Transcript ppt
Lightning: an electrical discharge in the atmosphere Lightning Facts •On average a lightning stroke is 5 km long and 2 to 3 cm in diameter. •Approximately 100 lightning strikes occur on the Earth every second. •The current in lighting strikes ranges from 15,000 to 30,000 amperes. (A typical household circuit is rated at 15 amperes, and a typical house has 100-200 amp service.) •The voltage in a lighting stroke ranges from 1 million to 3 million volts / meter. A 5km long lighting stroke would have 5 to 15 billion volts. (A typical household appliance is on a 120 volt circuit.) Lightning Facts •The temperature of a lightning stroke is 30,000 deg C (this is 5 times hotter than the surface of the Sun) •A lighting bolt has enough energy to power a typical house with for 2 to 22 months. •20 million cloud-to-ground lightning strikes hit the US each year. •Approximately 70 people are killed and 200 to 300 people are injured by lightning each year in the United States. Electricity and Charge Proton: positively charged particle Electron: negatively charged particle Ion: an atom with an unequal number of protons and electrons Electrical current: movement of an electrical charge (typically due to the movement of electrons) An electric field is present in any region where positive and negative charges exist. A charge will experience attractive and repulsive forces in the presence of an electricfield. What types of charges attract each other? What types of charges repel each other? Electricity and Charge The magnitude of this force of attraction is measured in volts (V). The strength of the electric field is measured in volts / meter (V/m). Current will move easily through conductors (such as metal or water) and will move poorly (or not at all) through insulators (such as plastic). Is the atmosphere a good conductor or a good insulator? What does this mean about the strength of electric fields that may build in the atmosphere? Fair weather electric field: The background electric field that exists in the lower atmosphere. The strength of the field is about 100 V/m In general the atmosphere is positively charged and the surface of the earth is negatively charged. Charging Mechanisms in Thunderstorms What is the charge distribution in and around a thunderstorm? How does a thunderstorm become charged so that lightning can occur? 1. Interface (noninductive) charging 2. Inductance charging 3. Other methods may also be important for creating the electric field in a thunderstorm Interface charging: The transfer of electrical charge from one conducting object to another when the objects are brought into contact Hail becomes negatively charged Ice crystals become positively charged. Induction charging: A feedback mechanism in which the background electric field causes positive and negative charges within a particle to migrant to opposite ends of a particle. This enhances the transfer of charge during collisions of The net result is that hail becomes even more negatively charged, and ice crystals become even more positively charged. Screening layer: Thin layer on the surface of thunderstorm of opposite charge due to charged ions created by cosmic rays. Image charge: The positively charged region on the ground that lies beneath the negatively charged region of a thunderstorm The Lighting Stroke Types of lightning strokes: 1. In-cloud lightning 2. Cloud-to-cloud lightning 3. Cloud-to-ground lightning Stages of a Lightning Stroke: (A) stepped leader: electrons moving towards the ground in a series of steps (A) return stroke: occurs when downward moving electrons meet upward moving positively charged atoms This process can be repeated multiple times over the same path. (A) dart leader: leader that occurs after the initial lightning flash Figure 20.6 Figure 20.7 Figure 20.8 Distribution of Lightening in the US Approximately 14,000 forest fires are started by lightning each year in the United States, burning roughly 2 million acres / year. Figure 20.10 Other Interesting Atmospheric Electrical Phenomena Lightning Safety Deaths: •28% occur in open fields •14% under trees •8% near water •5% on golf courses •3% on or near tractors •2.5% on corded telephones •40% unreported 84% are male What should you do if a thunderstorm is approaching? 1.Go inside if possible 2.Stay away from electrical appliances and corded telephones 3.Avoid taking a shower or coming in contact with indoor water supplies What should you do if you can’t go inside? 1.Crouch as close to the ground as possible and minimize the amount of your body that is in contact with the ground by staying on your toes or heels. 2.Do not go under trees (unless you are in a forest) 3.Do not lie on the ground 4.Remove metal objects if possible 5.A car with a metal frame is a very safe place in a thunderstorm. Figure 20.12 Figure 20A Figure 20B Figure 20C