Electricity

• Means a flow of charge (like a water current indicates flow of water)

• Electric current is based on quantity of charge per second• Current (I) = charge (q) / time (t)• I = q/t• Unit is coulomb per second, called ampere, or amp for

short

Electric current

Simple Circuit

• Substance that allows charges to flow easily• Metals- good conductors• Wood/plastic/rubber/diamond/glass- poor conductors

• Good conductors have low resistance and vice versa

Conductors

• Electrical resistance is property of opposing or reducing current

• Depends on four variables:• Material• Length (longer has greater

resistance)• Diameter (thinner has greater

resistance)• Temperature (increased temp

greater resistance)

Resistance

• An electrical circuit contains a device, such as a battery or generator, that acts as a source of energy as it forces charge out of one terminal, through the wires of the circuit, and back into the other terminal.

• Work is done by electric field generator (ie. battery)• Work done by device (ie. Lamp) is equal to work done on

battery• Voltage (V) is ratio of work to charge moved• V=W/q• 1 volt (V) = 1 joule (J) / 1 coulomb (C)

Voltage

Electric Circuits

Volt = joule/coulomb• 1 V = 1 J/1 C• Household circuits typically have 120 or 240 volts• V of 120 means that each coulomb of charge that moves

through the circuit can do 120 J of work in some electrical device

• Is there anything that might limit work?• resistance

Units of electric circuit

• If conductor offers small resistance, less voltage needed to push an amp of current through circuit

• More resistance requires more voltage• Resistance is ratio between voltage and resulting current • R=V/I………………….1 ohm = 1 volt/1 amp• Resistance units are ohms ()• Rearrange equation to get• V=IR, this is known as Ohm’s law

Resistance

Ohm’s Law

• Voltage (V)• Resistance (R)• Current (I)

• All circuits have three parts in common• Voltage source, device where work is done, conducting

wires• Resistance impacts work done (like friction), so we use

low resistance conducting wires• Power output is determined by voltage, current, time• Power =Voltage * Current• P = joule/coulomb times coulomb/sec yielding joule/sec• A joule/sec is called a watt

Electrical power and work

• Voltage usually 120 or 240• Lightbulb might have power of 100 W

• Cost varies, usually in cents per kilowatt-hour• Cost = (watts)(time)(rate)

1,000 watt/kW

Electrical power and cost