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.
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
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
HUGE NUMBER ALERT
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
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: kilowatthour
 Units and symbols for: electrical work, energy and power
 appliances are rated by power (Watts)
 measuring power in a circuit
 measuring electricity by the kilowatthour
 calculating the cost to run electrical devices
Series & Parallel Circuits. Safety. Circuit breakers. Kirchhoff’s laws.
 Series circuits and parallel circuits
 circuit breakers or fuseboxes 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.
As an analogy, consider that there are parallel (blood) circuits inside our body
Parallel circuit as a pump, two waterfalls, and waterwheels.
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:
Why do waterflow 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
HSPS29(MA). Evaluate simple series and parallel circuits to predict changes to voltage, current, or resistance when simple changes are made to a circuit
HSPS31. 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.}
HSPS32. 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.]
HSPS33. 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.}