You can turn it on and off with the same switch, or on with one and off to the other.ĭraw a diagram of a possible way of wiring the inside of the box, using 3-way switches. Either switch can turn the light on or off. You are given a box with an electric lightbulb and two switches (drawing).The power source is either a battery inside or a power cord (makes no diference). There is no other position-A must be connected to one or the other, it cannot be left open (as in an ordinary switch). (1) A 3-way switch is one that in one position connects the wire at A to the one at B, in the other position it connects it to the one at C. Actually, however, results derived here remain valid with alternating currents. Note: You may assume that the current in the examples below comes from a battery. If a path has resistance of R ohms and the voltage difference between its ends is V volts, then I amperes will flow in it. Ohm's law holds independently for any branch of the circuit. That equality leads to further conclusions, and we will return to them later. In this case, of course, the same current flows through all parts of the circuit, and denoting it by I, we get Ohm's law holds separately for the currents I 1 and I 2 flowing in the separate parts:
#TWO CIRCUITS IN ONE SWITCH BOX PLUS#
The total voltage drop from A to B equals V = 12 volt, and similar to a pressure drop in a pipe, it can be viewed as the sum of a voltage drop V 1 from A to C (across R 1), plus a voltage drop V 2 from C to B (across R 2). Imagine for example a circuit driven by a 12-volt battery (below), in which two resistances R 1 = 3 Ω and R 2 = 9 Ω follow each other as drawn ("connected in series").
What happens inside the battery (which is also part of the circuit) is not taken into account. In the drawing above, for instance, Ohm's law is applied (when the switch closes) between points A and B. Indeed, that is the way the law is usually used. If V is the voltage drop between any two points of the circuit, and R is the resistance between them, then the current I between these points satisfies Ohm's law holds separately for each part of the circuit. Two lines perpendicular to the wire, one short and thick, the other thinner and longer, are a chemical cell that produces voltage, and several cells may be combined to give a " battery." Wires that cross may either connect (dark spot) or cross without contact, often shown by having one line arch over the other. A simple line is a wire ("conductor"), a zig-zag line is a conductor with resistance, and a section on a pivot, breaking the continuity of the wire, is a switch. " fluorescent") or relying on solid-state effect have more complex behaviorĬircuit diagrams help visualize electric circuits. This way one may draw:Īn Edison-type lightbulb, emitting light from a hot glowing wire, is of course a resistance too, and will be drawn as such in circuit diagrams, unless we want to emphasize its special role. If two different batteries are part of the same diagram, the one with higher voltage could be drawn with a larger number of plates.Ī resistance (or a " resistor, a device introducing a specific resistance) is drawn as a zig-zag line, and its resistance in ohms is denoted by the Greek letter Ω (capital Omega). However, the number of cells may be large, so the symbol for an electric cell or for a 2-cell battery is often used for any battery-with the number of volts indicated in writing. (Do not confuse this with a capacitor, marked by two equal parallel lines.) With an obvious symbol for an electric lightbulb, the circuit of a simple flashlight can therefore be drawn thus:Ī battery consisting of two cells is sometimes shown thus A break in the wire with a short pivoted end is a switch, regardless of the way it actually works (the diagram suggests a "knife switch").Īn electric cell is marked by two parallel lines, perpendicular to the connected wires-a short fat one (the negative side) and a long thin one (the positive one-the button end, in the usual dry cell), each a symbol for one of the different materials (e.g. If they cross without being connected, the crossing is sometimes marked by a small arc in one wire, crossing the other like a bridge. When two lines in the diagram cross and they are connected at the crossing point, the point is usually drawn heavy, or with a small dark circle. At this point we might start drawing diagrams of electrical circuits.Ī line represents a wire (or in electricians' lingo, " an electrical conductor").
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