What I understand by voltage in a circuit is that an electric field causes electrons to move from the negative terminal to the positive terminal. As it does so it converts its electrical potential energy into other forms of energy.

However I cannot understand why the amount of energy that a coulomb of charge loses in one loop around the circuit is constant regardless of the components in the circuit.

Also I do not understand how a charge seems to know how much energy per coulomb to convert at each component.

For example if you consider a circuit with a 3 volt battery and 1 A of current flows, through the circuit with one 3 ohm resistor in series, each coulomb of charge will convert 3 joules of electrical energy into other forms of energy at this resistor.

However, if there were two resistors the amount of electrical energy that each coulomb transfers at each resistor is different. This to me seems unintuitive.

Also if voltage is the amount of electrical potential energy a coulomb of charge that is converted as the coulomb of charge moves around the circuit, then surely, if components are introduced into the circuit no electrical potential energy can be used to be converted to other forms of energy at the components as all the electrical potential energy is already being used to push the charge around the circuit.

As you can probably tell I am very confused by the concept of voltage especially in circuits. Please could somebody explain to me what voltage is and how it can be understood in circuits.

  • $\begingroup$ Can you clarify your two resistors example. There's several different ways to put two resistors into the circuit (in particular, series and parallel configurations), and it could be even more confusing if we were to talk to one configuration and you were thinking of the other. $\endgroup$
    – Cort Ammon
    Apr 10, 2017 at 17:48
  • $\begingroup$ Voltage is energy per unit of charge. V = J/C. Joules per coulomb. So a 3V battery can move 3J/1C of energy. The current in the circuit is not fixed. In your example the current is the voltage over the total resistance. 3V/3 Ohm or 1 Amp. IF you added a second 3 Ohm resistor(in series) the total current in the circuit would be 3V/6 Ohm or .5 Amps. There would now be a voltage drop between the first in second resistor of V = I * R or .5 Amp * 3 Ohm = 1.5V So as you can see less and less energy can be converted in each resistor as you add more. $\endgroup$
    – Brad S
    Apr 10, 2017 at 20:06

1 Answer 1


There are several things to unpack there. You have the definition of voltage right. But the energy loss in a circuit does depend on the elements on the circuit.

In your example, the battery is a fixed voltage source: it provides always the same voltage difference regardless of the circuit. This type of source, in the ideal case, can move a current of any value, but the current will be determined by the resistance of the circuit it is connected to, hence the energy loss (or the Power $P=V*I$, Energy per unit time) will depend on the current $I$ and therefore on the circuit's elements.

On the other hand, the loss of energy in a resistor depends on the Intensity passing through it. From Ohm's law we know that $R=\frac{V}{I}$ and that the higher the voltage applied, the higher the current passing. Hence the power loss in the resistor will be $P=R*I^2$.

Going back to your example, when you double the resistance by adding a new resistor, the Voltage remains the same (constant voltage source), but because there is more resistance, the current passing will be less (half actually). The total Power will then be half as well ($P=V^2/R$), and each resistor dissipates half of that power by its own.

However, the source I have talking about is an ideal one. In reality, sources can provide a constant voltage only for some range of current, because they cannot produce infinite power. So currents higher than the specified, the voltage provided will decrease.

So indeed you are right about your last statement: the more components you add the less energy is available, which is reflected on the lower current.

But there are also constant current sources, which provide the same current no matter how high voltage they need to produce. And in this case, the energy lost in each equal resistor will be the same, as you can verify for the relations above. And sure, in real scenarios this source has a limit for the amount of potential it can produce.

Bottom line, the confusion might arise from an unclear definition on the type of source.


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