Chapter 7 ELECTRICITY

Electric Charge Protons have a positive electric charge; electrons have a negative electric charge -In most atoms, the charge of the protons and electrons cancel each other out and the atom has no NET CHARGE. -Atoms become charged by gaining or losing electrons

Charges exerting force Electric charges exert a force upon each other. Like charges repel each other +, + or -,- Unlike charges attract each other +, - The force between any two charges that are electrically charged decreases as the objects get further apart. This force also depends on the amount of charge on an object. As the amount of charge increase, the electrical force also increases.

What is static electricity? Where does this occur?

Static electricity- the accumulation of excess electric charges on an object

Electrically charged objects obey the following rules: 1.Law of Conservation of charge- charge may be transferred from object to object, but it cannot be created or destroyed 2. Opposite charges attract, and like charges repel. 3. Charges can act on each other even at a distance, because any charge that is placed in an electric field will be pulled or pushed by the field. 4. Electrons move more easily through conductors, like metal. 5. Electrons do not move easily through insulators such as plastic, wood, rubber, and glass.

Insulators Conductors

Transferring Electric charge 1.Charging by contact-The process of transferring charge by touching or rubbing -Example: Static electricity 2.Charging by induction -The rearrangement of electrons of a neutral object caused by a nearby object. -Example: A negatively charged balloon near your sleeve causes an area of your sleeve to become positively charged. 3. Static Discharge -A transfer of charge through air between two objects because of a buildup of static electricity -example: lightning 4. Grounding- using a conductor to direct an electric charge to the ground safely.

Visualizing lightning

LIGHTNING (Text page 197 ) 1. convection currents in the storm cloud cause charge separation. The top of the cloud is positive, the bottom is negative. 2. negative charges on the bottom of the cloud attract to the positive charge on the ground. 3. when the bottom of the cloud accumulates enough negative charges, the attractions of the positive charges below cause electrons to move toward the ground. 4. when the charges from the cloud and ground connect you see a lightning flash. -The Earth is struck by lightning 100 times every second. -Lightning strikes can cause power outages, injury, loss of life, and Fires.

THUNDER The surrounding air during a lightning flash can reach 30,000 ‘C- several times hotter than the sun’s surface. The heat causes air in the bolt’s path to expand rapidly, producing sound waves that you hear as thunder.

Grounding

Electroscope- used to detect electrical charges

Do you see how the water flows down the cliff? Water takes the past of least resistance-it flows where its easiest for water to go. Electric currents flow and experience resistance too. 1. Which do you think has more energy, the waterfall in this picture or Niagara Falls? 2. How do people use the energy in water currents?

ELECTRIC CURRENT The flow of electric charges through a wire or conductor -Electric current is measured in amperes(A) -Charges flow from an area of high voltage to an area of low voltage -Current is the flow of electrons -A voltage difference is related to the force that causes electric forces to flow. This is measured in Volts.

Voltage A voltage difference is measured in volts(V) A voltage difference is the push that causes charges to move

Sources of electricity A dry cell battery- produces A VOLTAGE DIFFERENCE BETWEEN ITS ZINC CONTAINER AND ITS CARBON SUSPENSION ROD, CAUSING CURRENT TO FLOW BETWEEN THEM’ -Its components include two electrodes(carbon rod, zinc container)electrolyte (conducting chemicals in a moist paste)

What can you infer about battery capacity and battery life from this diagram?

A WET CELL Battery- contains two connected plates made of different metals in a conducting solution. Its components include two connected plates made of different metals in a conducting solution.

Wall sockets- have a voltage difference across the two holes of an electric outlet and a generator at a power plant provides this voltage difference.

Resistance - The tendency for a material to oppose the flow of electrons, changing electrical material into thermal energy and light. All materials have some electrical resistance Resistance is measured in Ohms Making wires thinner, longer, or hotter increases the resistance.

OHMS LAW- the current in a circuit equals the voltage difference divided by the resistance

Unknown Value Known Value Equation Current Voltage difference resistance I=v/r Resistance Voltage difference currentR=V/I Voltage difference Current resistance V=IR

ELECTRICAL CIRCUITS Circuits rely on generators at power plants to produce a voltage difference across the outlet, causing the charge to move when the circuit is complete. Two types of circuits- series and parallel

Series Circuit- The current has only one loop to flow through The parts of a series circuit are wired one after another, so the amount of current is the same through every point. Open circuit- if any part of a circuit is disconnected. No current flows through the circuit. For example- Christmas lights

Parallel Circuits -Contains two or more branches for current to move through. Individual parts can be turned off without affecting the entire circuit. Example- The electrical system in a house.

Household circuits use parallel circuits connected in a logical network Each branch receives the standard voltage difference from the electric company. Electrical energy enters your home at the circuit breaker or fuse box and branches out to wall sockets, major appliances and lights.

Does your home have a fuse box or circuit breaker? Why is it there? How does it work?

Electric Fuse- contains a small piece of metal that melts if the current becomes to high, opening the circuit and stopping the flow of current Circuit Breaker- contains a small piece of metal that bends when it gets hot, opening the circuit and stopping the flow of current.

FUSE VS. CIRCUIT BREAKER Similarities They are safety devices to stop hot wires from getting to hot They break the flow in the current when it is too high Differences In a fuse, a piece of metal melts when the current is too high, breaking the circuit In a circuit breaker, a piece of metal beds when the current is too high, flipping a switch to open the circuit.

Electrical energy is easily converted to mechanical, thermal, or light energy. Some examples are:

Electrical power- the rate at which electrical energy is converted to another form of energy. Electrical power is expressed in Watts(W) Power= current X voltage difference P(watts)= I(ampere) X V(volts)

Electric Power Unknown valueKnown ValueEquation Power Current Voltage difference P=IV Current Electric power Voltage difference I=P/V Voltage Difference Electric Power Current V=P/I

To Calculate the amount of energy an appliance uses: The unit of electrical energy is the kilowatt-hour, which equals 1000 watts of power used for one hour. Energy=power X time E(kWh)= P(kW) X t(h)

The cost of using home appliances

What is the difference between electric power and electrical energy? Electric power is the rate at which electrical energy is changed into another form of energy. It is measured in watts or kilowatts. Electrical energy is the electric power multiplied by the length of the time the power is used. It is usually measured in kilowatt hours.

What are some ways you can save on electrical use in your home?