Electricity

Static Electricity Static electricity is the buildup of excess electric

charge on an object

Why do you get shocked? Walking across the carpet moves electrons from

atoms in the carpet to your shoes, the carpet is no longer neutral and you have a negative charge stored on your body.

Conservation of Charge

Charge can be transferred from object to object, but it cannot be created or destroyed

Charges

Opposite charges attract, like charges repel

Electric Field and Force

An electric field is surrounds all electric charges and exerts a force on other electric charges

Materials

A conductor is a material throught which electrons pass easily

An insulator is a material that electrons do not move easily through

Induction

Induction is the charging of an object over a distance

Electroscope

An electroscope is a device to measure electric charge

Lightning Lightning is a large discharge of

static electricity Is caused by friction from the

movement of rain drops in clouds. It builds areas of positve and negative charges

Lightning can contain 100 million volts and can be hotter than the sun

Grounding – providing a path to the Earth for a charge to go through

Potential Difference

Is the differnce in electric potential energy between two points

It is measured in volts Electrons flow from high potential energy

to low potential energy.

Electric Current

What is electric current? Electric current is the flow of electrons

through a conductor It is measured in amperes (amps) The amount of current depends on the

number of electrons passing a given point in a given time

Circuit A circuit is a closed path through which current can flow

If there is no circuit, electricity will not flow

Think of electricity is like water in pipes Why does water flow through a hose? It goes from high pressure to low pressure Electricity goes from high potential to low

potential A battery is like a pump

Once there is no longer a pressure difference, water stops flowing

Once there is no longer a difference in electrical potential, electricty stops flowing

Batteries A battery is a device that converts chemical potential

energy into electrical energy. Batteries consist of two electrodes and a substance

that can conduct electricity

Types of Batteries – Dry Cell A dry cell contains two electrodes and a moist paste to

act as an electrolyte. Usually the electrodes are a carbon rod and the other

is zinc A reaction occurs between the zinc and the paste,

causing the zinc to build up a negative charge and the carbon to build a positive charge.

Examples: AA, AAA, C, D cell batteries

Types of Batteries – Wet Cell

A wet cell consists of two electrode metal plates in a solution of sulfuric acid

A wet cell battery consists of several cells connected together

Example: Car Battery

Resistance Resistance is the tendancy for a material to resist the

flow of electrons. It converts electrical energy into heat and light. Is measured in ohms (Ω) Copper has a low resistance, it is used for wire in

houses Tungsten has a high resistance and is used as a

lightbulb filament

Why would a thick wire have less resistance than a thin wire? It is easier for electrons

to flow through the thick wire

Ohm’s Law

Current in a circuit is equal to the voltage difference divided by the resistance.

I = V/R I = Current V = Voltage R = Resistance Twinkle Twinkle little star, I is equal to V over R

A battery with a voltage of 6V is in a circuit with a lamp of resitance 2 Ω. What is the current in the circuit?

V = 6 volts R = 2 ohms I = V/R = 6V/2 Ω = 3 amps

32 amps of current are flowing through a circuit with a battery of 8V. What is the resistance of the circuit?

I = 32 amps V = 8 volts I = V/R R = V/I = 8V/32amps = .25 Ω

Power and Circuits

Power Power is the rate at which work is done Electrical Power is the rate at which electrical energy is

converted into another form of energy Different devices use electricity at different rates Power Rating of an appliance

The amount of electrical energy needed to operate it

Measured in watts (W) or kilowatts (kW)

Power = current x voltage = I x V

The current in an electric clothes dryer is 15A when it is plugged into a 240 V outlet. How much power does the clothes dryer use?

I = 15 amps V = 240 V P = I x V = 15 A x 240 V = 3600 W = 3.6kW

Electrical Energy Electrical energy is measured by an electric meter The amount used depends on the power required by

the device and how long it is used. Measured in kilowatt hours (kWh) 1 kWh = 1 kW in 1 hour E= P x t kWh = kW x h

A microwave oven with a power rating of 1200 W is used for .25 hours. How much electrical energy is used by the microwave?

P = 1200 W = 1.2 kW t = .25 hours E = P x t = 1.2 kW x .25 hours = .3 kWh

Circuits

Two types of circuitsSeries Parallel

Series Circuit

The current only has one path to travel Amount of current is the same throughout

the circuit Example:

Christmas Lights Pasture Fence

What happens if a bulb burns out? All the lights go out Since there is only one path, any break stops the

current An open circuit is a circuit that is disconnected, with

no flow

Parallel Circuit Has many paths for the current to follow The amount of current can vary in each circuit because

the resistance can vary. More current will flow through the path of least

resistance Example

Houses are wired in parallel

Why are houses wired in parallel?

So you can turn off a light in one room and not turn off all the lights.

Each branch will have more current than in a series circuit

Symbols

Household Electricity Electricity flows from the

power company to the electric meter at your house.

Next it goes to a fuse box or circuit breaker

From there it goes to the appliances in your house

Standard voltage is 120V

Interrupting the Circuit

Fuse has a metal strip that melts when the current gets to high. It must be pulled out and replaced

Circuit Breaker has a metal strip that bends when it gets to hot and breaks the circuit. It can be reset.