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Currents and DC circuits

Electric current is the motion of electrically charged particles through a medium. Batteries can push electrons through thin wires of metal: that’s the type of electric current that you’re most familiar with, and that we’re examining in this chapter.

Here we atoms in a metal wire. The nucleus, and most of their electrons, are staying still.

But the valence electrons are free to move through the otherwise solid metal wire.

Electrons in metal Electric current

image from Francisco Esquembre , Universidad de Murcia; Maria Jose Cano; lookang http://weelookang.blogspot.sg/

A battery and a bulb

More examples at Electronics page by V. Ryan. TechnologyStudent.com

Circuit in a light bulb

On a bulb, the silver metal tip (“nub”) is a conductor.
The black ceramic ring above it is an insulating material (a resistor)

The larger metal screw above this is another conductor.

The only way for a bulb to light

The electricity in the circuit must go through the lightbulb, not around it. The electrons follow the red path shown here:

Image from Physicsclassroom .com

Circuit goes thru Light bulb

electrons come from a wire ->

into the base (made of metal) ->

follow a wire inside the ribbed side ->

up to the filament ->

through the filament ->

               {that’s the part which glows}

back down other half of the ribbed side ->

and then to the outside of the ribbed side->

The ribbed metal case is pretty much a screw. It lets you screw the bulb into a lamp.

Making a schematic diagram of a real circuit

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Electricity circuits intro

  • Look inside simple electrical appliances
  • What are circuit diagrams
  • 2 types of electrical charges
  • what is an electrical current

Static Electricity, Unit of Charge, Coulombs

  • What is static electricity? (and why is this term a misnomer?)
  • unit of electric charge is the Coulomb
  • How is the Coulomb defined?

Measuring Electricity. Recharging a battery.

  • Direct Current (DC) circuit : the water flow analogy
  • how to measure voltage drops
  • current is the flow of electrical charges
  • batteries don’t really lose charge – they lose energy

Measuring current. AC power. Resistance

  • Measuring current with an ammeter
  • households uses AC
  • conductors, insulators and semiconductors – their resistance
  • V, I and R relationships

In which direction do charges flow?

  • Do + charges move, or do – charges move?
  • Do all electrical currents flow in the same direction?
  • In ReDox chemical reactions, we can have two different currents, flowing in different directions, at the same time. Let’s see how.

Ohm’s law and resistors

  • Ohm’s “law” is true for many materials
  • Yet not all electrical devices obey Ohm’s law
  • Potentiometers (variable resistors)

Electric Power: kilowatt-hour

  • Units and symbols for: electrical work, energy and power
  • appliances are rated by power (Watts)
  • measuring power in a circuit
  • measuring electricity by the kilowatt-hour
  • calculating the cost to run electrical devices

Series & Parallel Circuits. Safety. Circuit breakers. Kirchhoff’s laws.

  • Series circuits and parallel circuits
  • circuit breakers or fuse-boxes as safety devices
  • Find voltage drops in a series circuit: conservation of energy
  • Kirchhoff’s law: In a circuit, voltage changes must add up to zero
  • Open circuits, closed circuits, and short circuits
  • Why short circuits are dangerous


Kirchhoff’s laws

Rules for understanding the behavior of :

   electric current (I) 
   potential difference (V) 

They were first described in 1845 by German physicist Gustav Kirchhoff. They are widely used in electrical engineering and physics; we’re studying them in class now. The ideas behind them can be found in chapter 35 of Conceptual Physics (Hewitt/Pearson)

Kirchoff’s two laws, by a different artist:

Animations of Kirchoff’s current law: kirchoff current GIF

Kirchoff voltage GIF

Animations of Kirchoff’s voltage law:


Analogies for electrical circuits

Series circuit as a pump, waterfall and waterwheel.

Water Flow Analogy Simple circuit 2

Parallel circuits inside our body.


Parallel circuit as a pump, & two waterfalls and waterwheels.

parallel circuit waterfall

Circuit labs

PhET Virtual lab: Series and Parallel circuits

Lab Measuring Voltage Current DC circuits

  • Learn how to build a simple circuit, measure voltage, and current

  • Build a DC series circuit and DC parallel circuit

Lesson on parallel circuits and equivalent resistance: Parallel circuits and equivalent resistance: PhysicsClassroom

What happens inside batteries?

