Electric Current and Potential Difference
Electric circuits
Electric circuits are systems where electric charge flows through a closed loop.
To understand how these circuits work, we must first define two fundamental concepts: electric current and electric potential difference.
Electric Current: The Flow of Charge
Electric current
Electric current is the rate at which electric charge flows through a conductor.
- Consider water flowing through a pipe.
- The amount of water passing a point per second is similar to the amount of electric charge passing through a wire per second.
Mathematically, current ($I$) is expressed as:
$$I = \frac{\Delta q}{\Delta t}$$
where:
- $I$ is the current in amperes (A).
- $\Delta q$ is the change in charge in coulombs (C).
- $\Delta t$ is the change in time in seconds (s).
The ampere (A) is the unit of electric current, defined as one coulomb of charge passing through a point in a circuit per second.
Direct Current (DC)
- In direct current (DC), charge carriers (usually electrons) flow in a single direction.
- This is the type of current produced by batteries and used in most electronic devices.
If 6 coulombs of charge pass through a wire in 2 seconds, the current is: $$I = \frac{\Delta q}{\Delta t} = \frac{6 \, \mathrm{C}}{2 \, \mathrm{s}} = 3 \, \mathrm{A}$$
Calculating Current
To calculate current, you need to know the total charge passing through a point and the time it takes.
TipTo find the number of electrons in a given charge, divide the total charge by the charge of a single electron ($1.6 \times 10^{-19}$ C).
A current of 1 A is established in a conductor. How many electrons move through the cross-sectional area of the conductor in 1 s?
Solution
- The charge of one electron is $1.6 \times 10^{-19} \, \mathrm{C}$.
- In 1 second, 1 coulomb of charge passes through the conductor.
- The number of electrons is: $$\frac{1 , \mathrm{C}}{1.6 \times 10^{-19} \, \mathrm{C/electron}} = 6.25 \times 10^{18} \, \mathrm{electrons}$$
Electric Potential Difference (Voltage)
Electric potential difference
Electric potential difference, or voltage, is the energy per unit charge required to move a charge between two points in an electric field.
Mathematically, voltage ($V$) is expressed as:
$$V = \frac{W}{q}$$
where:
