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I am interested in electronics as a hobby, so I have been reading up on the physics of electricity. I understand that difference in charge on battery terminals causes electrons to flow naturally through a circuit. (Like an object in a gravitational field.) I understand that electrons will flow through a circuit at a constant rate and not be consumed by the circuit. I understand that these particles have higher energy before being applied to a load and lower energy after being applied to a load.

I don't understand what is doing the work here, since the electrons are not being consumed. Does movement itself work on the load, like friction?

skibulk
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    Voltage, as you said below, is a potential difference. This is an exact analog to mechanical potentials. A mechanical potential is, as the name implies, the ability of a system to perform work by moving something. An electric potential is the ability of a system to perform work by moving charges, in wires it's electrons, but in electrolytes it could also be ions. The energy is stored in electromagnetic fields (or chemicals) and not in the movement of charges, though. – CuriousOne Jan 05 '16 at 17:21
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    When a ball rolls down a hill, the ball isn't consumed, but it still loses potential energy and gains kinetic energy. When water flows down a hill, it isn't consumed but it can still do work if it flows through a water wheel or turbine. Why do you think that electrons need to be consumed in order to do work on something? – The Photon Jan 05 '16 at 20:12

5 Answers5

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on battery terminals causes electrons to flow naturally through a circuit. (Like an object in a gravitational field.)

Probably better to think of it as pressure in pipes with no gravity, since circuits are loops.

hydraulic analogy

(The reservoir is not needed, though, and can be removed from the diagram without changing how it works.)

I understand that electrons will flow through a circuit at a constant rate and not be consumed by the circuit. I understand that these particles have higher energy before being applied to a load and lower energy after being applied to a load.

I don't understand what is doing the work here, since the electrons are not being consumed.

Think of the electrons like links in a chain (except pushing each other instead of pulling). The chain is not consumed, it circulates around forever, but it can carry energy from one wheel to another. As you turn a crank on one end, energy is transferred by the chain to the other end, where it can do work.

Does movement itself work on the load, like friction?

In a resistor, yes. The resistor converts electrical energy to heat energy through "friction", in the same way friction converts mechanical energy into heat energy. (Imagine that your chain is replaced by a belt, and the belt is rubbing against something at the other end to produce heat as you crank the local end.)

Technically, it's complicated and requires quantum mechanics to explain, but to oversimplify: Impurities in the metal crystals and vibrations from their temperature cause the electrons to collide with the metal and transfer energy to it. (That's why resistance increases with temperature, for instance.) But it behaves like friction. See How do you explain electrical resistance?

Community
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endolith
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Difference in voltage causes electrons to flow, but I digress - the battery is what is doing the work. A good analogy is that of a water pump. In a water pump, it is the pump that creates a pressure difference, causing the water to flow. In a circuit, it is the battery that creates a voltage difference, causing the electrons to flow. The electrons do lose potential energy after going around the loop, but the battery brings them back up to a high potential energy so they can go down the potential gradient again.

You can also think of the electrons being children sliding down a slide that spirals back down to the bottom of the ladder, where the parents (battery) pick them up (do work) and get them up to the top of the slide again. Whatever you find easier.

SSD
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  • Voltage is electric potential difference, right? And Electric potential difference is created by different charges in a circuit (a positive and negative terminal)? – skibulk Jan 05 '16 at 16:56
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    Rather than a water pump, you might compare an electric power system to a fluid power (i.e., hydraulic) system. In a typical fluid power system, the fluid flows in a closed circuit, carrying energy from a pump to one or more hydraulic motors or hydraulic actuators. – Solomon Slow Jan 05 '16 at 16:59
  • I understand that electrons move through a circuit and are "Pushed" or "pulled" by a power source. But how does a moving electron do work on a load? – skibulk Jan 05 '16 at 16:59
  • @jameslarge so the "pressure" of electrons does the work? – skibulk Jan 05 '16 at 17:03
  • I think it is different depending on the load. For a light bulb, the electrons release their energy through heat and light in the filament. Unfortunately that's the clearest example I can personally give you. For engines, I know that those use electromagnetic induction, but I'm not 100% sure how the electrons lose their energy during this process, but it has something to do with resistance due to induction. In all cases, I believe it is through heat and light being emitted that the electrons lose their energy. – SSD Jan 05 '16 at 17:11
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    @skibulk, yes. Electrons exert force on each other, and valence electrons move through a metallic conductor and interact with one another much like molecules of a gas, so "pressure" is a very good analogy. – Solomon Slow Jan 05 '16 at 17:16
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I understand that difference in charge on battery terminals causes electrons to flow.

