Current, Voltage and Resistance

Current  When charge (electrons) are built up at one end of a conductor, they will flow to an area where there are less built up electrons as long as a path (conductor) gives them a path to it.  The area to which the electrons flow is called a ground.

 A battery has built up electrons at one end. They will flow to the other if a conducting path is given to them (e.g. a wire).

How do electrons MOVE?  Imagine putting a line of bar magnets on small wheels, arranged so that the poles of the magnets repel each other.  If you push one end of the magnet “train” the whole thing will move, in fact the “car” at the far end will move almost instantly.

 This is how electrons move in a wire. Since all electrons repel, pushing one into a wire at one end pushes one out at the other!  Individual electrons move quite slowly through a wire, but the overall effect is that pushing one electron into the wire at one end pops one out at the other nearly instantly!

 Definition: The time it takes for a certain amount of electrons to pass a point.  The flow of electrons in a wire.  Electrons will move as new electrons are added.

The Coulomb  When measuring the millions and millions of electrons that are moving with current, the numbers can get very large.  Scientists use the COULOMB as a way to measure large amounts of electrons.  1 coulomb = electrons  electrons!

 The formula for finding current is: I is current measured in amps – Unit is A Q is the amount of charge measured in coulombs- Unit is C t is time in seconds – Unit is s

Example of Current A bulb in a flash light has 1.25 coulombs of electrons flowing through it in five seconds. How much current is flowing through the bulb?  Answer:

Voltage  Electrons can flow in a conductor.  Much like water flows down hill, highly energized electrons will flow through a conductor to areas of lower energy.  The difference between the areas of highly energized electrons and areas of lower energy is called potential difference.  Because the electrons in the highly energized state have the potential to do work (such as light a light or turn an electric motor) we say they have potential energy.

 Definition: How much energy it takes to move a certain amount of electrons (coulombs).  These electrons that are moving have potential energy as they move through a wire.

 For example, batteries are measured in VOLTS.  A 9 Volt battery has more potential energy than a 1 Volt battery.

 The formula for finding voltage is: V is voltage measured in volts – Unit is V E is the amount of energy measured in joules- Unit is J Q is the amount of charge measured in coulombs- Unit is C

Example of Voltage  Q: How many volts is there between the ends of a battery if there are 2 Coulombs of electrons in the battery and they have a total energy of 1200 joules?  Answer:

Resistance  Electrons encounter resistance to their passage as they travel in wires or through electrical devices.  Electrons must bump past the nuclei of other atoms to get to the end of the wire or circuit.  The more resistance to their motion; the lower the amount of electrons that can pass.

 Visualize electrons flowing through a length of wire. If you change the diameter or length of the wire, you change the amount of electrons that can flow through that wire based on the idea that the narrower and longer the wire, the more resistance the electrons in it will experience.  As resistance increases, you will require more energy electrons in order to continue to push the same amount of electrons through that wire.

 Definition: How hard a certain amount of electrons are being pushed through a circuit.  The higher the resistance the electrons encounter the more energy that is required to move them along.

 The formula for finding resistance is: R is resistance measured in ohms – Unit is Ω V is the amount of voltage measured in volts- Unit is V I is current measured in amps – Unit is A

 Q: Find the resistance if a 120V potential causes a 5A current to flow in an iron  Answer: