Current, Potential Difference and Resistance

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Current, Potential Difference and Resistance
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Electric Current and Charge

• Atoms contain positive and negative charges.
• Negatively charged electrons are free to move
  around the metal.
• The flow of electric current is due to the
  movement of electric charges.

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• Electric current is the rate of flow of charge
  and the SI unit is ampere (A).
• It is the amount of charge passing a given point
  in one second.
• One ampere (A) is the current when one coulomb
  (C) of charge passes a given point in one second
  (s).

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• Charge carried by electrons pass from the
  negative to the positive terminals in a circuit.

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• Conventional current flows from positive to
  negative.

Electrons flow in the opposite direction to that
of conventional current.
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If an amount of charge Q (in coulombs) flows past
a point in time t (in seconds), then the electric
current I (in ampere) is given by
Q
I
t
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Example 1 If 30 C of electric charge flows past
a point in the a wire in 2 minutes, what is the
current in the wire? Ans I Q/t 30 C/120
s (change 2 minutes into seconds) 0.25 A
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Example 2 The current in a lamp is 0.2 A. If the
lamp is switched on for 2 s, what is the total
electric charge passing through the lamp? Ans
I Q/t 0.2 A Q/2 s Q 0.4
C OR Q It 0.2 A x 2 s 0.4 C
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How to measure electric current?

• An ammeter can be used to measure the size of an
  electric current.
• The ammeter must be connected in series to the
  circuit.

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Where to place the ammeter ?

• The ammeter can be placed anywhere in the circuit
  if the circuit consists of only one loop.

A1
A6

-
A5
A2
A4
A3

• The ammeter would measure the same current at the
  different locations.

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• Some ammeters allow us to measure different
  ranges of current.

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Video clip on internet

• Go to this website
• Introduction to Electricity
• http//www.youtube.com/watch?vEJeAuQ7pkpcfeature
  fvw