Actually, difference in electric potential (called voltage) between the terminals causes them to flow.

I understand that electrons will flow through a circuit at a constant rate and not be consumed by the circuit.

Electrons, like any form of matter can't be consumed, they can at best transform into something else during a nuclear reaction. So, you may like to give up that notion of getting consumed.

I understand that these particles have higher energy before being applied to a load and lower energy after being applied to a load.

Energy in an electric field is given by charge times potential difference. Since batteries are a fairly constant potential difference source (unless on the verge of getting dead), and the charge of an electron is of course constant, and these are the only two things on which energy depends, so the energy of electrons remains the same irrespective of whether there is or isn't a load attached.

I don't understand what is doing the work here, since the electrons are not being consumed. Does movement itself work on the load, like friction?

Answer to that question would vary with specifics of the question. Let's take the example of heating instruments. Most of such instruments consist of a conducting coil with its end-points plugged into AC (It won't work on DC, find out why). Now, we know that our perception of heat comes from random walk vibration of sub-atomic particles, so in this case the work is done by moving electrons colliding with particles of the coil and transferring some kinetic energy to the particles of the coil. So, the particles of the coil start vibrating faster, and this is perceived by us as increased temperature of the coil. An electric motor which produces motion rather than heat would have a different explanation.

Volker Siegel
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Soham
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  • Voltage is electric potential difference, right? And Electric potential difference is created by different charges in a circuit at different places (a positive and negative terminal)? – skibulk Jan 05 '16 at 17:08
  • Right. As the name says Electric potential difference will exist if two points in space have different electric potential - they can both be constant negative (one more negative than the other), both be constant positive (one more positive than the other), one constant positive and one constant negative, or one at zero potential and the other varying sinusoidally from negative to positive, as is the case with AC. – Soham Jan 05 '16 at 17:12
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    "It won't work on DC"? Resistive heating elements definitely work on DC. – endolith Jan 05 '16 at 19:44
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Voltage is electrical pressure or force. Voltage is sometimes referred to as Potential. Voltage Drop is the difference in Voltage between the two ends of a conductor through which current is flowing.

Ohm's Law A set of rules that show the relationships among Current, Voltage, Power and Resistance. Given any two of the above, one is able to calculate the other two using the following formulas: E = I x R I = E / R R = E / I P = E x I

An electrical current can flow in either of two directions through a conductor. If it flows in only one direction whether steadily or in pulses, it is called direct current (DC). Almost all the projects in class will be powered by DC electricity. In order to be able to work with DC we need to convert the alternating current (AC) from the outlets into a direct current, which we use to power our circuits. A wall adapter transforms AC into DC, the wall adapter in our lab kit transforms 120 VAC into 9/12VDC. The maximum current it can provide is 1000mA (1A). The wall adapter has two wires that go to our circuits - one for positive power supply and one for negative (ground, or GND).

Since direct current only flows in one direction, we have to be able to easily determine the positive and negative side of the power supply. Remember that we assume the conventional current flow from positive to negative when we work with circuits!

varunk
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I think electrons can gain higher energy and is not only dependent on charge. Mainly, the electron can gain kinetic energy, movement energy. I do not know how it applies to a load, I am only saying electron energy can be higher, with kinetic energy for example.

Jean-Michel Lambert
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