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15 Kirchoff's Laws


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Unformatted text preview: follow the convention we discussed for passive circuits. With the current shown in the above figure we say that the battery generates P V0I watts or consumes P V0I watts. The same notation between voltage and current is followed in current sources as well: 15.5 2. The diamond symbol is used for dependent sources. The following is an example of a voltage‐controlled current source (VCCS). Many times these dependent sources are used to model some more complex circuits (e.g. transistor amplifiers). All different combinations exist for dependent sources, i.e. output is voltage or current and are controlled by voltage or current (all different combinations are possible) To summarize, a circuit element CONSUMES power P = VI (or p(t ) v(t )i(t ) in the time varying case) if the V and I are defined as follows: 15.6 This is called passive sign convention and you need to remember it. If it happens that the voltage or current does not follow this, then you can change them so they do follow the convention and you can apply the formula P = VI. For example: The circuit CONSUMES P 3V (1A) 3W or the circuit PROVIDES 3W of power. You may wonder why don’t we always define voltages and currents according to the passive sign convention. There are (at least) two answers to this question: 1) As we will soon see, in many cases you won’t know a prior, the direction of a current or the polarity of a voltage. You will need to assume one and after solving the circuit you may need to change the assumed direction or polarity...
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