Switch
Description
In the open state, an ideal switch represents an infinitely large resistance; no current then flows through it (I = 0 A). When the switch is closed, its resistance is RSchalter = 0 Ω. In the closed state, there is no voltage drop across the switch (U = 0 V), no matter how large the current is. It then behaves like an ideal conductor.
In the closed state, a real switch has a very small resistance which – as with a real conductor – is in the milliohm range. Depending on the type of switch, no current at all flows through real switches in the open state. With some types of switch, a residual current in the nA range flows.
Power and energy
For a switch, either I = 0 A applies in the open state or U = 0 V in the closed state. Power and energy at an ideal switch are therefore always
At an ideal switch, too, no electrical energy is converted into another form.
Use in a circuit
The switch connects or disconnects other components as required. Without a switch, a lamp in your room that is connected to a power plant via cables would light up continuously. To switch it, you would always have to disconnect the local cable from the lamp. Manually operated switches therefore make operating loads easier and safer (protection against contact).
Switches can also be controlled by electronics. This makes it possible, for example, to supply the loudspeaker of your smartphone with current only when you play music in an app.
A switch usually separates parts of a circuit from the rest of the circuit. In the example in the figure above, there is a main switch that disconnects all parts of the system from the socket. When it is closed, the stirrer and the heater are connected to the socket. The subsystems lamp and pump can be switched on or off via separate switches.