31.1 Electric Current

Electric current: is the net amount of charge per unit time passing through an element of surface area of any particular location in the conductor.

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Direct current (DC): A circuit is which the current through each branch in the circuit is always in the same direction.

Battery

• The function of a battery in a circuit is to maintain the potential difference that enables the flow of charges.
• The battery is not the source of electrons. The internal chemical reaction inside the battery is what allows electron to gain energy (when moving from the negative electrode to the positive one).
• When we say a battery is “drained,” we do not mean that it has “used up” its supply of electrons; instead, we mean that the battery has exhausted the source of energy (often a chemical reaction) that is responsible for raising the energies of the electrons.

<aside> 💡 The direction of current is the direction that positive charges would move, even if the actual charge carries are negative.

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<aside> 💡 The current $I$ is the same for all cross sections of a conductor, even though the cross sectional area may be different. The current density $j$ and drift velocity $v_d$ will change as the area change. When area decreases, drift velocity increases, current density increases.

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31.2 Electromotive Force

An external energy is required to drive the current around the circuit. The circuit must include a device that undergoes a discharging processes (maintains a potential difference between two points) to convert chemical energy into electric energy. Devices that perform the task is called a source of electromotive force. Common examples of a source is electromotive force includes: ordinary batteries and power plants.

31.21: Direction of electron flow

Electrons are also known as charger carries. Because electrons are negatively charged, they are attracted to the positive electrode of a battery and repelled by the negative end. When a battery is connected to a circuit, electrons flow from the negative terminal to the positive terminal .