L4 - Current in a Simple Circuit

objectives

Current in a Simple Circuit

• An electric cell (commonly called a battery) can make electrons move, but only if there is a conductor connecting its two terminals.
• Then, chemical reactions inside the cell push electrons from the negative (-) terminal round to the positive (+) terminal.

Simple Circuits

• The cell below is being used to light a lamp.
• As electrons flow through the lamp, they make a filament (thin wire) heat up so that it glows.
• The conducting path through the lamp, wires, switch, and battery is called a circuit.

• There must be a complete circuit for the electrons to flow.
• Turning the switch OFF breaks the circuit and stops the flow.

• The previous circuit can be drawn using circuit symbols:

Measuring Current

Some Typical Current Values

• Current in a small torch lamp 0.2 A (200 mA)
• Current in a car headlight lamp 4 A
• Current in an electric kettle element 10 A
• Putting ammeters (or milliammeters) into a circuit has almost no effect on the current. As far as the circuit is concerned, the meters act just like pieces of connecting wire.

Charge and Current

• For example, if a current of 2 amperes flows for 3 seconds, the charge delivered is 6 coulombs.

Current Direction

• Some circuit diagrams have arrowheads marked on them.
• These show the conventional current direction: the direction from + to - round the circuit.
• Electrons actually flow the other way.
• Being negatively charged, they are repelled by negative charge, so are pushed out of the negative terminal of the battery.
• The conventional current direction is equivalent to the direction of transfer of positive charge.
• It was defined before the electron was discovered and scientists realized that positive charge did not flow through wires.
• However, it isn't ‘wrong’.
• Mathematically, a transfer of positive charge is the same as a transfer of negative charge in the opposite direction.