© john parkinson 1 © john parkinson 2 © john parkinson 3 e l e c t r i c c u r r e n t electrons...
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©JOHN PARKINSON
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©JOHN PARKINSON
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E L E C T R I C C U R R E N T
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+
Current (positive charge) flows from
POSITIVE to NEGATIVE
+++++++
Charge Q is measured in Coulombs [ symbol C ]
Current I is measured in Amps [ symbol A]
CURRENT = RATE OF FLOW OF CHARGE
+ve-ve
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+ -I I
Conventional flow of Current in a Circuit
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e-+ve-ve
Electrons flow from NEGATIVE to POSITIVE
Conventional Current - I
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CURRENT =
THE CHARGE FLOWING
PER SECOND
t
QI
AMPS = COULOMBS per SECOND
tIQ
tIQ AMPS = COULOMBS per SECOND
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1. A Conducting Path2. 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
cold hotV
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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.
IV
V = I R
R is the Resistance in the circuit In Ohms (Ω)
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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.
V = I R
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
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Ohmic and non-ohmic conductors
V-V
-I
IOhmic eg resistor
or a piece of wire
Gradient RV
I 1
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Ohmic and non-ohmic conductors
V-V
-I
INon-ohmic eg diode
forward bias
reverse bias
switch on pd for silicon = 0.6V
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Ohmic and non-ohmic conductors
V-V
-I
INon-ohmic
eg filament bulb
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RESISTORS
Colour codes are used to identify resistance value
Circuit symbol R
The ResistorColour Code
Colour Number
Black 0
Brown 1
Red 2
Orange 3
Yellow 4
Green 5
Blue 6
Violet 7
Grey 8
White 9
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
TOLERANCES
BROWN 1%
RED 2%
GOLD 5%
SILVER 10%
NONE 20%
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AA
B
A = ?
B = ?
47 OOO , 1% Tolerance
47 OOO , 1% Tolerance
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Resistivity ()
depends on the type of material and temperature
Units of ohm m (m)
R
How does resistance depend upon length?
How does resistance depend upon cross sectional area?
R 1/A
A
lR
A
lR
AA
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Temperature / 0C
Res
ista
nce
/
Variation of Resistance [resistivity] with temperature
semiconductor
insulator
metal
Metal – more ion vibration impedes electrons
Semiconductor – more ion vibration outweighed by more charge carriers
Insulator – thermal energy releases more charge carriers
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PowerPower, Voltage and Current, Voltage and Current
The CurrentCurrent indicates how many CoulombsCoulombs flow each Second
The VoltageVoltage indicates how much energy each coulomb carries
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 )
The EnergyEnergy carried is measured in JoulesJoules
So work done W = Q V ( = I t V ) JoulesJoules
P = I VP = I V
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PLUGS AND FUSES
3 A 13 A
LightSound
HeatMotion
5 A
up to 720W 720-3000W
liveneutralearth
230
P
V
PI