A Plus Physics

Electric Current

Flow of Charge

Regents Physics Waterfall Cascade

Electric current is the flow of charge, much like water currents are the flow of water molecules. Water molecules tend to flow from areas of high gravitational potential energy to low gravitational potential energy. Electric currents flow from high electric potential to low electric potential. And the greater the difference between the high and low potential, the more current that flows!

In a majority of electric currents, the moving charges are negative electrons. However, due to historical reasons dating back to Ben Franklin, we say that conventional current flows in the direction positive charges would move. Although inconvenient, it's fairly easy to keep straight if you just remember that the actual moving charges, the electrons, flow in a direction opposite that of the electric current. With this in mind, we can state that positive current flows from high potential to low potential, even though the charge carriers (electrons) actually flow from low to high potential.

Electric current (I) is measured in amperes (A), or amps, and can be calculated by finding the total amount of charge (∆q), in Coulombs, which passes a specific point in a given time (t). Electric current can therefore be calculated as:

electric current

 

Question: A charge of 30 Coulombs passes through a 24-ohm resistor in 6.0 seconds. What is the current through the resistor?

Answer: Regents Physics Current Solution

 

 

Question: Charge flowing at the rate of 2.50×1016 elementary charges per second is equivalent to a current of

  1. 2.50×1013 A
  2. 6.25×105 A
  3. 4.00×10–3 A
  4. 2.50×10–3 A

Answer: current calc solution

 

 

Question: A 1.5-volt, AAA cell supplies 750 milliamperes of current through a flashlight bulb for 5 minutes, while a 1.5-volt, C cell supplies 750 milliamperes of current through the same flashlight bulb for 20 minutes. Compared to the total charge transferred by the AAA cell through the bulb, the total charge transferred by the C cell through the bulb is

  1. half as great
  2. twice as great
  3. the same
  4. four times as great

Answer: (4) If Δq=It, and both cells supply 0.750A but the C cell supplies the same current for four times as long, it must supply four times the total charge compared to the AAA cell.