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# Currents and DC circuits

### Valence electrons move through the otherwise solid metal wire. 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 how much water is flowing through pipes, or out of a faucet, we usually use gallons/minute. ### The smaller moving circles are valence electrons that are not tightly bound to any one atom. ## A battery and a bulb

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

## The only way for a bulb to light

### Image from Physicsclassroom .com ## Making a schematic diagram of a circuit http://www.technologystudent.com/elec1/bulb1.htm

### 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

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

### There are electrical behaviors that don’t match what some of our analogies may suggest. Analogies have limits, As Dogbert illustrates here: ### Electric charges flow inside the wire, but their electricmagnetic fields flow outside the wires! http://amasci.com/elect/poynt/poynt.html

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