What is an Electric Circuit?
An Electric Circuit is a closed path for transmitting an electric current
through the medium of electrical and magnetic fields
. The flow of electrons across the loop constitutes the electric current. Electrons enter the circuit through the 'Source' which can be a battery
or a generator. The source provides energy
to the electrons, by setting up an electrical field which provides the electromotive force.
Fig. 2: Active and Passive circuit elements.
These are the most common components of electric circuit:
Construction of a Circuit Model
Suppose we want to construct a circuit model for a flashlight. What are the important
- Batteries: $v_s$
- Filament (lamp): $R_L$
- Case: $R_c$
- Coil: $R_1$
We can model the batteries with a voltage source. The other components can be modeled with
resistors, except for the switch. The switch is nothing more than a connection that can be either
open (off) or closed (on).
The filament is more than a resistor: it gives off light and heat. However, we are interested in
only electrical behavior, so we will ignore those properties and stick with the resistor
as a model.
How do we connect these elements together? An examination of a flashlight shows that these
things are connected in series (more on that later), which is to say they are connected end-to-end.
The following diagram results.
Fig. 2: A circuit model for a flashlight.
This is a circuit model for a flashlight. We have made some assumptions here…
- The switch is ideal and has no resistance when it is closed, and an infinite resistance
when it is open. Real switches have some small resistance when closed, and a large but
finite resistance when open.
- The batteries are ideal: the voltage does not depend on the fact that current is flowing
through them, which is not the case for real batteries.
- We use a resistor, which has constant R, to model the lamp. In fact, the resistance of a
real flashlight lamp changes a little when current flows through it. We will ignore that
Now what? We want to “solve” this circuit. That means we want to know all the voltages and
currents associated with all the circuit elements. To do that, we need to know something more
about the voltages and currents in this circuit. That information comes from Kirchhoff’s Laws