Ohms law

Hi Mitchell,

I think your doubts stem from the confusion between Potential Energy (measured in Joule) and difference of Potential between 2 points (measured in Volt).

When a Potential of 1 V is applied to a charge of 1 Coulomb, you have a potential energy of 1 Joule in this system. This is static energy, stored in the system (if you think of mechanics, it is like when you hold an object in your hand: the object has potential energy, due to its weight, its position and the gravity), and in this situation the charge is not moving.

Once you let the charge move, by connecting the 2 points and creating a circuit (i.e. you let the object in your hand go, and the object falls), the energy is transformed in work, which will make the charge flow into the circuit. The flow of charge in the unit of time is your current (measured in Ampere).

When you are in this situation (potential applied to a circuit, giving rise to a current flow), then you can define how "easy" or "hard" this charge can flow in the circuit (i.e. how much charge can flow, depending on physical properties of the circuit): this is what Ohm's law will tell you. Resistance measures just that.

jw0752 example explains perfectly how Ohm's law works in a circuit.

I hope this helps,

Fabio.

Fabio,

So in order to make electrons flow you want to make it where one point is positive and one point is negative right? So is voltage really a measurement of the positive/negative ions that are placed at opposite ends of a circuit? Because it takes energy to separate charges from neutral atoms is voltage a measurement of the amount of energy stored in the process of separating the electrons? Then when the two points are connected this makes a path for charges to equalize. How dopes ohms law take into account the physical properties of the circuit?

Hi Mitchell,

in general, I am all for simplifying things, in order to make easier to grasp the concepts. Sometimes, though, you need to formally define entities and relationships, otherwise you end up with a lot of confusion. So, with this in mind, I will try answering your questions the best I can, while also keeping as simple as I can.

In order to have a current flowing in a conductor/circuit, it is enough that the ends of such conductor are connected to points at different potential, not necessarily to a positive and a negative. The fact that we mark the ends as + and - is a convention that has been established, just like the conventional flow for the current is established to be from positive voltage point to the negative one (while, actually, the electrons move the opposite way). Applying a voltage at the ends, let's say by connecting a battery to the conductor, you are creating a condition where there is a difference in potential.

Voltage (difference of potential between 2 points) does not represent a measurement of the quantity of positive/negative charge stored at the two ends, but rather a measurement of how much work is needed to move one unit of charge from one end to the other. In a battery, for example, the voltage of its poles does not depend on how much charge it has stored, but it is defined by the chemical elements used to build it (for the chemical reaction). It is the energy of a battery that tells you how much charge is stored in it (anyway, as you said, electrically the battery is neutral, because the amount of positive charge equals the amount of negative ones).

When you apply a voltage to the ends of a conductor, you create an electric field between the two ends. While normally, in a conductor, free electrons move randomly in all directions, with no net flow of charges (no current), when the electric field is applied, an electric force is applied to the free electrons, forcing them to ordinately move, following the direction of the field, and so generating a net flow of charge - the current.

The physical structure of the conductor determines how much current can flow:  while moving, some of the electrons collide with the conductor's ions. In the collision, the electron loses part of its kinetic energy, which is transferred to the conductor's ion. The ions of the conductor are bonded together, so this transferred energy will cause them to vibrate more (an increase in vibration means an increase in temperature for the conductor). Ultimately, the "resistivity" of an object is a measure of how much electrons are "taken out" by the collisions, and so how much the conductor "resists" to the flow of charges.

The physical properties of the conductor are taken into account with the value of the resistance R itself. In particular, for any material we can define a property called resistivity, which only depends on the physical characteristics of the material. Then, the resistance of an object R is directly proportional to the resistivity and the lenght of the object, and inversely proportional to the cross-section area of the object. In Ohm's law, R does not depend on either V or I, but it is a constant, which means that, for an obkect of know resistance R, any change in the voltage across it will result in a change is the current flowing through it.

I hope this helps,

Fabio.

Hi Mitchell,

in general, I am all for simplifying things, in order to make easier to grasp the concepts. Sometimes, though, you need to formally define entities and relationships, otherwise you end up with a lot of confusion. So, with this in mind, I will try answering your questions the best I can, while also keeping as simple as I can.

In order to have a current flowing in a conductor/circuit, it is enough that the ends of such conductor are connected to points at different potential, not necessarily to a positive and a negative. The fact that we mark the ends as + and - is a convention that has been established, just like the conventional flow for the current is established to be from positive voltage point to the negative one (while, actually, the electrons move the opposite way). Applying a voltage at the ends, let's say by connecting a battery to the conductor, you are creating a condition where there is a difference in potential.

Voltage (difference of potential between 2 points) does not represent a measurement of the quantity of positive/negative charge stored at the two ends, but rather a measurement of how much work is needed to move one unit of charge from one end to the other. In a battery, for example, the voltage of its poles does not depend on how much charge it has stored, but it is defined by the chemical elements used to build it (for the chemical reaction). It is the energy of a battery that tells you how much charge is stored in it (anyway, as you said, electrically the battery is neutral, because the amount of positive charge equals the amount of negative ones).

When you apply a voltage to the ends of a conductor, you create an electric field between the two ends. While normally, in a conductor, free electrons move randomly in all directions, with no net flow of charges (no current), when the electric field is applied, an electric force is applied to the free electrons, forcing them to ordinately move, following the direction of the field, and so generating a net flow of charge - the current.

The physical structure of the conductor determines how much current can flow:  while moving, some of the electrons collide with the conductor's ions. In the collision, the electron loses part of its kinetic energy, which is transferred to the conductor's ion. The ions of the conductor are bonded together, so this transferred energy will cause them to vibrate more (an increase in vibration means an increase in temperature for the conductor). Ultimately, the "resistivity" of an object is a measure of how much electrons are "taken out" by the collisions, and so how much the conductor "resists" to the flow of charges.

The physical properties of the conductor are taken into account with the value of the resistance R itself. In particular, for any material we can define a property called resistivity, which only depends on the physical characteristics of the material. Then, the resistance of an object R is directly proportional to the resistivity and the lenght of the object, and inversely proportional to the cross-section area of the object. In Ohm's law, R does not depend on either V or I, but it is a constant, which means that, for an obkect of know resistance R, any change in the voltage across it will result in a change is the current flowing through it.

I hope this helps,

Fabio.