Summary for Chapter 23: Electric Current

 


Current

An electric current is a flow of electric charge. The flow or movement of charge is usually driven by a difference in potential. Consider two points A and B. If the potential at A is greater than the potential at B, positive charge will want to move from A to B and negative charge will want to move from B to A. If positive charge moves from A to B we say there is a current flowing from A to B.

 


Circuits


Resistance

If the voltage across a resistor is 3V and the current is 0.1A, the resistance is 3V/(0.1A) = 30(V/A) or 30 Ohms.

Series and Parallel Circuits

When current flows in circuits, there is often more than one element in the circuit that the current can go through.  There are two common configurations, parallel and series.  

In series circuits, all the current must flow through each element.  Then the voltage across each element will be different than the total voltage, but the total voltage will be the sum of the individual voltages.  (One can say they have equal currents but different voltages.)  In the picture at the right, all the current that goes out the + terminal must pass through BOTH A and B to get to the - terminal.

In parallel circuits, the voltage across each element is the same, but the currents are different.  The total current in the circuit is the sum of the currents through each element.  In the picture at the right, the current coming from the + terminal can divide at point 1 and go through either Bulb A OR Bulb B to get to the - terminal.  Here the voltage across each bulb is the same, the voltage of the battery, VB.  In household wiring the circuits are usually parallel circuits, so each element will have the same voltage across it.


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