Current Electricity © JOHN PARKINSON

? Electricity What is it ? © JOHN PARKINSON

E L E C T R I C C U R R E N T A F l o w o f C h a r g e Electrons Positive Ions Negative Ions A F l o w o f C h a r g e Positive Holes © JOHN PARKINSON

+ + + + + + + + Current (positive charge) flows from POSITIVE to NEGATIVE +ve -ve + + + + + + + + Charge Q is measured in Coulombs [ symbol C ] Current I is measured in Amps [ symbol A] CURRENT = RATE OF FLOW OF CHARGE © JOHN PARKINSON

Conventional flow of Current in a Circuit + - I I © JOHN PARKINSON

Electrons flow from NEGATIVE to POSITIVE Conventional Current - I © JOHN PARKINSON

+ + + AMPS = COULOMBS per SECOND AMPS = COULOMBS per SECOND PER SECOND CURRENT = THE CHARGE FLOWING + + + PER SECOND AMPS = COULOMBS per SECOND AMPS = COULOMBS per SECOND © JOHN PARKINSON

e.g. Cell [battery], Generator or Thermocouple REQUIREMENTS FOR A CURRENT TO FLOW 1. A Conducting Path 2. A Source of Potential Difference – p.d. i.e. a source of VOLTS - V A source of volts is referred to as an e.m.f. …… an Electro Motive Force source e.g. Cell [battery], Generator or Thermocouple COLD cold hot V © JOHN PARKINSON

RELATIONSHIP BETWEEN I and V Applying a Potential Difference between two points on a conductor produces a current Over a limited range of V, the current is sometimes proportional to the voltage. Another way to look at current is the charge that moves through a material with resistance R across which is placed a potential difference V. This is sometimes known as Ohm’s Law although it is not a Law of Physics as such, it is rather a limited term relationship that describes what happens over a limited range of V for some materials. Nevertheless it does introduce the concept of RESISTANCE which is the ability of an object to allow charges to pass thought it in response to an applied potential difference. The units of R are ohms. V = I R R is the Resistance in the circuit In Ohms (Ω) © JOHN PARKINSON

V = I R Resistance can be thought of as a proportionality constant between I and V if Ohm’s Law applies. It is also the opposition to the flow of current. OHM’s LAW “The current through an ohmic conductor is directly proportional to the potential difference across it, provided there is no change in physical conditions e.g. temperature.” V I R © JOHN PARKINSON

Ohmic and non-ohmic conductors Ohmic eg resistor or a piece of wire -V V Only a few materials exhibit ohmic behaviour (see the blue line on the graph) - this means “i” increases in proportion to increasing applied “V”. Some materials like wood, living tissue, plastics and even air are not ohmic. Some material combinations like pairs of semiconductors (eg Germanium/Silicon) exhibit a V - i relationship such as shown by the yellow curve. This is in effect an on-off type of material. That means they will pass current in one direction but not the other. This special property makes them extremely useful in semiconductor switches such as are found in computers and transistors that make up solid state amplifiers or mobile phones. Gradient -I © JOHN PARKINSON

Ohmic and non-ohmic conductors Non-ohmic eg diode -V V switch on pd for silicon = 0.6V reverse bias forward bias Only a few materials exhibit ohmic behaviour (see the blue line on the graph) - this means “i” increases in proportion to increasing applied “V”. Some materials like wood, living tissue, plastics and even air are not ohmic. Some material combinations like pairs of semiconductors (eg Germanium/Silicon) exhibit a V - i relationship such as shown by the yellow curve. This is in effect an on-off type of material. That means they will pass current in one direction but not the other. This special property makes them extremely useful in semiconductor switches such as are found in computers and transistors that make up solid state amplifiers or mobile phones. -I © JOHN PARKINSON

Ohmic and non-ohmic conductors eg filament bulb -V V Only a few materials exhibit ohmic behaviour (see the blue line on the graph) - this means “i” increases in proportion to increasing applied “V”. Some materials like wood, living tissue, plastics and even air are not ohmic. Some material combinations like pairs of semiconductors (eg Germanium/Silicon) exhibit a V - i relationship such as shown by the yellow curve. This is in effect an on-off type of material. That means they will pass current in one direction but not the other. This special property makes them extremely useful in semiconductor switches such as are found in computers and transistors that make up solid state amplifiers or mobile phones. -I © JOHN PARKINSON

The Resistor Colour Code RESISTORS Circuit symbol R Colour codes are used to identify resistance value The four colour code bands are at one end of the component. Counting from the end, the first three (or sometimes four or five) bands give the resistance value and the last the tolerance The Resistor Colour Code Colour Number Black Brown 1 Red 2 Orange 3 Yellow 4 Green 5 Blue 6 Violet 7 Grey 8 White 9 TOLERANCES BROWN 1% RED 2% GOLD 5% SILVER 10% NONE 20% Resistors are represented in circuit diagrams using the well known zig-zag symbol. In many circuit diagrams the “W” symbol is omitted and resistor values are often shown as numbers eg 230 for 230 ohms or 230k for 230 thousand ohms. Sometimes “K” is used for kilo ohms as well. Discrete resistors are identified by a colour code. These are internationally accepted stripes of colours coded to numeric vales with the first 2 or 3 representing numbers, the 3rd or 4th stripe represent an order of magnitude and the last one a tolerance or uncertainty. Most texts list all the colours and numbers. © JOHN PARKINSON

A A B A = ? 47 OOO  , 1% Tolerance B = ? 47 OOO  , 1% Tolerance © JOHN PARKINSON

r depends on the type of material and temperature Resistivity (r) l How does resistance depend upon length? R µ l A How does resistance depend upon cross sectional area? R µ 1/A Units of r = ohm m (Wm) Resistance is the property of a specific component or object. A more general property of a material is the concept of resistivity which is the resistance-length (Wm) for a specific material. We all know that longer objects will have greater R, and that objects with narrower cross sections also have greater R. These two factors are combined with the resistivity factor “r” (NB this is the Greek letter “rho”) which is specific to a material at a given temperature. If we know the length, area and resistivity of a material we can design an object of known R. This is how many resistors are built. EG: What is the resistance of a copper “electrical kettle cord” of length 1 m, cross sectional area 2 x 10-6 m2 and a resistivity of 2 x 10-8 Wm - if you do this correctly you should get about 0.01 W . This low value is what you would expect for a such a device, after all a high resistance would affect the operation of the kettle. r depends on the type of material and temperature © JOHN PARKINSON

Variation of Resistance [resistivity] with temperature Temperature / 0C Resistance /  insulator semiconductor metal Metal – more ion vibration impedes electrons Semiconductor – more ion vibration outweighed by more charge carriers Insulator – thermal energy releases more charge carriers © JOHN PARKINSON

Power, Voltage and Current The Current indicates how many Coulombs flow each Second The Voltage indicates how much energy each coulomb carries The Energy carried is measured in Joules So work done W = Q V ( = I t V ) Joules The energy carried per second is the POWER The power (in joules per second) = coulombs per second x joules per coulomb Current (I) Voltage (V) A joule per second is called a watt ( W ) P = I V P = I V © JOHN PARKINSON

PLUGS AND FUSES live neutral earth 3 A 5 A 13 A Light Sound Heat Motion up to 720W 720-3000W © JOHN PARKINSON