WHAT IS CURRENT?
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Electrons are charge carriers
Unit of charge is the Coulomb (C)
Current is the rate of flow of charge
1C of negative charge = total charge carried by
6.242×1018 electrons
Charge of 1 electron = 1/ 6.242×1018 = 1.6×10-19C
Charge can either be positive or negative
Electric current exists when there is a net transfer of
charge in a material.
For example: If you inject electrons into a copper wire,
they travel through the wire and emerge at the other
end → current in the wire
• Current = rate at which charge is transferred
• Unit of Current = Ampere (Amp)
• 1A = rate of flow of charge of 1C in 1 second i.e.
• Current (I) = Charge (Q) / Time (s)
1. Current, Voltage and
Resistance
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WHAT IS VOLTAGE?
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To establish a flow of charge through a conductor, we
need to exert a force on the electrons that carry the
charge
This is called electromotive force (emf)
To sustain flow, electrons need a destination such as
the positive and negative terminals of a battery
Unit of EMF is the volt
Named after Alessandra Volta
The greater the voltage of a source of EMF, the greater
the current it can produce
EMF is also called electric potential, which is the
same as talking about the ability (potential) of a voltage
source to produce current
We say E volts across the voltage source or component
Symbol for voltage source and its terminals
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1. Current, Voltage and
Resistance
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WHAT IS RESISTANCE?
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It is the measure to the extent to which a material
interferes with, or resists, the flow of current through it
A conductor (metals) has small resistance, an
insulator (rubber, wood) has high resistance
Semi-conductors (e.g. silicon) not as conductive as
metals, more conductive than insulators – form the
basis for modern electronic devices such as diodes,
transistors and integrated circuits
Unit of resistance is the Ohm (George Ohm) - R
The symbol of the Ohm is Ω
For a perfect conductor, its resistance is 0 Ω
A perfect insulator has a resistance of ∞ Ω
Conventional current needs a complete path to flow
There must be a destination that will accept electrons,
and there must also be a source of electrons
E
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I
R
1. Current, Voltage and
Resistance
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RESISTIVITY
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Resistance of a material depends on its length l, cross
sectional area A, and its resistivity ρ which is
dependent on the material
R=ρ
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l
A
Good conductors have low resistivity. Poor conductors
have high resistivity.
Example Find the resistance of 6.5m of wire with a
diameter of 0.6mm, and a resistivity of 430nΩm.
Example Find the diameter of a wire with resistivity of
50nΩm needed for a resistance of 15Ω.
1. Current, Voltage and
Resistance
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LINEARITY
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An electrical device is linear if its V v I graph is a
straight line
Many circuit analysis techniques can only be applied to
circuits composed of linear devices, such as resistors.
Graph of a linear component such as a resistor, where
the gradient ∆V / ∆I = R
The voltage across a device is directly proportional to
the current through it
V
∆V / ∆I = R
I
1. Current, Voltage and
Resistance
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OHM’S LAW
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The greater the voltage at a source, the greater the
current it can produce
Current produced in a resistor is directly proportional
to the voltage of the source
Resistance reduces the flow of current
Current is inversely proportional to resistance, i.e. the
greater the resistance, the less the current
For a fixed resistance R, the current I increases with an
increase in the voltage V at the source
This is Ohm’s Law, which is a linear relationship
V = IR; I = V/R; R = V/I
V
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24V
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8Ω
∆V /∆I = R
I
Question:
How much voltage is necessary to create a flow of 0.24C
in 0.8s through a resistance of 500 Ω?
1. Current, Voltage and
Resistance
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THE VOLTMETER
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Used to measure
voltage across a
component
Always connected in
parallel to a component
The voltmeter in the first
three circuits will always
read 6V
The voltmeter in the
forth circuit reads -6V
Voltmeters have black
(negative or common)
and red (positive)
terminals
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6V
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V
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6V
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V
R
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6V
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R
V
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6V
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R
V
+
1. Current, Voltage and
Resistance
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THE AMMETER
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Used to measure
current
To measure current
flowing in a resistance,
you must disconnect
the resistance and
insert the ammeter in
such a way so that all
the current flowing in
the resistance also
flows through the
ammeter, i.e. in series
with the resistance
The ammeter in these
circuits will read 3A
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3A
24V
8Ω
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8Ω
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24V
A
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A
+
24V
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8Ω
1. Current, Voltage and
Resistance
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MULTIMETERS
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In the lab, multimeters are used to carry out
measurements of voltage, current and resistance –
depending on the mode selected
Ammeter connections
mA/20A range
Voltmeter/resistance
measurement connection
Ground connection – return path back
to multimeter
1. Current, Voltage and
Resistance
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