# CH 7 Electricity.

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CH 7 Electricity

Section 1: Electric Charge
You know from experience that walking across a carpet floor and touching something can often result in a shocking experience. What causes this sometimes painful experience? Static electricity is the build-up of electrical charge in an object. Sometimes static electricity can suddenly discharge, like when a bolt of lightning flashes through the sky. Other times, static electricity can cause objects to cling to each other, like socks fresh out of the dryer. The static cling is an attraction between two objects with different charges, positive (+) and negative (−). You can create static electricity by rubbing one object against another object. The rubbing produces friction which releases negative charges, called electrons, that build up to produce a static charge. When you shuffle your feet across a carpet the friction causes a static charge to build up inside of you. You can suddenly discharge this static electricity when you touch a friend to send them a shock. When you rub a balloon on your head the friction causes opposite static charges to build up in your hair and in the balloon. You can see these two opposite static charges attracting each other when you pull the balloon slowly away from your head making your hair stand up Give every two students a balloon. Have them try to create static electricity by rubbing their balloon on their hair. Static electricity is the accumulation of excess electric charges on an object.

Remember—opposite charges attract and like charges repel.
According to the law of conservation of charge, charge can be transferred from object to object, but it cannot be created or destroyed. Remember—opposite charges attract and like charges repel. After 1st: Charges are actually moving from one object to another—they are being transferred, not lost. Electrons move from one object to another. The object that gains electrons becomes negatively charged. The one that loses electrons becomes positively charged.

Electric Field An electric field surrounds every electric charge. The electric field exerts the force that causes other electric charges to move. Any charge that is placed in an electric field will be pushed or pulled by the field. Before 1st bullet: Read 1st paragraph on page 196. Electric fields are represented by lines with arrows drawn away from positive charges and toward negative charges. The arrows show how the electric field would make a positive charge move.

Conductors and Insulators
An excess of electrons can move more easily through conductors. What are the best conductors? metals An insulator is a material that doesn’t allow electrons to move through it easily. What could happen to you if you walk across a carpet and then reach for a metal doorknob? Shock --The spark is caused by excess electrons moving from your hand to the doorknob. --Excess electrons were transferred from the carpet to your shoes. After 1st bullet: Read after conductors on page 196. After 2nd bullet: This is why electric wires are usually made of metals, such as copper. Gold and silver are also very good conductors but are much more expensive than copper. After 3rd bullet: Most plastics are insulators. This is why plastic coats electric wires so that a dangerous electrical shock is prevented when you touch the wire. Other insulators include wood, rubber, and glass. Read the discussion on page 196.

Transferring Electric Charge
The process of transferring charge by touching or rubbing is called charging by contact. The rearrangement of electrons on a neutral object caused by a nearby charged object is called charging by induction. Rubbing two materials together can result in a transfer of electrons between the objects. Then one object is left with a + charge and the other with an equal amount of – charge. Before 1st bullet: Read 1st paragraph on page 198.

Lightning and Thunder Lightning is a large static discharge—transfer of charge through the air between two objects because of a buildup of static electricity. Thunder is powerful sound waves generated by lightning. It is estimated that Earth is struck by lightning more than 100 times every second. It can cause power outages, injury, loss of life, and fires. Read the rest of the lightning paragraph on page 198. Read the rest of the thunder paragraph on page 198. Before 3rd bullet: The sudden discharge of so much energy can be dangerous. Read Grounding on page 200. Discussion on page 198 Misconceptions and Science Journal What is an electroscope? HW page

Section 2: Electric Current
Charges flow from high-voltage areas to low-voltage areas. Voltage is like an electrical pressure that pushes charge. A voltage difference causes charges to flow and is measured in volts (V). Before 1st bullet: Water and air move from high pressure areas to low pressure areas.

SI unit for current (I)—amperes (A)
The flow of charges through a wire or any conductor is called electric current. SI unit for current (I)—amperes (A) After 2nd bullet and with pic: Current is almost always the flow of electrons. (Read last two sentences of closed circuits on page 203).

Closed and Open Circuits
A circuit is a closed, conducting path. Electric charge flows only when the wire makes a closed loop. When any part of a circuit is disconnected, no current flows through the circuit (called an open circuit). Before 1st bullet: Have students look at the water pipe in Figure 12 on page Read 1st paragraph.

Sources of Voltage Difference
In order to keep the current moving continuously through a circuit, a device must be used to maintain a voltage difference. One common source of this is a battery. Another source is wall sockets. Read about wall sockets in 2nd paragraph on page 205.

