1. Current Electricity
JOHN PARKINSON

2. ?
Electricity
What is it ?
JOHN PARKINSON

3. 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

4. + + + + + + + + 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

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

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

7. + + + 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

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

16. 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

17. 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

18. 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

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