Chemical reactions occur within battery.
e- are stripped away from the carbon electrode.
e- try to flow from – terminal to + terminal, if a conducting circuit exists.

Here’s an amazing explanation: How A Battery Works – John Denker Av8n.com

In Physics forums we find these details.
Contributor leright writes:

Inside the battery, the negative charges flow IN THE DIRECTION OF THE E-FIELD, which means the negative charges are going AGAINST the electrostatic force set up inside the battery.
The electrons are able to flow against the electrostatic force because of an opposing chemical potential.
Normally, a battery which is not shorted out or connected to a load is under equilibrium conditions, meaning the chemical potential inside the battery exactly equals the electrical potential. Under these conditions, no charge carriers flow.
If you connect the positive terminal of the battery to the negative terminal, through some load, the electrons at the negative terminal of the battery flow through the wire to the positive terminal by the electric field set up by the E-field external to the battery.
When these electrons reach the positive terminal, the E-field inside the battery is momentarily reduced which in turn upsets the equilibrium between the chemical and electrical potential.
The chemical potential then dominates and allows the negative charge to continue flowing from positive terminal to negative terminal until equilibrium is once again established.
Notice that the electrons flow AGAINST the coulomb force inside the battery. They are able to do this because of the chemical potential, which is slightly greater than the electrical potential when equilibrium is disturbed.
Contributor vanesch adds:
The whole point is that the flow of electrons (and ions) is not controlled by the electrostatic potential, but by the ELECTROCHEMICAL potential.
That electrochemical potential is also function of the concentrations of chemicals and a battery is exactly such a structure, where the gradient in electrochemical potential and the gradient in electrostatic potential are in opposite directions.
Hence, it is the electrochemical potential which drives electrons and ions against the electrostatic force.
Of course, the electrostatic potential is a PART of the electrochemical potential.
So it is true that the electrostatic force tends to diminish the tendency to flow against the E-field, but if the concentration gradients can overcome this, then nevertheless, the charges flow against the electrostatic force.
The price to pay is that this flow will change the concentrations of chemicals in exactly the way which is necessary to “drop” the gradient of the electrochemical potential.
The system reaches a static condition when the electrochemical potential is equal everywhere: in that case, charges are not “motivated” to move anymore.
This situation can STILL contain both a gradient in electrostatic potential and a gradient in concentrations.

This is BTW, what happens in a PN junction in a semiconductor. There, you DO have an E-field, and NO charges flowing (because they are pushed exactly the same amount in the opposite direction by the concentration gradient, and both cancel).

E-field-inside-of-a-battery: Physics Forums


Is Ohm’s law (V = I·R ) really a “law”? Are all of our analogies really spot-on? Nope. Ohm’s law is useful approximation that works in many situations. But the complete laws of electricity and magnetism are in Maxwell’s equations. Understanding them requires a year of college physics and calculus, but a brief overview can be found here: Maxwell’s laws of electromagnetism.

There are electrical behaviors that don’t match what some of our analogies may suggest. Analogies have limits, As Dogbert illustrates here:

Dilbert Wisdom bad analogies

Why do water-flow analogies break down?
Water flows inside a pipe, but electricity doesn’t completely flow inside a wire.

Electric-magnetic fields flow outside wires, like this:

In a simple circuit, where does the energy flow? – William J. Beatty

Misconceptions spread by textbooks about Electricity: By William Beaty

Electricity apps Molecular expressions


Learning Standards

Massachusetts 2016 Science and Technology/Engineering (STE) Standards

HS-PS2-9(MA). Evaluate simple series and parallel circuits to predict changes to voltage, current, or resistance when simple changes are made to a circuit
HS-PS3-1. Use algebraic expressions and the principle of energy conservation to calculate the change in energy of one component of a system… Identify any transformations from one form of energy to another, including thermal, kinetic, gravitational, magnetic, or electrical energy. {voltage drops shown as an analogy to water pressure drops.}
HS-PS3-2. Develop and use a model to illustrate that energy at the macroscopic scale can be accounted for as either motions of particles and objects or energy stored in fields [e.g. electric fields.]
HS-PS3-3. Design and evaluate a device that works within given constraints to convert one form of energy into another form of energy.{e.g. chemical energy in battery used to create KE of electrons flowing in a circuit, used to create light and heat from a bulb, or charging a capacitor.}

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