DC Circuits

This section lists the main components of d.c. (direct current) circuits that you will encounter in your physics classes.
This is a source of emf (electromotive force), with a voltage of , measured in units of volts, V. The most common source you will see will be a battery. 
This is a resistance, measured in units ohms ohms, . Most often it will be a resistor. However, wires connecting parts of a circuit are really not resistance-free. We can model this real-life case by putting a 'resistor' in the circuit which has the same resistance as the wires would have. Thus the wires can still be considered resistance-free since their resistance is represented by an additional resistor. 
This is a capacitor, with capacitance C measured in units of farads, F. The perfect capacitor will have no resistance (or inductance; see next), and we will assume that this symbol represents such an ideal capacitor. If a capacitor is not ideal, that is, it has some resistance (or inductance) associated with it, then we can model this by putting a 'resistor' (or inductor) in parallel with the capacitor. This resistor (or inductor) will have the same resistance (or inductance) as the non-ideal capacitor. 
This is an inductor, with inductance L, measured in units of henrys, H. To model a non-ideal inductor in a circuit (an inductor with an associated resistance or capacitance) we can put a resistor in series or a capacitor in parallel with the inductor. Giving these the same values as the inductor will represent an imperfect inductor.

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