11.2: Introduction to Electric Circuits

 

Voltage and Direction of Current Flow

A voltage drop (V) across two points creates an electric field (E) that impacts a force on a charged particle (F=qE) and causes it to move. Note, the voltage drop (V) is often called the "Electromotive Force" and this can cause some confusion as it is not a force, but a measure of the electric potential difference. See 5.5: Open Stax Physics and 10.2: Electromotive Force for a discussion of electric fields and Electromotive force. As the particle moves the potential energy across the field is converted to kinetic energy and the field strength decreases. In electronics we consider the current to flow from the region of high potential to the region of low, as if the particle had a positive charge, where in reality, the particle is a negative electron and flows towards the higher potential. This causes a lot of confusion for chemistry students, who like to visualize the current at the electron moving towards the positive voltage, which it does, but the current is defined as moving in the other direction, as in animated gif of figure \(\PageIndex{1}\).

Figure \(\PageIndex{1}\): Copy and Paste Caption here. (Qniemiec ; cc 4.0, Wikimedia commons)

There are several things to note from figure \(\PageIndex{1}\)

• P represents the power from the perspective of the system doing work on the surroundings, which is the opposite perspective taken in thermodynamics. That is, in thermodynamics positive energy is when energy is added to the system and negative is when the system does energy on the surroundings. There are two facets to the energy flow, on the left you have a power source, and on the right a load.
  • Power Source: On the left (p<0) is the power source that creates the positive voltage. This is a non-spontaneous endothermic process in that energy is being added to the system to pump the charge up the gradient. This could be a device like a generator connected to a hydroelectric turbine that uses the energy of water flowing down hill to move coils in an electric field, of a solar cell that uses the energy of a photon from the sun to excite electrons into higher energy states.  A battery is a power source that is driven by a spontaneous chemical redox reaction.
  • Load: On the right is the load (p>0), and this represents spontaneous flow of charge across the electric field that could be converted to mechanical work like running a motor, or the generation of heat or light, just to describe a few processes that can be driven by the flow of current.
• i represents the flow of current, here being depicted by the moving positive charged particles
• v represents the voltage, the potential drop across the load that is created by the power source. The current flows from high to low potential
• E is the electric field that the charged particle interacts with, it is a result of the potential drop (voltage)
• The current must form a closed loop to maintain flow or the accumulation of charge would decrease the electric field that interacts with the charged particles to cause the current to flow. 

Electric Circuits

• Current flows through wires and electrical components in a circuit
• Flow is driven by the voltage drop and goes from high to low potential. 
  • In a battery the high potential is the [+] electrode and the low potential is the [-] electrode.
• Circuits must form a closed loop or charge will accumulate and create an opposing electric field that stops the flow
  • Ground wires can complete the circuit

The following Falstad Circuit Simulator has two representations of identical circuits, the first shows wires completing the loop, while the second uses two ground wires to complete the loops. These simple circuits have 4 types of components; power source, load, switch and wires. If you hover your mouse over them the applet will provide information on them. The components are:

• Power source (two parallel plates) representing the electodes of an electrochemical cell.  The larger (green) one is the high (positive) voltage and this device uses some form of process to create the voltage difference.
• Resistors (series of zig-zag lines) are the load, and here you have 100 and 400 \(\Omega\) resistors connected in series.
• Switch (white line) can open or close the circuit.  By default it is open and you need to click on the switch to close it.
• Wires (lines)
  • Ground wires (three triangle lines on right circuit) show places in the diagram that are connected.  

Be sure to play with this diagram and open and close the switch of the two different representations of identical circits.

Falstad Circuit.JS \(\PageIndex{1}\): Two identical simple circuits, one showing a closed loop and the other using ground terminals.  Hoover over one of the grounds, right click and choose Delete.  The circuit stops because you no longer have a closed circuit.  Now hit <ctrl> Z (undo) and the ground returns.

AC vs DC Circuits

There are two basic types of electric circuits; direct current (DC) and alternating current (AC). Most of the electronic devices we will work with use DC, but the power grid and outlets in your house are AC.

• AC- Alternating Current
  • Oscillating change in diretion of flow
  • Easier to increase voltage which results in less power loss over long distance transmission, which is why power lines use AC
• DC - Direct Current
  • Typically used in consumer projects
  • We will use DC for our projects

The Falstad simulation \(\PageIndex{2}\) below shows 40 V AC and DC  power supplies hooked up to 200 and 400 \(\Omega\) resistors in series.  At the bottom is an oscciloscope reader across the 400 \(\Omega\) resistor showing the voltage being a constant 26.67 V for the DC power source, and sinusoidally oscillating between 26.67 abd -26.67 V for the AC power source. Note, the scales of the two scopes are different

Falstad Circuit Simulation \(\PageIndex{2}\): 40V DC and AC power supplies.

 

Tutorials

• SparkFun: What is a Circuit?
• SparkFun: Series and Parallel Circuits
• SparkFun: Alternating Current vs. Direct Current