Chapter 7Chapter 7

Electricity

Electricity• An atom is the basic unit of matter and is

made of protons, neutrons, & electrons

– protons: + charge

– electrons: - charge

– neutrons: no charge

Electricity• An atom is the basic unit of matter and is

made of protons, neutrons, & electrons

– when atoms contain the same number of protons & electrons they have a neutral charge

– positive charge – occurs when electrons are lost from atoms to other objects

– negative charge – occurs when electrons are gained from atoms of other objects

Electricity• positive charge – occurs when electrons are

lost from atoms to other objects

• negative charge – occurs when electrons are gained from atoms of other objects

Electricity• static electricity – the accumulation of

excess electric charges on an object

– example: as you walk on carpet, some electrons that are loosely held by the carpet are transferred to your shoes creating opposite charges

Electricity• law of conservation of charge – states that

charge can be transferred from object to object, but it can’t be created or destroyed

– similar to the law of conservation of mass or the law of conservation of momentum

ElectricityLike & Opposite Charges

• opposite charges attract

• like charge repel

ElectricityConductors and Insulators

• Electricity is caused by the movement of electrons, from a negative to a positive region

– conductors - materials in which electrons can move through easily

• examples: metals & water

• responsible for shocks when an excess of electrons are transferred from one surface to another, such as from your hand to a doorknob

ElectricityConductors and Insulators

• Electricity is caused by the movement of electrons, from a negative to a positive region 

– insulators - materials in which electrons cannot move through easily

• examples: wood, plastic, rubber, & glass

ElectricityTransferring Electric Charge

• charge by contact – the process of transferring charge by touching or rubbing

– causes a transfer of electrons from one object to another, leaving one object with a positive charge and the other one with an equal amount of negative charge

– example: socks being attracted to each other when coming out of the dryer because of rubbing during drying

– touching a door handle and getting shocked

Warmup 12-1-11• What is Electricity?

• What is a Positive Charge?

• What is Static Electricity?

• What is the Law of Conservation of Charge?

ElectricityTransferring Electric Charge

• charge by contact – the process of transferring charge by touching or rubbing

ElectricityTransferring Electric Charge

2.charging by induction – rearranging of electrons on a neutral object caused by a nearby charged object

– example: bringing a negatively charged balloon near clothing and having it cling to the clothing by an electrical force

ElectricityTransferring Electric Charge

3.lightning - caused by a large static discharge of electrons through the air that meet with the positive charge that is built up on the ground

– collisions of atoms and molecules in the air during

– this discharge account for light being given off

Electricity• Lightning

Electricity• Lightning

ElectricityTransferring Electric Charge

4. thunder - sound wave generated by lighting

– the heat from lightning causes air to expand rapidly,

– producing sounds waves that you hear as thunder

Electricity• grounding – a way to transfer any excess

electric charge to the ground before it builds up and creates lightning

– examples: lightning rods, pipes, plumbing fixtures, metal faucets, etc.

Electricity• The presence of electric charges can be

detected by a device called an electroscope

Electricity7.2: Electric Current

• How is it that electrons can move from one object to another or move through an object? The answer lies with a difference in voltage

Electricity• voltage difference – the push that causes

electrical charges to flow through a conductor

– measured in volts (v)

– Just how heat moves from a high temperature area to a lower temperature area, charge flows from a high voltage area to a low voltage area

Electricity• circuit – closed, conducting loop through

which an electric current can flow

– If the circuit is not closed, the electrons would have nowhere to go, just as if a water line broke, the water would not be able to flow through the pipe.

Electricity• electric current – the flow of electric charge

through a wire or any conductor

– electric current is measured in amperes (A)

ElectricityTypes of Batteries

• dry cell - involves a chemical reaction between zinc and a chemical paste that cause the central carbon rod to become positive and the surrounding lower region to become negative

– once the two terminals are connected through a circuit, electricity flows

– these types of batteries are the most common type (in flashlights or calculators)

Electricity• Dry Cell Battery

ElectricityTypes of Batteries 

• wet cell - involves two metal plates and typically an acid solution with a “bridge”

– these types of batteries are used in cars

Electricity• resistance – the tendency for a material to

oppose the flow of electrons, changing electrical energy into thermal energy and light

– resistance means, how hard it is to get electricity through

– resistance in measured in ohms ()

– all materials have some electrical resistance

– electrical conductors have much less resistance than insulators

Electricity• resistance – the tendency for a material to

oppose the flow of electrons, changing electrical energy into thermal energy and light

– list some factors that affect resistance in a wire

• Material of the wire, length of the wire, and thickness of the wire

Electricity• Ohms’s Law – a formula that is used to

measure the amount of voltage or current in a circuit

V = I R

– V: voltage (measured in V)

– I: current (measured in amps)

– R: resistance (measured in )

Electricity• Ohms’s Law – a formula that is used to

measure the amount of voltage or current in a circuit

V = I R 

• Calculate the voltage difference across a 25- resistor if a 0.3-amp current is flowing through it.