately above the valence band. The ease of exciting electrons in the semiconductor from the valence band to the conduction band depends on the band gap between the bands.
The size of this energy band gap serves as an arbitrary dividing line roughly 4 eV between semiconductors and insulators. With covalent bonds, an electron moves by hopping to a neighboring bond. The Pauli exclusion principle requires that the electron be lifted into the higher anti-bonding state of that bond.
For delocalized states, for example in one dimension — that is in a nanowirefor every energy there is a state with electrons flowing in one direction and another state with the electrons flowing in the other. For a net current to flow, more states for one direction than for the other direction must be occupied.Charge, Current and Voltage - GCSE Physics - Doodle Science
For this to occur, energy is required, as in the semiconductor the next higher states lie above the band gap. Often this is stated as: However, as a semiconductor's temperature rises above absolute zerothere is more energy in the semiconductor to spend on lattice vibration and on exciting electrons into the conduction band.
The current-carrying electrons in the conduction band are known as free electrons, though they are often simply called electrons if that is clear in context.
Charge, Current & Potential Difference
Current density and Ohm's law Main article: Current density Current density is a measure of the density of an electric current. It is defined as a vector whose magnitude is the electric current per cross-sectional area. In SI unitsthe current density is measured in amperes per square metre.