Need clarification on basic concepts

Discussion in 'Homework Help' started by TheCircuitsHaveEyes, Mar 19, 2016.

  1. TheCircuitsHaveEyes

    Thread Starter New Member

    Mar 19, 2016
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    Just started learning how circuits work. I'm currently having problems understanding a number of concepts. I will just ask them one at a time and in the spirit of this forum, also post my current understanding :)

    1) What is the difference between electromotive force (E) and voltage (V)? Based on what I've read, E is the maximum potential force, while voltage is the real one. In the case of a battery, voltage will decrease as it's being drained, but E stays the same. But how about in voltage sources, where they provide steady voltage. Is E equal to V in those? What exactly is the explanation behind this stranger Ohm's Law: E - U = R * I. I know regular Ohm's Law with just the U, so how do you explain the "expanded" version?

    2) What is the difference between voltage source and current source? I understand that voltage sources offer constant voltage(allowing the components to draw however much current needed), while current sources offer constant current. How would this second source offer constant current? Can it know what voltage to push through to maintain the current? Why do we need both kinds of sources?

    3) Why would you need multiple sources in a circuit? What's the difference between having two sources pushing in the same direction and them pushing in opposite directions?

    4) Based on question 2, I've seen circuits with multiple voltage sources and a current source. In these circuits, the current flow is unknown and must be determined with Kirchoff. How on Earth can currrent flow opposite the voltage drop? I'm guessing since there are multiple sources, they push against each other and the stronger one sends current in the direction of ITS voltage drop.

    Please explain these concepts to me. I can actually understand the calculations so far, but the explanations as to why the circuit functions that way elude me.

    Thanks.
     
  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    1) Electromotive force (EMF) is a Voltage generated by a battery or by any other source of a EMF without any load connected across this source.
    http://hyperphysics.phy-astr.gsu.edu/hbase/electric/elevol.html#c2
    And voltage is a more general term and can be applied to the voltage drop across the resistor.
    Does the voltage source need to know the load current to be able to provide a constant voltage ? The answer is no. The load resistance determines how much current is drawn from the voltage source. So we have the same situation with a current source. Current source is a circuit element that maintains a "preset" Is current regardless of a voltage across its terminal. In response to the current (Is) the load will develop some voltage across the current source terminal.
    Ideal_current_source.jpg
     
  3. crutschow

    Expert

    Mar 14, 2008
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    E(MF) is the Electro Motive Force of a source which is measured in volts similar to current (I) being measured in amperes.
    So the difference is that E is the force which is measured in volts (V).
    E and V are often used interchangeably but that is not proper notation.
    I don't know what U is. :confused:
    A current source doesn't "know" voltage, it knows current. It maintains a given amount of current and the voltage then becomes whatever is needed to maintain that current through the load resistance.
    We need both types of sources since some loads (such as LEDs or gas discharge lamps for example) require a constant current for proper operation. [We often make a poor-man's current source by adding a resistor in series with a voltage source, which is good enough for many applications that need a (near) constant-current.]
    (Many a poor LED has been zapped by users trying to apply a constant voltage to them). :eek:
    We need sources based upon the circuit requirements.
    For example, some circuits require a plus and minus voltage, or some require difference voltages.
    If two sources push in the same direction the voltages add, if they push in opposite directions the voltages subtract.
    If you have two voltages of opposite polarity in series, then the one with the higher voltage can force current through the lower voltage source in the opposite direction.
    That's how batteries are charged.
     
    Last edited: Mar 20, 2016
  4. TheCircuitsHaveEyes

    Thread Starter New Member

    Mar 19, 2016
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    Thanks for the replies.

    Also U is used for voltage in my country. That's why I'm confused at seeing equations with both E and U at the same time.
     
  5. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    In my country we also used a letter U for voltage "drop" across component because we do not have the letter V in our alphabet.
     
  6. BR-549

    Well-Known Member

    Sep 22, 2013
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    Electro Motive Force used to be a description of the source of the voltage.

    It's usual context was that the voltage potential has been generated by mechanical or chemical means.

    But EMF can be thought of energy and not just voltage.

    So EMF is an energy term, referenced to voltage.

