An electric circuit is a closed pathway for the transfer of energy. While the movement of charge, known as current, is the visible manifestation of a circuit operating, the energy itself does not originate from this flow. Instead, energy is supplied by a source that converts stored energy into electrical energy, pushing the charge carriers through the circuit to do work.
The Necessary Condition: Creating Potential Difference
The entire process of supplying energy hinges on the creation of an electrical potential difference, often called voltage. This difference represents the electrical potential energy available per unit of charge between two points in a circuit. Without this differential, charge carriers would move randomly but would not flow in a concerted, directed manner to power a device.
A simple way to understand this concept is through a gravitational analogy, like a water slide. A pump must perform work to lift water to the top of the slide, granting it potential energy. Similarly, an electrical source does work to accumulate positive charge at one terminal and negative charge at the other, creating a high-energy and low-energy side.
This separation of charge establishes the potential gradient, which acts as the “push” or pressure that drives the charge around the circuit. This potential difference provides the incentive for electrons to move from the high-potential side, through the external circuit, and back to the low-potential side.
The Physical Sources of Electrical Energy
Energy is supplied by physical devices that perform the necessary conversion to establish and maintain the potential difference. These sources fall into two main categories: those that use chemical reactions and those that use mechanical motion. Both types convert non-electrical energy into the electrical potential energy required for the circuit to function.
Chemical Energy Conversion
Batteries are electrochemical cells that store energy chemically and convert it into electrical energy. The stored energy lies within the chemical bonds of the materials used for the two electrodes and the electrolyte solution. When the circuit is closed, a spontaneous oxidation-reduction (redox) reaction begins.
This reaction causes one electrode, the anode, to release electrons while the cathode is ready to accept them. Since the electrons cannot pass directly through the electrolyte, they are forced to travel through the external circuit. This movement constitutes the electrical current, powered by the potential energy difference created by the chemical reaction.
Mechanical Energy Conversion
Generators and alternators supply energy by converting mechanical motion into electricity, a process known as electromagnetic induction. This principle states that moving an electrical conductor through a magnetic field will induce an electromotive force. In a generator, a source of mechanical power rotates a component called a rotor.
The rotor, which contains magnets or wire coils, spins within the stationary magnetic field of the stator. This continuous relative motion causes the magnetic field lines passing through the conductor to change constantly, and the rate of this change determines the magnitude of the induced voltage, which pushes the charge carriers through the external circuit. This method produces the vast majority of the electrical power distributed across power grids.
Transformation and Consumption of Energy
Once the source has supplied the electrical potential energy, the energy is transferred by the rest of the circuit. Any component in the circuit that converts the electrical energy into another form is referred to as the load.
When the charge carriers flow through a load, they encounter resistance to their movement. Pushing the charges against this resistance transforms the electrical energy into other energy forms. For example, in a heating element, the energy is primarily converted into thermal energy, while in a light bulb, it is transformed into both light and heat.
The rate at which this energy transformation occurs is called electrical power, measured in watts. Although the charge itself is conserved and continues to circulate back to the source, the energy it carried is dissipated into the environment through the work performed by the load.