[Physics] Does Ohm’s law $V = IR$ mean voltage causes current, or does it just say that voltage and current are related

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I have read that Ohm's law, $V=IR ,$ just means that $V$ is equivalent to $IR ,$ not that voltage is the cause of current.

This is similar to the interpretation of Newton's second law, $\mathbf{F} = m \mathbf{a} ,$ where force is not the cause of acceleration.

Question: Is it true that Ohm's law is merely a relationship between voltage and current without it implying that voltage causes current?

Best Answer

As usefully pointed out in the comments, Ohm's law just states that voltage $V$ and current $I$ have a relation. Therefore, Ohm's law per se means equivalence and not causation. This could be not immediate to deduce, there is plenty of Physics laws which imply both a relation and a causation, but this is not the case for neither Ohm's law, nor Newton's second law, as you correctly mentioned.

The causation is established according to the specific circuit and its features.


For example, a battery is a device which tries to always impose a fixed voltage $V_b$ between its terminals $V_+$ and $V_-$. In this example, $V_b$ is the cause and the independent quantity. If you connect a lamp between the battery terminals, it can be represented as a resistance $R_{\mathrm{lamp}}$. The voltage across $R_{\mathrm{lamp}}$ is $V_b$. The current on $R_{\mathrm{lamp}}$ is then the effect of the presence of the battery, and the dependent quantity. This is a little circuit which consists in a voltage generator (the battery), a resistor (the lamp) and a single loop. As an effect of the imposed voltage, a current $I$ will flow through the resistor. $I$ is the dependent quantity: to determine its value, you must know both $V_b$ and $R_{\mathrm{lamp}}$; Ohm's law can be written as

$$I = \frac{V_b}{R_{\mathrm{lamp}}}$$

Of course, the battery may not be able to erogate enough current to create or maintain a $V_b$ across $R_{\mathrm{lamp}}$, or the current may be so high that $R_{\mathrm{lamp}}$ is damaged, and so on, but these real cases are not relevant here.


On the other hand, transistors can be used as current generators in several configurations. Current generators try to always impose the value $I_b$ of current between their terminals $I_+$ and $I_-$. If you connect a resistor $R_{\mathrm{lamp}}$ between these two terminals, you already know the current $I_b$ flowing into the resistor: $I_b$ is the independent quantity and the cause. This is again a simple circuit consisting in a current generator, a resistor and a single loop. As an effect of the current $I_b$, a voltage $V$ will appear across the resistor: in this case, $V$ is the dependent quantity. To determine its value, write Ohm's law as:

$$V = R_{\mathrm{lamp}} I_b$$

There are again some real scenarios when $I_b$ can not be imposed by the current generators, but they can be discarded here.


The same Ohm's law is used in both the examples, but with exchanged roles between $V$ and $I$, according to the circuit.


In any circuit, regardless of its structure, if you know the value of a resistor $R$ and the current $I$ flowing through it, you can determine the voltage between the resistor terminals as $V = RI$. If instead you know the voltage existing between the resistor terminals, you can determine the current as $I = V / R$. In the previous cases, the cause was always the independent quantity and the effect was the dependent quantity. In this case instead, you don't know what is the cause, but you can still use Ohm's law: the dependent quantity is now, for you, the one you still do not know.

As correctly pointed out by Whit3rd's answer, Ohm's law mainly refers to a resistor in a circuit and it involves three quantities: the voltage across it, the current through it and the resistance value. If you know two of them, regardless of what is the cause and what the effect, you can determine the third one.