Theory of electric current Flashcards
Electricity, when flowing along a wire (known as a conductor) is called a
current
What are Amperes
a measure of the number of electrons passing a particular point in a conductor. This rate of flow is measured in units called amperes (symbol A).
in basic terms what are Volts
Pressure must be provided to cause the electrons to flow and this pressure, which may be derived from a number of sources, is termed the applied voltage or electromotive force (EMF). This is measured in volts (symbol V); the greater the applied voltage, the greater the current flowing
describe insulator s and conductors
Electricity is always trying to find a path to earth, that is, to escape from its conductor and reach the ground or a conducting path which is connected to the ground. Some materials offer such a high resistance to the flow of electricity that the current cannot force its way along them; they are then said to act as insulators. Other materials offer little resistance and are said to be good conductors; copper and aluminium are two examples of good conduc- tors and so are used extensively for electric cables. Water is also a good conductor when impure, so a firefighter with wet clothing or holding wet hose who touches a live conductor could form an electrical path to earth and receive a shock which could be fatal
Most insulated cable nowadays is …. and the advantages
oil impregnated paper or by PVC (polyvinyl chloride) or other plastics such as PCP (polychloroprene) or CSP (chloro-sulphonated polyethylene). These plastics are extremely durable and, whilst not strictly non-flammable, will only burn whilst a source of heat such as a naked flame is continuously applied
What cables are laid in hot places, such as near furnaces or boilers or in situations where circuit integrity is vital e.g., alarms also used in general situations where extra physical protection is required. what are they made of
MICS (mineral insulated copper-sheathed) or alternatively MICC (mineral insulated copper-clad) cables A mineral powder (generally magnesium oxide) within a copper sheathing
Describe electric current as a flow of electrons
The electric current flowing is actually the movement of the atoms electrons, negativley cherged they move from crowded areas to areas with fewer electrons. Electrons move from positive to negative Volts rae the pressure Amps the flow at any point on the conductor
there are 2 basic types of electricity what are they
AC DC
main differences between AC and DC give examples of uses
DC flows in 1 direction only Easly looses energy with distance frequency is zero batteries, wind turbines, pylons AC changes direction easy to step up or down with transformer safe to carry over long distances frequency is 50 hrtz uk
DEfine and describe Volts
In the water flow analogy sometimes used to explain electric circuits by comparing them to water-filled pipes, voltage (difference in electric potential) is likened to difference in water pressure In any circuit there are components that put energy in to the circuit and components that take energy out. From now on, we will say that any device putting energy into a circuit is providing an electo-motive force (emf) and any device taking it out has a potential difference (pd) across it. Both emf and pd are measured in volts, V, as they describe how much energy is put in or taken out per coulomb of charge passing through that section of the circuit.
Volts equation
V = E - Ir Where: V = pd across the external circuit (V) E = emf of the cell (V) I = current through the cell (A) r = value of the internal resistance (Ω) (Ir = the p.d. across the internal resistor) V is sometimes called the terminal pd as it is the pd across the terminals of the cell
Define the Watt and the equation
The unit, defined as one joule per second, measures the rate of energy conversion or transfer. Joules coz its worh done v = W A
equation for current
I = C ( ammount of charge ) T ( ammount of time ) Basically the number of electrons per second ! I is current, C is Coulombs, T is time secs answer in Amps Each electron carries a charge measured in Coulombs Current is simply the flow of charge
