Electric Current (I)
Section E: Electricity & Magnetism
Static Charge & Electric Current
Static Charge & Electric Current
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The metallised ball will swing quickly to and fro
between the plates. As this occurs the current meter
reads an electric current passing through it. The
frequency of the swings can be increased by moving
the metal plates closer together; the effect is to
increase the reading on the ammeter.
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The positive charge on the van de Graaff generator
dome flows along the wire conductor giving the metal
plate A a positive charge. The positive charge on plate
A induces and equal and opposite charge on plate B
by attracting negative electrons to the surface of B.
Static Charge & Electric Current
Static Charge & Electric Current
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When the metallised ball touches plate A, it becomes
positively charged (due to loss of electrons) and is
immediately repelled from plate A and attracted to
plate B. (We could say that in the electric field
between the two plates, a force acts on the ball
towards B.)
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On touching plate B the ball gives up its positive
charge and collects a negative charge which is
immediately repelled from plate B and attracted to
plate.
Static Charge & Electric Current
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Thus each swing of the ball transfers electric charge between
plates. The swinging ball completes a continuous path, round
which electric charge is made to flow by the generator. The
path of the positive charge can be traced from the dome of the
generator round the circuit returning finally to the dome of the
generator again. It is found that electric charge can flow
continuously only round unbroken conducting paths called
closed circuits.
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This demonstration confirms that the static charge carried by
the ball is the same charge which flows through the meter as
an electric current. Static charge and the charge which flows in
an electric current have the same origin – the electron.
Electric Current (I)
Conductors
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Metals are good conductors of electricity, because their
atoms have one or more loosely held (free) electrons in their
outer shells. These electrons are easily shifted from atom to
atom in the metal structure, and are considered to be equally
shared among all the atoms.
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However, unless a potential difference is applied, the free
electrons drift randomly through the structure and there is no
net movement of charge in any one direction.

When a potential difference is applied across the ends of a
conductor, the electrons drift or produce a flow of negative
charge towards a higher potential (positive potential), that is,
from the negative to the positive terminal of a power supply.
To sustain an electric current, charge needs a continuous path
or circuit to flow around.
Electric Current (I)
Examples of Conductors:
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Metals
Graphite
Electrolyte
Water
Electric Current (I)
Insulators
In some materials, all electrons, including the
outer ones, are tightly bound and are therefore
not free. In these materials, called insulators,
the electrons do not move from atom to atom
under normal conditions. Large amounts of
energy must be supplied to free the electrons
from the attractive forces of the nucleus.
Electric Current (I)
Conventional Current

Before electrons were discovered, scientists agreed
to think of current as the flow of positive charge,
moving through a circuit from the positive to the
negative terminal of a battery.

This convention is still used today to explain
concepts in electricity and magnetism. So, arrows are
included on the wires in circuit diagrams, pointing
out of the positive terminal and towards the negative
terminal of the battery
Electric Current (I)
Electric Current (I)
Electric Current (I) is the rate of flow of electric charge.
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Electric Current (I) is measured in Amperes (A).
A current of 1 ampere is a flow of charge at the rate of 1
Coulomb per second.
The ampere is that constant current which, if maintained in
two straight parallel conductors of infinite length, of negligible
circular cross-section, and placed 1 meter apart in vacuum,
would produce between these conductors a Force of 2x10-7N
per meter of length.
Electric Current (I)
Quantity of Electric Charge (Q) is measured in Coulombs (C).
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A Coulomb is the charge which flows in 1 second past any
point in a circuit in which there is a steady current of 1
ampere.
Each electron has a minute charge of -1.6x10-19C, so about
6x1018 electrons are needed to make up just 1 coulomb of
charge.
Q = It
Electric Current (I)
Charge Carriers
Type of Charge
Polarity
Type of Current
Where applicable
1.6x10-19
negative
Electron flow
In metallic conductors, graphite and
n-type semiconductors
Ions
Multiples of the
charge on the
electron
negative
or
positive
Ion current
In gases and liquids, including
electrolyte
holes
1.6x10-19
positive
hole
In p-type semiconductors
Electrons
Charge / C
Electric Current (I)
Electric Current is measured with an ammeter.
The ammeter should:
 Have Zero Resistance (R)
 Not change the current at all
 Be connected in series in the circuit, at any
location.

Electric Current (I)
Current Through Series Circuits: The current is
the same throughout the circuit.
Electric Current (I)
Current Through Parallel Circuits:
 The current splits at the junction, where the sum of the current
through each branch equals the total current entering/exiting
the junction.
 IT = I 1 + I 2 + I 3 + …
 Kirchoff’s First Law:
The total current entering a junction in a circuit must equal the
total current exiting it.
Electric Current (I)
Effects of Current:
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Heating & Lighting
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Magnetic
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Chemical
Electric Current (I)
Direct Current
Fixed direction
Alternating Current
Direction alternates
May be steady in magnitude from a Varies between reversals of polarity
given source
Its value cannot be stepped up/down
Its value can be stepped up or stepped
down by a transformer
Has an associated heating effect
Has an associated magnetic effect
Can be used in electrolysis, Cannot be used
electroplating and battery charging
---
Can be converted
rectification
to
dc
using
Electric Current (I)
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Direct Current (dc)
In direct current the electrons flow in one
direction only.
Electric Current (I)
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Alternating Current (ac)
In alternating current the electron flow
reverses regularly or periodically.
Electric Current (I)

The maximum value for an ac or alternating
voltage is known as its peak value.
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Frequency (f) of an Alternating Current is the
number of complete alternations or cycles
made in one second.
Electric Current (I)
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Root Mean Square (rms) Values
The peak value of an ac (or alternating V) is only
reached momentarily, and is therefore greater than the
effective value of the supply.
A simple average or mean value of an ac would
give a zero (0) result, because during each revolution
the current has equal positive and negative values.
Instead ac is measured by its effective or root mean
square (rms) value.
The effective or root mean square (rms) value is
the steady direct current (or voltage) which would
give the same heating effect.
Electric Current (I)
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The root mean square value of current is that
value of alternating current which will give the
same heating effect had it been steady direct
current.
Electric Current (I)
Electric Current (I)
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Diodes
Semiconductors are solids whose electrical conductance lies
between that of good conductors & insulators. A semiconductors
diode is made by special processes which increase the conductance
of semiconductors. Two different types of semiconductors are then
joined forming a ‘junction’. At this junction, current will flow in
only one direction. Germanium & Silicon are best known as
semiconductor elements.
Diodes, when forward biased, have reasonably low resistance and
conduct an electric current. The reverse-biased diode has high
resistance and negligible current passes. So, diodes function as oneway electrical valves.
Electric Current (I)
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Overheating and other unsuitable treatment will
‘destroy’ diode properties.
* An ‘open diode’ has infinite resistance in both
directions. It does not conduct electricity in either
direction.
* A diode which has been shorted has low
resistance in both directions. It conducts equally
well in both directions.
Electric Current (I)
Rectification
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Semiconductor diodes are used to rectify ..
ie to convert ac to dc.
 This is called rectification.
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A rectifier in an ac circuit cuts off the negative
half-cycles of the current.
This is called half-wave rectification.
Electric Current (I)
Electric Current (I)
Electric Current (I)
Electric Current (I)
Electric Current (I)
Electric Current (I)
Sources of dc:
 cells / batteries
 dc generators
Sources of ac:
 ac generators
 ac is better for the transmission of power
through the mains
Electric Current (I)
The End
Section E: Electricity & Magnetism