Chapter 21 Electricity

21-1 Electric Charge Static Electricity is the accumulation of electric charges on an object: One object loses electrons while another object gains electrons. Objects with “opposite” charges will be “attracted” Objects with “like” charges will “repel” each other.

21-1 Electric Charge Charged objects can cause electrons to rearrange their position on a neutral object by a process called induction. (this can cause your hair to “stand up” from static electricity) The electrons create an electric field that exerts a force on objects that have an electric charge. It is strongest near the electron and becomes weaker as distance from the e- increases.

21-1 Electric Charge Conductors are materials that allow electrons to move easily through them. (carry electricity) Metals make good conductors because they only have one or two electrons in their outer shells so they don’t hold on to them very well. Copper is used in wiring. Silver is a good conductor. Gold is also a very good conductor but it is very expensive. Metals are also used as cooking utensils and pans since they conduct heat very well. Stainless steel is an alloy of metals often used.

21-1 Electric Charge Insulators are materials that DO NOT allow e- to move through it easily; EX: plastics, wood, rubber, and glass. Earth contains a large supply of e- therefore it functions as a conductor. An object connected to Earth is grounded and will carry a current, such as lightning to Earth.

21-1 Electric Charge The Electroscope is a device that is able to detect electric charges; made of two thin metal leaves/strips attached to a metal rod with a knob at the top. (p.549) When a charged object touches the knob the electrons travel down the rod to the leaves. Both leaves become negatively charged as they gain electrons. Since they have similar charges, they repel each other. Rubbing a glass rod with silk will cause e- to leave the glass rod and build up on the silk, leaving the glass rod positively charged. This will positively charge the electroscope leaves and cause them to repel each other.

Electroscope

21- 2 Lightning – Causes & Effects Lightning is a very large discharge of static electricity. Air currents in a cloud build up areas of positive and negative charges. If the bottom portion of a cloud has a negative charge it can induce a positive charge on Earth’s surface. As the difference in charge increases, electrons are attracted toward the positively charged ground. A lightning bolt occurs when many electrons are transferred at the same time. Each lightning bolt that strikes Earth may contain several billion billion electrons.

21- 2 Lightning – Causes & Effects Most lightning doesn’t strike Earth, just moves from the negative area of one cloud to the positive area of another cloud. The lightning ionizes atoms and produces great amounts of heat which causes the air to expand rapidly, producing thunder. Lightning strikes can cause power outages, fires, injury, and death. Many buildings have lightning rods to attract the lightning bolt and carry it harmless to the ground.

21- 2 Lightning – Causes & Effects Lightning plays an important role in the nitrogen cycle of the Earth’s forests. Many, if not most, forest fires are started by lightning. Most often the flames travel along the ground and don’t harm healthy trees. This clears the dead debris from the forest floor and reduces the risk of rapidly spreading fires that can become out of control.

21- 3 Electric Current Electrons flow from one object to another because of a potential difference. (One object has more e- than the other object.) Static electricity isn’t a constant flow of electrons and cannot keep a bulb lit or an appliance working. This potential difference between two places is measured in volts (V) and is often called voltage. It is measured by a voltmeter.

21- 3 Electric Current The closed pathway through which the electrons flow is called a circuit. If a switch is turned on, closing the pathway, a bulb is lit, or an appliance starts working because electrons have a “path to flow”. The “flow” of e- through a wire or conductor is called current.

21- 3 Electric Current The amount of current depends on the number of e- passing a point in a given time. Current is measured in amperes (A). One ampere = one coulomb flowing past a point in one second. One coulomb = the charge of 6.24 x electrons. Current is measured with an instrument called an ammeter.

