Electrical properties of Materials

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Electrical properties of Materials
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Starter

• Draw symbols for the following electrical
  components.
• Battery
• Variable resistor
• Potential divider
• Fuse
• Ammeter
• Voltmeter
• Diode
• LED
• Thermistor

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Learning objectives

• Define Drift velocity
• Recall and use the formula for resistivity
• Recall and use the formula for conductance

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Drift velocity

• In many types of conductor, there are free charge
  carriers. In metals, these free charge carriers
  are electrons. When there is n o current, the
  charge carriers move at high speeds (up to
  106ms-1) and quite randomly.

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• Under the influence of a potential difference,
  there is an additional velocity along the
  conductor the Drift Velocity.
• The drift velocity of the charge carriers in a
  metal is of the order of 10-4 ms-1

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2 definitions

• E.m.f. is the energy transferred per unit charge
  when one other type of energy is converted into
  electrical energy
• P.d. is the electrical energy transferred per
  unit charge when electrical energy is converted
  into another form of energy

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Resistivity

• Resistivity describes a property of the material
  rather than an electrical component
• The resistivity of a material is defined as the
  resistance of a 1m length with a 1 m2 cross
  sectional area. It is measured in ohm-meters ( Om)

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• ? RA/l
• ? resitivity Om
• R resistance O
• A cross sectional area m2
• l length m

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Resistivity of different materials
Material Resistivity at 20oC/ Om
Silicon 1 x 10-2 2.3 x 103
Germanium 1 x 10-4 1 x 101
Nichrome 150 x 10-8
Constantan 49.0 x 10-8
Iron 9.71 x 10-8
Tungsten 5.6 x 10-8
Aluminium 2.65 x 10-8
Copper 1.68 x 10-8
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• Conductance is the inverse of resistance
• Conductivity is the inverse of resistivity

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• s Gl/A
• s conductivity Sm-1
• G conductance S
• l length m
• A cross sectional area m2

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• 8.94 S

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• B(i) G OA/L
• Total length of the two cables 60m
• G (5.9x107 Sm-1) x ( 1.8 x 10-6 m2)
• 60m
• G 1.8 S 2 sig figs

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• B (ii) R 1/G
• V I R I/G
• V 13A/1.8 S
• 7.2V
• B (iii) Power I V
• 13 x 7.2
• 93.6 W