© 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 Positive Ions Negative Ions Positive Holes.

©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