Current, Voltage & E.M.F.
Resistors & Circuits
Electric Current
Electric current is the flow of electrons in a conductor. A conductor can be any material (usually a metal) that has an atomic structure that allows electrons to be easily detached from their parent atom by an electric force (which we call a voltage or an elecric potential). These "free electrons", which are naturally negatively charged are attracted towards a positive electric charge. This movement is called ELECTRON FLOW is what we call a flow of current. So current flows from the negative terminal to the positive terminal in an electrical circuit. We could also consider that the atoms that are now short of the negatively charged electrons attracted away by the electric potential must be positively charged. Un this state they are called positive ions and they will be attracted towards a negative electric charge. Therefore we can also consider current (in the form of positive ions) flowing from positive to negative.
Before the discovery of the electron at the end of the nineteenth century it was thought that current only flowed from positive to negative. Now we know that it depends whether you consider current to be due to the movement of electrons or to the movement of positive ions. Both are correct and both ways of considering current can be used in practice.
To make sure we know which way current flows we say that;
CONVENTIONAL CURRENT (the original idea) FLOWS FROM POSITIVE TO NEGATIVE, but...
ELECTRON FLOW TAKES PLACE FROM NEGATIVE TO POSITIVE
Whether you consider current flowing from negative to positive or from positive to negative depends in many cases on where you live. In the USA text books and digrams show current flowing from negative to positive (electron flow) In Europe current is considered to flow from positive to negative( called Conventional Current flow). Which system you use doesn´t really matter, so long as you know which system you are using! For most purposes at www.learnabout-electronics.org we will use CONVENTIONAL CURRENT for our explanations of how a circuits work, only using electron flow when the flow of current is entirely or mostly made up of moving electrons only. (As it is in devices such as transistors). Therefore, unless specifically stated otherwise you can assume that current flows from positive to negative. This flow is normally shown in diagrams by a small arrow head placed on the conductor and labelled I as illustrated below.
Indicating Current Flow in a Simple circuit
Current is measured in Amperes, (often abbreviated as "Amps") or commonly in milliAmperes or microAmperes in electronic circuits.
An Ampere can be defined as;
The amount of electric charge, measured in Coulombs which passes a given point in a circuit, per second.
1 Ampere = 1 Coulomb per second.
1 Coulomb is the amount of charge carried by approximately 6.24150948 x 1018 electrons, or to be a little more exact: 6,241,509,479,607,717,888 electrons!
The measurement of the Ampere is not made, believe it or not, by sitting there and counting electrons! It is actually defined by calculating the force exerted between the magnetic fields around two parallel wires. If you are really keen to get into the numbers and method of defining the Ampere try this link to NIST, the National Institute for Standards and Technology at the U.S. Department of Commerce.
Voltage and E.M.F.
Whenever a current is flowing, a voltage must be present. Voltage is sometimes described as an electrical pressure; the force that drives current through the circuit, just as water pressure drives water around a circulating pipe. In electrical terms, a voltage is actually the difference in electric charge at two points in a circuit. This difference in charge at two points will always try and equalise by causing the electrons to flow around the circuit. With no potential difference between different points in a circuit there will be no currrent flow. Equally if there are potential differences, but the circuit is incomplete (i.e. there is a break in the circuit) there will be no current.
What causes the charge difference is therefore the force which drives our circuit. This may be a device such as a CELL or a BATTERY (a battery is just several interconnected cells) or alternatively we may derive our source of electric potential from the mains (line) supply. Whatever the source of energy used we can call the driving force for our circuit current the ELECTRO-MOTIVE FORCE or E.M.F.
We only use the term E.M.F. to describe that difference in charge or difference in "voltage" that is the actual source of power for our circuit. Differences in voltage between any other points in the circuit are called "potential differences" (abbreviated to p.d). Both EMF and potential difference are measured in Volts and so are often both (inaccurately) called "voltages".In addition voltages may each sometimes be labelled E just to add to the confusion. Strictly speaking:
The Electrical potential which drives the circuit is called an EMF (measured in volts) and labelled E.
The difference in electric potential between any other two points in the circuit is called a POTENTIAL DIFFERENCE or p.d.(also measured in Volts) and labelled V.
A voltage in a circuit (either EMF or potential difference) may be shown by an arrow alongside the two points in the circuit (often the two ends of a component) at which the potential difference or EMF exists. Conventionally the arrow head is at the more positive potential. Multiple voltages and currents may be labelled V1, V2, I1, I2 etc. as shown below.
Labelling Voltages and Currents.
