What is Ohm’s law?
Ohm’s law is a relationship between voltage and current. It says that the amount of electrical charge passing through an electric circuit in any given time is directly proportional to the difference in potential between two points on the circuit.
A way to describe it is if you double the voltage across a resistor, then the current will be doubled. If you halve the voltage across a resistor, then the current will be halved. This means that if you want to increase or decrease the flow of electrons through a circuit, you need to change either the voltage or the resistance.
This means that as long as the resistance remains constant, doubling or halving the voltage will always result in doubling or halving the current.
The reason why we care about Ohm’s law is because it tells us something interesting about how electrons flow in an electric circuit.
When we talk about how much current flows through a wire, we are really talking about the number of electrons flowing per second. We call this quantity current.
Ohm’s law example
But what happens when we connect our wires together? What happens if we have a battery connected to one end of a wire and another end is attached to a light bulb?
Well, the first thing that happens is that there is a voltage drop from the positive terminal of the battery all the way down to the negative terminal. That means that the voltage at the negative terminal of the battery is lower than the voltage at the positive terminal.
If we were to draw a diagram of the situation, we would see that the line connecting the positive and negative terminals has been split into two parts. One part goes to the positive side of the battery and one part goes to the negative side of the battery.
But what does it mean for there to be more or less current flowing through a wire? Well, it means that there are more or fewer electrons moving from one point to another at any particular moment. In other words, the number of electrons flowing through a wire changes over time.
And when we say that the number of electrons flowing changes over time, we are saying that the number of electrons at some point in space changes over time.
We call this change in electron numbers density. So, when we say that the current is changing, we are really saying that the electron density is changing.
Now, here’s where Ohm’s law comes into play. When we say that the number or the density of electrons is changing, we are saying that there is a difference in voltage between two points in space.
So, if we imagine drawing a diagram with a battery at one end and a light bulb at the other end, we would see that there is a voltage gradient along the length of the wire.
Now, let’s think about what happens when we put a resistor in series with the wire. The voltage drops proportionally across the resistor, but the current doesn’t change. Why not? Because the voltage drop across the resistor is exactly equal to the voltage drop across the wire. So, the voltage drop across the whole circuit is the same as the voltage drop across the entire wire.
Now, what happens if we put a load in parallel with the wire? That is, if we attach a light bulb instead of a battery to the wire, we get a different kind of voltage drop.
Because the light bulb has a very high impedance compared to the wire, most of the voltage drop occurs across the light bulb.
So, now, the voltage drop across both the resistor and the wire is reduced by the amount that the voltage drop across the light bulb is increased.
In other words, the total voltage drop across the circuit is still the same as before, but the voltage drop across the resistors is smaller and the voltage drop across the wires is larger.
So, the current through the circuit is unchanged.
The key idea behind Ohm’s law is that the resistance of a conductor depends on how much current is flowing through it.
So, if you know the current going through a wire, then you can calculate its resistance. And if you know the resistance of a wire, you can figure out how much current will flow through it.
This is why Ohm’s law is so important. It lets us measure things like power dissipation in circuits.
What can Ohm’s Law be used for?
The answer to this question is a simple one: it can be used to calculate the resistance of an unknown circuit. Ohm’s law states that V IR where R is the resistance and I is the current through the resistor. The voltage across any component will always equal the sum of all currents flowing into or out of the component. If you know the value of the current then you can use Ohm’s law to find the resistance of the component. In practice, however, the calculation is usually carried out using a multimeter rather than by hand.
For example, if you measure the voltage across two resistors connected in series with each other, you can use Ohm’s law to work out the resistance of both parts of the circuit. You do this by measuring the potential difference between them and dividing the result by the total current passing through the circuit.
When doesn’t Ohm’s law work?
Well, there are many situations when Ohm’s law doesn’t apply.
For example, the most obvious case is when the circuit consists of only one wire. Then, the current is infinite, so the voltage drop across the circuit is infinite too.
Another situation arises when the resistance of the circuit is very high compared to the resistance of the wires within the circuit. This happens when the circuit has a low impedance, such as a capacitor or inductor.
A third situation occurs when the circuit contains no resistive elements. For example, a light bulb is often modelled as a perfect conductor. So, the current is zero, and the voltage drop across the bulb is also zero.
The temperature can also have an impact on the resistance. For example, metals become more conductive at higher temperatures.
Finally, the resistance of some materials depends upon their orientation relative to the direction of current flow.
What else?
So, what else do we need to understand about Ohm’s law? Well, we also need to know that the voltage drop across a resistor is proportional to the current flowing through it.
But, what does that mean? Well, it just means that the relationship between voltage and current is linear. It’s linear because it looks like a straight line on a graph. But, it’s actually curved. It’s curved because the relationship between voltage and charge is non-linear. In fact, the closer you look at it, the more complicated it gets.
For example, if you have an electric field pointing towards a charged particle, the force exerted on the particle is directly proportional to the strength of the field. So, the stronger the field, the greater the force. But, the weaker the field, the less the force.
If you want to find the potential energy associated with a charged particle, you multiply the force times the distance from the source of the field. But, if you want to find the kinetic energy associated with a charged object, you must add up all the forces acting on the object. That is, you must sum up the forces due to the electric field, the magnetic field, and any gravitational fields.
The total net force on an object is always zero. And if the net force is zero, the velocity of the object is constant. So, if I have a battery connected to a resistor, the voltage across the resistor isn’t constant. Instead, it varies according to the rate at which charges pass through the resistor. If the resistor is large compared to the size of the battery, then the variation in voltage across the resistor is small. On the other hand, if the resistor is small compared to the size of a battery, then the variation is large.
So, the relationship between voltage and electric charge is non-linear, but the relationship between voltage and electricity is linear. That’s why we can use to find the relationship between voltage and currents.
Ohm’s law says that the total voltage drop across a circuit is equal to the product of the current through the circuit and the resistance of the circuit.
