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 circuits  Series  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.