Electrons and work

Discussion in 'Physics' started by jamo1989, Aug 9, 2012.

  1. jamo1989

    Thread Starter New Member

    Aug 9, 2012
    Hi everyone its my first post here, i'm an apprentice electrician and i'm having a little bit of trouble grasping the physics related to voltage, current and work done. Its probably a stupid question but here goes anyway.

    Using a 12v battery for example this is my understanding of voltage.

    A grouping of electrons (charge) builds up on the positive terminal of the battery due to a chemical process in the battery. This charge then has a larger potential to do work or push then the negative terminal? once a closed circuit with a resistor is attached the charge of electrons push's electrons all the way around the conductor through the resistor to the negative terminal.

    Current is basically how many electrons pass a given point in a second?

    Now my question is if the current is the same before and after the resistor, and the same amount of electrons are entering the resistor as coming out at the same rate. Physically, what is converting to heat and light energy given off by the resistor? Something must be changing state to produce heat and light right?
  2. bretm


    Feb 6, 2012
    A tiny amount of electrons build up on the negative terminal and are removed from the positive terminal by the battery chemistry.

    The excess at one end and deficit at the other end creates an electric field. This field causes electrons in the circuit to redistribute themselves in such a way that the electric field inside the wires follows the direction of the wires.

    The electric field is stronger inside the resistor. Electrons flowing inside the resistor are pushed harder, but they lose this extra momentum when they collide with the resistive material, causing heat.
  3. nsaspook

    AAC Fanatic!

    Aug 27, 2009
    The electrons in the wire and resistor are the charge carriers of electrical energy moved by electromagnetic fields that are confined in space near the wire from source to load. What you see as heat and light is the effect of that energy on the matter in the resistor.

    Last edited: Aug 9, 2012
  4. Austin Clark

    Active Member

    Dec 28, 2011
    Very good question. Basically, the electrons are indeed moving at the same charge/second before and after the resistor, this is thanks to the conservation of current (kirchhoffs current law), HOWEVER the energy/charge of those electrons is lower after passing through the resistor. If you look at a simple circuit with a voltage source and a resistor, you'll notice that the voltage difference before and after the resistor is equal to the source voltage, the entire voltage was dropped over the resistor. Each charge lost some energy. Now, the charges don't need much energy to pass through the wire back to the source, theoretically, if the resistance back to the source is zero, zero energy per charge is needed to keep them in motion, that's why the point leading back to the sourec on the negative terminal is considered to be at 0V.

    Voltage can be defined as Joules (energy) divided by the number of charges. J/C = V
    Current can be defined as charges per second. C/S = I

    You probably know that power is equal to Voltage times Current. If you do the math, (J/C * C/S), you get J/S, which is the definition of a watt (one joule of energy per second of power)

    The charges at the negative terminal of a battery or source don't physically have zero energy, they just have zero energy relative to the rest of the circuit. Current will only flow if there's a DIFFERENCE in energy/charge, sometimes called electrical potential, or EMF (electro-motive force).

    Also, imagine the electrons are being pushed by other electrons and their electric field, this is what causes voltage and thus current. The electrons flowing tend to "bump" into other electrons, in conductors that's ok and the electrons that are bumped simply begin to move on their own, helping to carry the current further, HOWEVER, in a resistor, many of the electrons are more reluctant to start moving, and so they actually absorb some of that energy and literally yank their mother atom, creating heat.
    Last edited: Aug 9, 2012
  5. nsaspook

    AAC Fanatic!

    Aug 27, 2009