Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

B Movement of electrons in AC

  1. Aug 12, 2017 #1
    In DC, we have learnt that the electrons move from their position through the conductor, resulting in a current. They leave their positions on the application of a potential difference, and move or flow in the direction of higher to lower potential, thereby causing a current.

    But what about AC? Say the frequency is 50Hz. What is altering in AC? It is said that the sine graph of AC is positive for 50 times and negative for 50 times in a second. It is also said that the direction of AC keeps changing. Does this mean that the live will become the neutral in our house wires? But that doesn't happen. Then what is the thing that changes direction 50 times a second? Is their actually any flow of electrons in AC, or the electrons just vibrate about their mean positions, resulting in a current? If so, how can a current result from harmonic vibrations of electrons?
     
  2. jcsd
  3. Aug 12, 2017 #2

    FactChecker

    User Avatar
    Science Advisor
    Gold Member

    These are not "vibrations of electrons". 50 Hz gives enough time for electrons to move very far back and forth between atoms. The frequencies to vibrate atoms is much higher. Water resonates at about 22 GHz.
     
  4. Aug 12, 2017 #3
    Can you elaborate a bit on this part? What does back and forth in a conductor mean?
     
  5. Aug 12, 2017 #4

    FactChecker

    User Avatar
    Science Advisor
    Gold Member

    50 Hz gives you current in one direction for a long time before it switches to the other direction. So however you want to visualize electric current, you can consider 50 Hz as being DC for a long time in one direction, then DC for a long time in the other direction.

    PS. I am using "long time" as a relative term. Relative to KHz or GHz.
     
  6. Aug 12, 2017 #5
    How does the direction change physically? If it was flowing from A to B (say) at first, does it now flow from B to A after a change? If the direction changes completely, then shouldn't the live wire in our household become a neutral wire and vice-versa?
     
  7. Aug 12, 2017 #6

    FactChecker

    User Avatar
    Science Advisor
    Gold Member

    Yes.
    Well, it averages out to neutral, but it is only 0 voltage for the instant that it is switching between + and -. All other times, it is hot. (The voltage is really a sine wave so the voltage does decrease smoothly to 0 when it is switching rather than making a sudden jump between positive and negative.)
     
  8. Aug 12, 2017 #7
    If it averages out to neutral, and if current actually flows from both live and neutral wires, then why do we get a shock on touching the live and not the neutral, as the neutral should have a current as the direction of flow of electrons changes physically?
     
  9. Aug 12, 2017 #8

    CWatters

    User Avatar
    Science Advisor
    Homework Helper

    AC stands for Alternating Current but actually both the voltage and the current is alternating polarity/sine.

    Correct. The voltage of one wire relative to the other does change polarity (sine) 50 times a second.

    In some countries the two wires dangerous wires (for want of a better term) are called Live and Neutral. The Neutral is typically but not always connected to earth somewhere but for now lets consider it to be our reference voltage (0V). The voltage on the live wire relative to 0V will change sign so it goes positive and negative. In the UK where the average voltage is 240V it actually goes down to about -330V and upto +330V fifty times a second. That causes the current to change direction fifty times a second.
     
  10. Aug 12, 2017 #9
    Doesn't a negative value of voltage indicate a physical change in direction of flow of current? In that case, current should now flow through the neutral wire to the device and then back through the live wire. How does connecting the neutral to earth allow such a case?
     
  11. Aug 12, 2017 #10

    CWatters

    User Avatar
    Science Advisor
    Homework Helper

    As I said above the Neutral is frequently connected to Earth (0V). However for safety reasons you should always treat the neutral wire as dangerous because one day there might be a fault and can become dangerous.

    Correct.

    0V is more positive than say -100V.

    Consider a 9V DC battery. If you connect the +ve terminal to Earth (0V). Then the -ve terminal becomes -9V. It will still work just fine.
     
  12. Aug 12, 2017 #11

    CWatters

    User Avatar
    Science Advisor
    Homework Helper

    Going for lunch. Back later.
     
  13. Aug 12, 2017 #12

    FactChecker

    User Avatar
    Science Advisor
    Gold Member

    A neutral or ground wire can be kept at the same potential as the ground (and your body). So you should never get a shock from it no matter what the voltage of the AC hot wire is. (Please don't take this too literally. If you don't wire things correctly, you can get killed by a neutral wire.)
     
  14. Aug 12, 2017 #13
    @CWatters then as you are saying, the neutral at both the dynamo station and at our homes are connected to the earth. When there is a negative voltage, then the current reverses direction and flows through the earth, through the neutral, device, and back to live?
     
  15. Aug 12, 2017 #14
    I understood the concept of earthing it, but why should we earth it? Can't it be carried back to the dynamo station, like the phase wires are brought to our homes?
     
  16. Aug 12, 2017 #15

    FactChecker

    User Avatar
    Science Advisor
    Gold Member

    It can be done either way and different countries do things differently. A return voltage wire requires twice as much wire.
     
  17. Aug 12, 2017 #16
    @FactChecker then as you are saying, the neutral at both the dynamo station and at our homes are connected to the earth. When there is a negative voltage, then the current reverses direction and flows through the earth, through the neutral, device, and back through live?
     
  18. Aug 12, 2017 #17

    jim mcnamara

    User Avatar

    Staff: Mentor

  19. Aug 12, 2017 #18
  20. Aug 12, 2017 #19

    jim mcnamara

    User Avatar

    Staff: Mentor

    It is what happens when:
    1. you flip a switch
    2. current reverses
     
  21. Aug 12, 2017 #20

    CWatters

    User Avatar
    Science Advisor
    Homework Helper

    There are different ways to model the behaviour of electrons in a wire depending on how much physics you have studied. One model is to think of electrons like a cloud of bees that wiz about in random directions. When a voltage is applied the cloud drifts along in one direction but the bees still wiz about in random directions within it. When the voltage is reversed the direction of the cloud drifts in the other direction. The speed with which the cloud drifts is very slow, typically only a quarter of a millimetre per second. So in 1/50th of a second they don't drift very far.

    People then ask how come when I switch on a light it turns on virtually instantly? To explain that consider a garden hose that is already full of water. When the tap/faucet injects a little more water all the water in the pipe moves and excess water comes out of the other end virtually instantly.

    These models aren't the best but until you learn about the quantum theory of the atom they are good enough.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Movement of electrons in AC
  1. AC Electronics (Replies: 1)

Loading...