Dry-cell and Wet-cell Batteries
Each cell of a battery has an electrolyte (a chemical that conducts charge) and two electrodes (terminals). One electrode provides electrons to the electrolyte. The other electrode takes electrons from the electrolyte. In dry cell batteries, the electrolyte is the paste. The electrodes are the carbon rod and zinc container. In wet cell batteries, the electrolyte is a liquid. The electrodes are the lead dioxide and lead plates. The individual batteries you are most familiar with are dry cells. Tell the students to examine Figure 13 on page 204. After 2nd bullet: See the moist paste.

Resistance Electric current loses energy as it moves through the filament because the filament resists the flow of electrons. Resistance is the tendency for a material to oppose the flow of electrons, changing electrical energy into thermal energy and light. SI unit for resistance = ohms (Ω) After 2nd bullet: All materials have some electrical resistance. Conductors have much less resistance than insulators. After 3rd bullet: Ohms are symbolized by the greek letter omega. Read last paragraph on page 205. Ask students: Should the heating elements on a stove tope be made of a high-resistance or low-resistance material? high resistance (You want the electrical energy changed into thermal energy). Read Resistance in Wires on page 206 and click on pic.

Ohm’s Law Ohm’s Law is related to the flow of charges, or current, in a circuit. Ohm’s Law: current (A) = voltage difference (V) resistance (Ω) If the resistance in a circuit increases, the current decreases. If the voltage difference increases, the current increases. Draw on board I (A) = V (V) R (Ω) Show circle: V=IR or R=V/I After 3rd bullet: Think about this in terms of water flow in a pipe. If you place an obstruction in the pipe (ie. create resistance) then the water will not flow well (or the current will decrease). After 4th bullet: Remember voltage is like an electrical pressure that pushes charges. If you increase the water pressure in a pipe, the water will flow faster (or the current increases). Homework: page

Section 3: Electrical Energy
Circuits typically include a voltage source, a conductor such as a wire, and one or more devices that use the electrical energy to do work. Before 1st bullet: Look around. How many things in this room or at your home do you see that are plugged into wall sockets? After 1st bullet: Read second paragraph on page 209.

Series Circuit In a series circuit, the current has only one loop to flow through. Series circuits are used in flashlights and some holiday lights. Several kinds of circuits exist. After 1st bullet: Notice how these circuits have a voltage source (battery), a conductor (wire), and one or more devices that use the electrical energy (light bulbs). At end: What happens to the brightness of each bulb as more bulbs are added? (the brightness decreases)

Parallel Circuits Parallel circuits contain two or more branches for current to move through. The current splits up to flow through the different branches. At end: Read last paragraph on page 210. Examine Figure 19 and read all of page 211. Complete Quick Demo on page 211.

Fuses and Circuit Breakers
contain a small piece of metal that melts if the current becomes too high When it melts, it causes a break in the circuit, stopping the flow of current through the overloaded circuit. Circuit Breaker contains a piece of metal that bends when it gets hot The bending causes a switch to flip and open the circuit. After 2nd Fuses bullet: To fix this, you must replace the blown fuse with a new one. Too many appliances in use at the same time is the most likely cause for the over-heating of the circuit. After 2nd Circuit bullet: Circuit breakers can usually be reset by moving the switch to the “on” position.

Electrical Power The rate at which electrical energy is converted to another form of energy is electrical power. power = current x voltage difference P (watts) = I (amperes) x V (volts) SI unit for power: watts (W) Read 1st paragraph and 2nd paragraph under Electrical Power on page 212. For example: A hair dryer might draw 10 A of current at a voltage difference of 120 V. The power rating of the hair dryer is then 10 A X 120 V = 1200 W.

Calculating Energy energy = power x time E (kWh) = P (kW) x t (h)
unit of electrical energy: kilowatt-hour (kWh) One kilowatt is equal to 1000 watts. You can calculate your energy bill, by multiplying the energy used by the cost per kilowatt-hour. Read Electrical Energy on page 214. So, one kilowatt-hour is equal to 1000 W of power for 1 h.

Example Problems A dryer draws 40 A of current at a voltage difference of 240V. What is the power rating for the dryer? 9600 W A 100 W lightbulb has a power rating of 100 W. How much energy in kWh is used when you leave it on for 5 h? 0.5 kWh Determine the cost of using a 100 W lightbulb for 20 h at a cost of \$0.09/kWh. \$0.18 1st—40 A x 240 V = 9600 W 2nd—Convert watts to kW—100 W= 0.1 kW 0.1 kW x 5 h = 0.5 kWh Homework page 215 questions 1-5 #7 as bonus