Resistance

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Resistance

• Electricity Lesson 3

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Learning Objectives

• To define resistance.
• To know what causes resistance.
• To know how to measure resistance.

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Prefixes
Prefix Symbol Value
nano
106
k
109
µ
milli
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Prefixes
Prefix Symbol Value
nano n 10-9
mega M 106
kilo k 103
giga G 109
micro µ 10-6
milli m 10-3
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Resistance

• Resistance is a measure of how difficult it is
  for current to pass through a component.
• What causes it?

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Resistance

• Resistance is caused by the collisions between
  the charge carriers in the material with each
  other and the fixed positive ions in the material
  through which the current is flowing.

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Resistance

• The resistance of a component is defined as-
• Or in symbols-

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Unit of Resistance

• The unit of resistance is the ohm (O) which is
  equal to 1 volt per ampere.

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Example 1

• If the current through a component is 3.4 nA when
  the voltage across it is 4.3 µV.
• Calculate-
• A) The resistance at this current.
• B) The pd across the component when the current
  is 4.3 MA, assuming its resistance is unchanged.

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Measuring Resistance

• A resistor is designed to have the same
  resistance regardless of the current flowing
  through it.
• Build a Circuit.
• Plot a graph! V vs I. Should get a straight line
  with the gradient equal to 1/R.

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Assumptions

• The same current flows through the ammeter as the
  resistor.
• No current passes through the voltmeter.
• The resistance of the resistor remains constant
  as the current flowing through it changes.
• The gradient of the line only 1/R if the
  resistance remains constant.

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Ohms Law

• Ohms law states that the p.d. across a metallic
  conductor is proportional to the current through
  it, provided the physical conditions do not
  change.
• If a conductor obey Ohms law then V is
  proportional to I and the gradient of the graph
  1/R.