21- 3 Electric Current In order to keep the current moving through a circuit, a device must maintain the potential difference. One device that can maintain this potential difference (measured in volts) is a battery. Small batteries are dry cell batteries. Dry cells are electron pumps because they create a potential difference between the (+) and (-) terminals/end of the batteries. When the two ends are connected in a circuit, e- are released from the carbon rod, making the C rod (+), and e- accumulate on the zinc, making it negative (-). The potential difference between these two ends causes current to flow through the circuit as long as the chemical reaction occurs between the carbon and the zinc.

21- 3 Electric Current Connecting dry cells (batteries) can produce higher voltage. Most radios require six or more 1.5V batteries to operate. Appliances can also operate from a wall socket. The potential difference between two holes in a wall socket is usually 120 V. The electricity from the wall socket is provided by an electric generator. Batteries can also be wet cells which contain two connected plates of different metals in an electrolyte solution. EX: car batteries; most car batteries use lead plates in a sulfuric acid solution.

21- 3 Electric Current Resistance is the tendency for a material to “oppose” the flow of electrons. Most all conductors have some resistance. It is measured in Ohms (  ). As a current flows through a light bulb part of the electrical energy is converted by the Tungsten filament that glows white-hot as current passes through. It has a high resistance and the current loses electrical energy as it moves through the filament. Copper is an excellent conductor because it has low resistance. It is used in household wiring because very little electrical energy is converted to thermal energy as current passes through the wires.

21- 3 Electric Current The size (diameter and length) of the wire affects the resistance. Thick wires allow more electrons to travel so they have less resistance. Thin wires have greater resistance to electron flow because fewer electrons can travel. The longer the wire, the greater the resistance because it takes longer for the electrons to travel through it. In most conductors, resistance increases as temperature increases.

21- 3 Electric Current George Simon Ohm found experimentally that the current in a metal conductor is directly proportional to the potential difference across its ends and inversely proportional to the resistance. This statement is expressed as Ohm’s Law: Potential difference = current X resistance V (volts) = I (amperes) x R (ohms)

21- 3 Electric Current A light bulb with a resistance of 160  is plugged into a 120V outlet. What is the current flowing through the bulb? Given:resistance: R = 160  voltage: V = 120 V Unknown: Current: (I) Equation: I = V/R = 120V / 160  = 0.75 A

21- 3 Electric Current 1) (a.) Find the current flowing through a 20  wire connected to a 9 V battery. (b.) What if it were connected to two 1.5 V batteries? 2) The current flowing through a lamp is 4.5 A. It is plugged into a 120 V outlet. What is the resistance of the lamp?

21- 4 Electrical Circuits  Series circuits have only one path for the current to travel.  When any part of the circuit is turned off or goes out, or “disconnected”, no current can flow through the circuit, creating an “open” circuit.  The electrons will have no complete path to flow.

21- 4 Electrical Circuits  Parallel circuits contain separate branches (paths) for current to move through.  More current flows through the paths of lowest resistance.  Because all branches connect the same two points of the circuit, the potential difference is the same in each branch.  When one branch is open (light switch off, appliance off), the others remain working because electrons can still flow.  A fuse or a circuit breaker is wired between every parallel circuit and the main switch box as a safety device.

21- 5 Electrical Power and Energy  Power is the rate at which electrical energy is converted to another form of energy such as mechanical energy, light, or heat energy.  Power is expressed in watts (W) or kilowatts (kW). The amount of power used by an appliance can be calculated by multiplying the potential difference (V) by the current (I). P = I x V units: watts = (amperes) (volts)  Appliances use different amounts of energy depending upon their electrical needs.  Solve practice problems 1 & 2 page 566

21- 5 Electrical Power and Energy  Electrical energy in your home is measured by an electrical meter.  The amount of electrical energy you use depends on the power required by the appliances in your home and how long they are used. E = P x t energy = (power) (time) kWh = (kW) (t)  The unit of electrical energy is the kilowatt-hour (kWh).  One kilowatt-hour is 1000 watts of power used for one hour. The power company charges for each kWh you use.  Solve practice problems 1 & 2 on page 568.