Electrical Current Study Guide
In this lesson, our attention will be on moving charges and the effects they produce on their surroundings, We will define electrical current, electrical resistance and capacitance, Ohm's law and Kirchoff's law, resistor series, and parallel circuits.
A new equilibrium situation arises in electricity. In thermal phenomena, as particles move around and collide with each other, we say they reach thermal equilibrium when the temperature becomes the same across all parts of the system. In the case of electrical phenomena, the charges are the ones determining a situation of equilibrium. Charges move between two points when a potential difference (electrical potential energy per charge) exists between the two ends of a conductor. If the ends are at the same potential, there is no flow of charge through the conductor. The flow of charge rapidly stops if we do not insure that the potential difference is maintained between the ends of the conductor by supplying energy to the conductor through a battery or an electrical generator such as a Van der Graaff generator. The maximum potential difference given out by a battery is called electromotive force (emf), is symbolized by E, and is measured in volts by an instrument called a voltmeter. A device that can measure more than one electrical property (such as resistance, voltage, and current) is called a multimeter.
A battery is usually symbolized by a diagram such as that shown in Figure 14.1, where the two electrodes are charged with positive and negative charges, and the current, as the convention we agreed upon tells us, travels from the positive electrode out in the circuit toward the negative electrode.
The charges moving through the conductor are, in a majority of cases, the free electrons in the conductor; for this reason, they are also called conduction electrons. Positive charges moving through the conductor are usually positive ions because the protons are strongly bonded to the nucleus.
The flow of charge can be continuous and at a constant rate, and the current is called direct current, or dc. Or it can flow in alternating directions, and then the current is called alternative current, or ac.
The electrical current is the amount of charge that passes through the cross-sectional area of the conductor per unit time.
where Δq is the total charge that passes through the area and Δt is the time considered. Electrical current is measured in coulombs per second, or amperes, A, named after Andrè-Marie Ampère (1775–1836).
Historically,itis considered that the flow of electrons is opposite to the electrical current, because in the beginning, it was believed that positive charges–and not electrons–determine the electrical current through a conductor.
In a conducting wire, electrons move as shown in Figure 14.2. Determine the electric current if 108 electrons move through the cross-sectional area A in 1 ms.
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