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Alternating Current

When a DC cell is connected in a circuit, the potential difference between the terminals stays constant. This is called direct current (hence DC). In a wall outlet, things are more complicated.

In the United States, the long slit is the
ground, connected to the Earth itself, which is a large supply of charge. This ensures that this terminal stays at a constant potential which we call 0 volts. The short slit is “hot”. Its potential, relative to ground, varies like a sine wave between +170 volts and –170 volts. The voltage goes from high to low to high about 60 times a second, that is, with frequency 60 Hz. This is called alternating current, since the current in the wire is changing back and forth (Figure 15-29).


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Figure 15-29


Consider your toaster plugged into the wall. The power company pulls electrons out of the Earth and pushes them onto a wire. The resulting electric field pushes electrons on down the wire, until electrons are pushed into your toaster. Electrons are pushed out of the toaster into the wire, out of the wire into the Earth. Any one electron does not go very far, but the signal goes from the power company to your toaster. (See Figure 15-30.)

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Figure 15-30


Then the power company pulls electrons out of the wire and pushes them into the Earth. The resulting electric field pulls electrons along the wire, out of your toaster. Electrons in the other wire go into the toaster and are replaced with electrons pulled from the Earth. Sixty times a second. Until your bread is toasted. Note that only one wire needs to go from the power company to your house, since the Earth itself completes the circuit.
Generally, we do not talk of the line current as being 170 volts (the maximum). Instead we talk of a kind of average (root mean square) which is Vrms = 120 volts for the line current. The following equations hold for alternating current:

So there are no new equations to memorize.
Things begin to get complicated when we connect alternating current to capacitors (and to other things), but that is beyond the scope of the MCAT.
In this chapter we built on the concepts of the previous chapter to study simple circuits. When you solve problems involving circuits, it is helpful to visualize the flow of charge as the flow of a fluid and the potential in the wires as a height above a standard. Each piece of wire is at one potential. Each individual resistor has a current I through it and a potential ∆V across it such that ∆V = IR (Ohm’s law). It is important to be careful when using Ohm’s law. Think about the circuit first. The power dissipated by a resistor is Pi = IiVi.
A capacitor is two parallel conducting plates which store charge when a potential is applied to them. The capacitance of a capacitor is its ability to hold charge C = Q/∆V. The capacitance is determined by the dimension and material of the capacitor and the stored charge depends on the applied potential. The electric field between the plates is given by ∆V = E∆x, where ∆x is the separation of the plates.

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