Polarity and direction of current flow Earlier you learned about the term polarity, referring to the charge at one point with respect to another. When working with electrical circuits, we often refer to the polarity between different points in the circuit. Understanding polarity is important for connecting the leads of polarity-dependent devices such as some meters and motors. Polarity is also important for determining the direction of current flow. In Figure 10 the current leaves the source at the negative terminal, travels around the circuit in a clockwise direction, and re-enters the source at the positive terminal. It is important to notice that current flows through loads from negative to positive, and current flows through sources from positive to negative. A more precise way of stating this is that outside the source, current flows from negative to positive, but inside the source current flows from positive to negative.
Answer
Hint:Electric circuit is an arrangement of several conducting wires, connected to different electrical devices like a battery, an ammeter, a voltmeter. Electric current is nothing but a uniform flow of electrons.
Complete answer:
Conductivity is an innate property of certain materials. These materials have high values for conductivity$\left( \sigma \right)$ and low values of resistivity$\left( \rho \right)$. The reason behind these values is the presence of free charge carriers. These charge carriers can be anything. In aqueous solutions of electrolytes, these charge carriers are ions; whereas in semiconductors these charge carriers are holes left in the materials because of excitation of electrons.In case of conductors, these charge carriers are free electrons present in the material. Most of the conductors are solid metals. In solids the structure in which its atoms are placed is known as its lattice. These electrons move from one atom to another randomly in the lattice.Now since electrons are negatively charged entities, they tend to move towards the positive charge. But in a neutral conductor, where there is no positive and no negative, and it is not connected to any battery of any sorts, these electrons move randomly around the lattice, producing zero net displacement. But when the conductor is connected to both the terminals of a battery, some changes occur.A battery acts as a source of electromotive force, which is a force required for uniform motion of electrons. What a battery does is it applies an external electric field inside the conductor. The direction of this electric field is from the positive terminal of battery to the negative terminal of battery. But since electrons are attracted to positive charge, the electrons move in the direction opposite to the electric field, which is from the negative terminal to the positive terminal of the battery. Since electric current is a uniform flow of electrons, its direction is opposite to the flow of electrons.Therefore the direction of electric current in an electric circuit is from the positive terminal to the negative terminal of the battery. Its SI unit is ampere$\left( A \right)$.
Note:The functioning of electric current is analogous to the mechanics of free fall. In a free fall, the object moves from a position with higher gravitational potential to a position with lower gravitational potential. And the force responsible for creation of this gravitational position gradient is the Gravitational Force. Similarly, electric current moves from a position with higher electrical potential to a position with lower electrical potential. And the force responsible for the creation of electrical potential gradients is Electromotive Force.
It depends a lot on the circuit. Some circuits are insanely complex to such an extent that it is next to imposible to do the calculations mentally. In fact, most of us use simulators to solve complex circuits.
Anyway, the best tool you've to work on simple resistor & multi-battery circuits mentally is superposition principle.
The Superposition Principle
According to the principle, you can treat a circuit containing many sources to be made up of several different circuits with single source each. You can solve the circuit containing multiple sources by considering each source to be acting alone and add the effects of all the sources to get the final answer. The following example will illustrate the idea.
Lets say you had to solve (find the currents in each branch) the following circuit,
According to the superposition principle, the circuit can be thought to be made up of two separate circuits each with one of the source.
We are basically selecting a particular source and remove rest of the sources to get a sub-circuit. We keep doing until we have a sub-circuit for each source. In our case, there are two sources and hence there shall be two sub-circuits.
The two battery circuits is now a problem consiting of two separate single battery circuit. Solve each of them separately and you get the currents in each case to be the following,
For the first circuit (at the left),
$I_{R1} = 1.5A\space$ downwards, +ve terminal of the battery to the -ve terminal
$I_{R2} = 3.0A\space$ downwards, +ve terminal of the battery to the -ve terminal
For the second circuit (at the right),
$I_{R1} = 1.5A\space$ upwards, +ve terminal of the battery to the -ve terminal
$I_{R2} = 0.0A\space$ shorted
All that is left is to add the corresponding currents from the two sub-circuits to get the current through the resistors of the combined circuit. If the current is in opposite directions, then they will cancel as much as possible and the net current will be in the direction of the sub-circuit current which was higher.
So the final answer will be,
$I_{R1} = 0.0A\space$ This was expected. Wasn't obvious? Think why.
$I_{R2} = 3.0A\space$ downwards
This idea can be extended to circuits containing any number of sources.
The direction of electric current can be a bit confusing topic. Here we have addressed your query. Hope this article shall be helpful to you in understanding the direction of current flow.
Every particle in the nature other than the insulators* has a large number of free electrons in them. These electrons move randomly in all directions within the material under normal conditions. If a certain amount of voltage is applied across these materials, all these electrons start moving from the region of higher potential towards the region of lower potential. This movement of electrons from the region of higher potential to the region of lower potential under electric field constitutes the electric current.
*there will be no free electrons in insulator provided that it is maintained at a normal temperature or room temperature.
Definition of current
Electric current is normally referred to as the flow of charges through a conductor. It can be defined as the amount of charge that flows past a cross-section area in a conductor. In other words, the term “current” can be defined as the rate of flow of charges through a conductor. Read more about Electic Current
Electric current is measured by the number of electrons flowing past a particular point in a conductor or a circuit per unit time.
Where Q is the charge of electrons flowing through the conductor. t is the time of flow in seconds.
In which direction does electric current flow?
The direction of electric current flow is a little difficult to understand to those who have been taught that current flows from positive to negative. There are two theories behind this phenomenon. One is the theory of conventional current and the other is the theory of actual current flow. When Benjamin Franklin was studying charges, the structure of an atom and atomic particles were unknown. Hence he assumed the point of charge accumulation as positive and the point which is deficient of charges as negative. Therefore, the charge is said to flow from positive to negative. This is called conventional current.
But in reality, an electric current is nothing but the flow of electrons. Electrons are negatively charged particles and are attracted towards the positive charge. Also, many experiments have revealed that it is free electrons in a conductor that flows. Negatively charged electrons move from the negative terminal to the positive terminal. This is the direction of the actual current flow.
Direction of current flow in circuit analysis
In terms of circuit analysis, we normally consider the direction of electric current from positive to negative. Mathematically, negative charge flowing in one direction is equivalent to positive charges flowing in the opposite direction. Hence it does not make a difference. One can either consider the flow of current from positive to negative or vice versa during circuit analysis. In fact, positively charged ions can be attracted by negatively charged electrons.
Unit of current
The unit of current is ampere or A. one ampere is equal to the one coulomb per second whereas one coulomb is equal to 6.25 x 1018 electrons. By saying that one ampere of current is flowing through a circuit, it is meant that 6.25 x 1018 electrons are crossing a point in the circuit per second.