If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

### Course: Electrical engineering>Unit 2

Lesson 1: Circuit elements

# Ideal circuit elements

Explore the world of circuit analysis with the three fundamental circuit elements: resistors, capacitors, and inductors. Understand their unique symbols, properties, and the key equations that connect voltage and current for each component. Master Ohm's law for resistors, the capacitor equation, and the inductor equation to build a strong foundation in circuit design and analysis. These ideal component equations will unlock the door to creating and understanding a wide variety of electrical circuits. Created by Willy McAllister.

## Want to join the conversation?

• So I was looking through my EE notes from last semester and I see an inconsistency:

@ when you start labeling the Voltage sources direction for current you show that the direction of the current is into the source as opposed to out of the source! I verified with the textbook and for a voltage source the current points out from the positive terminal.
• Depends how you want to look at the system. Conventional current (which I have been taught in college/high school classwork) does have current flow from the positive terminal, but the actual electron current flows from the negative to positive. I admit the conventional current is easier to work with in circuit analysis, but when I do electron flow models, I do have to use current flow coming out of the negative terminal.
• Why do we need a current source?
If we have a voltage source, current automatically flows, if the circuit is closed
• Ideal constant voltage and constant current sources are mathematical conceptions. Both of them create both voltage and current. One keeps voltage constant, the other keeps current constant. In the real world, we are more familiar with voltage sources such as batteries or wall plugs. We build real-world voltage sources by packaging chemical reactions (batteries) or by driving generators with steam or falling water. A power supply (a complicated circuit) as another kind of source of constant voltages. There are plenty of real-world current sources, too. Constant current sources can be designed just like power supplies for constant voltage. Inside your cell phone, the analog amplifiers that send and receive the signals all have 2-transistor circuits called "current mirrors", which act as a current source. When we model a bipolar junction transistor (BJT), this transistor has a tiny current source inside.
• I didnt exactly understand what a capacitor does Can anyone please clarify??
• Capacitor holds an electric charge so it can discharge it later.
It charges and discharges itself, basically.
• Is a solar panel a circuit?
• No, you can consider a solar panel as a voltage supply from the video above. It's an element to be part of a circuit and it supplies voltage to the circuit based on the amount of light it is receiving.
• what is the use of an inductor ?
• A use of an inductor is in a low pass filter. Also 2 inductors side by side makes a transformer.
(1 vote)
• But what about an inverter? I know that with wind and solar power you produce DC and store it in batteries and if you want to have a much higher voltage AC(DC voltage is usually around 6 volts per solar battery whereas AC at home is usually around 100-200 volts) produced by the low voltage DC you need an inverter which is something that converts DC to AC and raises the voltage in the process.

How would you represent that in a circuit?
• Inverters are much more complex circuits, designed using other types of circuit elements, including microcontrollers, gotta work up
• When are uppercase V and I used, and when lowercase v and i? I have noticed lowercases are often used with capacitors and inductors (when V and I they are not constant). Can anybody confirm this or am I wrong? If so, what is the difference?
• Hello Joost,

Confirmed!

As a rule the uppercase V and I are used for DC circuits. The lowercase v an i are used for time varying or AC circuits. In DC we talk about V and in AC we talk about v(t).

However, later in your studies you will be introduced to phasors and then once again we will see V and I except this time they are in bold face type.

Enjoy,

APD
• How would you create an operational amplifier with a gain of unity? I believe this is called a unity follower and am assuming no resistors would be required. How would you wire an oscilloscope to measure the input and output voltage?
• An op amp whose output feeds directly into its inverting (-) input will have a gain of 1 respective to the voltage at its non-inverting (+) input.
In an ideal op amp this configuration will always result in a gain of 1 regardless of the input voltage. In a real world component the output voltage may not be able to reach the same voltage as the non-inverting input. Due to for example power supply limits, if you power the op amp from +5V and 0V don't expect the output to be able to track the input above +5V or below 0V (or even to exactly +5V or 0V as the output of a real op amp can't reach to exactly equal to its power rails).

Also a real world op amp will not be able to provide a stable output in a unity gain configuration at all input frequencies. This is because there is a propagation delay from the input to the output.

Regarding an oscilloscope, a basic oscilloscope probe measures between two points the reference being the probe tip, with respect to the ground (usually a crocodile clip). If the scope has multiple channels the grounds of the probes will be connected together. This means that for the most part you can only measure voltages in your circuit with respect to 0V. To measure a unity gain op amp circuit, you would use one channel to measure the input with respect to ground (so the probe tip connected to the non-inverting input and the ground to 0V) and the second channel to the output in the same manner.
• what is the use of capacitor and inductor in a circuit ?
• Capacitors have two main jobs in a circuit.
When connected to signals that are changing, capacitors are used as filters. A filter allows some frequencies to pass through while other frequencies are discouraged.
When a capacitor is connected to a voltage that is supposed to remain steady (a power supply voltage for example), the capacitor acts as a reservoir or bucket to hold charge close to where it is needed.

Inductors are used in filters in a similar way as capacitors. This is becoming less common because inductors are invariably large objects. They are hard to fit inside modern gadgets. The other job for inductors is in power supply circuits that convert high voltage AC from a wall plug into low voltage DC used inside plug-in equipment like desktop PCs or monitors. When you see a power cord with a cylinder-shaped lump in it, that lump is a type of inductor (called a ferrite core).