    So we describe a power source as a certain voltage power supply.

    We can reference the current instead of the voltage.

    And we can describe a power source as a certain current power supply.

    When current begins to flow because of the power supply's EMF, the current's own movement generates a COUNTER EMF. The current will stabilize when the counter EMF equals the power supply EMF. So we have a balance, voltage and current. We can use either one to control the other.

    In a voltage power supply, the voltage determines the current, in a current power supply, the current determines the voltage.
     
  7. Sinus23

    Member

    Sep 7, 2013
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    1) If we are using the same terminology.
    E=U(voltage "seen by" the load)+URi(Voltage drop of the internal resistance of a battery).

    So E is the potential of the battery without any load.
     
    Last edited: Mar 20, 2016
  8. anhnha

    Active Member

    Apr 19, 2012
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    Well, it is same here (in Vietnam). We also use U for voltage and V for potential. By the way, the comma for decimal point as well.
     
  9. TheCircuitsHaveEyes

    Thread Starter New Member

    Mar 19, 2016
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    So If I added the voltage read by my multimeter with the voltage drop across the internal resistance of a battery, I would get the EMF? That actually makes a lot of sense.
     
  10. WBahn

    Moderator

    Mar 31, 2012
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    Lots of attention given to (1), so I'll offer some thoughts on the others.

    How does a voltage source know how much current to deliver?

    A constant voltage source will deliver however much current is needed (even if that current is negative) in order to maintain a constant voltage at its terminals.

    The same notion applies to a constant current source. It will produce whatever voltage is needed (even if that voltage is negative) at it's terminals in order to maintain a constant voltage flowing through it.

    Yes, we need both kinds. Lots of circuits, large and small, rely on having constant current sources for their proper operation.

    Different parts of circuits tend to like different voltage levels. Your motor controller might use logic components that need 3.3 V to operate but the motor itself needs several hundreds of volts. Your computer has several different voltage levels for different types of components. A radio or audio system often has low power and high power circuitry that operate at different voltage levels.

    You seldom design circuits so that multiple sources interact in a way so that they are pushing in the same direction or in opposite direction, though sometimes you do. What you are seeing at this stage are mostly contrived circuits intended to help you learn the basic analysis techniques.

    If current is flowing through a device in the direction of the voltage drop, then the device is acting like a load and is absorbing power. If it is flowing through a device in the direction opposite the voltage drop, then the device is acting like a source and is supplying power. Some devices, like a resistor, can only absorb power. Many other devices can do either, at least for a while. When a capacitor is being charged it is acting like a load and when it is discharging it is acting like a source. Same for an inductor. When a battery is powering a circuit it is acting like a source and when it is being recharged it acts like a load.
     
  11. Sinus23

    Member

    Sep 7, 2013
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    Yes.

    And to correct myself a little since I posted this just before I went to bed. It's not just the potential of the battery without the load. When current runs in the circuit the internal resistance will drop voltage that adds to the voltage dropped over the Load. (All this is assuming just one external resistor)

    So when you measure the voltage dropped over the Load you don't really get the full picture of the power that the battery is "outputting"...Plus the current drawn could be a bit off from what you calculated and so on...So it's a trap for young players, myself included.

    Hope it makes a little bit of sense, I really need to stop posting before I go to sleep;)
     
  12. TheCircuitsHaveEyes

    Thread Starter New Member

    Mar 19, 2016
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    I forgot to ask this, but I don't understand what you mean by counter EMF. I thought that in circuits with a SINGLE source of DC voltage/current, voltage drop and current go in the same direction. It's only when multiple sources push against each other that current can flow opposite some. So how exactly is current generating a counter EMF?
     
  13. BR-549

    Well-Known Member

    Sep 22, 2013
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    Assemble a battery and resistor circuit. After connection and current is flowing, measure the voltage across the battery. A positive voltage.

    Now measure the voltage across the resistor, NOT the circuit. In other words, measure the voltage across the resistor in the SAME DIRECTION as you did when you measured the battery.
    (the negative meter probe will be on the top of resistor, positive meter probe on bottom.)

    You will read an negative voltage, equal to the supply voltage.

    This negative voltage comes from the current flow, not the supply voltage.
     
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