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Currents and DC circuits

Electric current is the motion of electrically charged particles through a medium.

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

But the atom’s valence electrons are free to leave on atom and go to the next.

Valence electrons 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/

What units do we use for measuring electrical current?

To measure distances we usually use meters.

To measure volumes we usually use liters or cubic centimeters

Laboratory beakers and graduated cylinder on lab table

To measure how much water is flowing through pipes, or out of a faucet, we usually use gallons/minute.

To measure the flow of electric current, what units should we use?

Well, the most common thing we see in our daily lives carrying electric current would be metal wires. Like the wires that bring electricity to our homes, school, and places of work. Like the wires carrying charge within our car engine, smartphones, or computers.

What exactly is flowing here? Electrons are flowing through (seemingly) solid metal!

So we measure the # of electric charges transported by a current of 1 ampere, in 1 second.

The unit of measurement is called a coulomb.  (unit symbol: C)

This is the SI (Metric) unit of electric charge.

The symbol for charges is either Q or q.

1 coulomb = # of electric charges transported by a current of 1 ampere, in 1 second


1 coulomb = 6.2415 × 10 18 charged particles moving through something, per second.

We can imagine electrical current looking like this:

Red circles are metal atoms – including the nucleus, and almost all of the electrons. S

The smaller moving circles are valence electrons that are not tightly bound to any one atom.

coulombs -> amount of charges

amperes -> flow of charges

How much current in a typical ‘shock’?

If you rub two different fabrics together, you might get a static electric shock.

How much electrical charged when this happens?

Only a very tiny amount, even for a large shock.

1 microcoulomb = 1 μC = 1 x 10 -6  C

So how much charge is on just one electron,
compared to a coulomb of charge?

e = 1.602 x 10 -19  C

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 {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 circuit

Click to download these Word documents

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

Analogies for electrical circuits

Series circuit as a pump, waterfall and waterwheel.

Water Flow Analogy Simple circuit 2

As an analogy, consider that there are parallel (blood) 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

Is Ohm’s law (V = I·R ) really a “law”?

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. 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?

While water always and only flows inside a pipe, electricity doesn’t completely flow inside a wire.

Electric charges flow inside the wire, but their electricmagnetic fields flow outside the wires!

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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