Flow of Charge
When a battery is connected to a light bulb by wires, the bulb lights up. A current flows through the wire.
The wire is usually copper covered with plastic. Some of the electrons in copper are free to move through the copper. What actually flows are these electrons (and not the copper atoms).
The electrons carry negative charge. A current is a flow of charge. It is defined as the amount of charge flowing pass a point per unit time.
Charge is measured in the unit of coulomb, or C. An electron has the very small charge of -1.6 x 10-19 C.
Current is measured in the unit of ampere, or A. If current in the wire is 1 A, it means that 1 C of charge flows pass a point on the wire in 1 second. In the above circuit, current at any point in the circult is the same.
Current does not have to be just a flow of electrons. It can also be a flow of other particles that have electrical charge. One example is the flow of protons coming out from a cyclotron that is used to treat cancer. This is also a current.
A flow of electrons in one direction gives a current that is equivalent to a flow of protons in the opposite direction. This is because they have opposite charges.
If there is a flow of protons to the right, we say that the current is to the right. If there is a flow of electrons to the left, we say that the current is to the right.
So for negative charges like electrons, the current is defined to be opposite to the flow of the particles. This may sound strange.
One practical reason for adopting this definition is because a flow of electrons to the right produces magnetic field in the same direction as a flow of protons to the left.