Me! you!
q 1. suppose the unit horsepower actually replaced watt as the unit of power for electric appliances. what would the power rating of a 500-w refrigerator be in horsepower?
q 2
an electric field tends to be strongest at the ends of pointed objects. how does this explain the fact the st. elmo's fire appears on pointed objects such as the masts of ships?
compare and contrast st. elmo's fire with lightening.
q 3
an electric field tends to be strongest at the ends of pointed objects. how does this explain the fact the st. elmo's fire appears on pointed objects such as the masts of ships?
compare and contrast st. elmo's fire with lightening.
q 4 the red glow of a neon light is also produced by a plasma. how might the glow of the light change if the neon gas inside it were replaced by air?
q 5 why does st. elmo's fire only occur during thunderstorms?
q 6 based on what you have learned about lightening and st. elmo's fire, do you think air is a good conductor of electric charge?

st. elmo's fire

st. elmo's fire is bluish glow sometimes seen during stormy weather on the tops of masts of ships, church steeples, and other tall pointed objects. despite its name, st. elmo's fire is not a flame and does not burn the objects on which it appears. it is a type of static discharge, like lightening. st. elmo's fire can last for several minutes.

you know that electrons accumulate on the bottoms of clouds during thunderstorms and induce a positive charge in the ground. if enough charge builds up in this way, atoms in the air can be stripped of their electrons, producing a plasma. a plasma is a glowing gas with no net charge. it contains positive ions and free electrons. st. elmo's fire is a plasma. the color of light given off by a plasma depends on the gas involved. the air in earth's atmosphere is mostly a mixture of oxygen and nitrogen gas. as a plasma, this mixture gives off a bluish glow.

Question 1. To solve this we are going to take advantage of the fact that 1 horsepower = 745.7 Watts, so to convert watts to horsepower, we just need to multiply by the conversion factor .
Lets convert the power of our refrigerator from Watts to horsepower:


We can conclude that the power rating of a 500 W refrigerator in horsepower is 0.67 horsepower.

Question 2. An electric field tends to be strongest at the ends of pointed objects; the strongest the electric field the more charge it builds up, so at the ends of pointed objects there is enough charge to strip the atoms in the air form its electrons producing plasma. That glowing of that plasma is what we call St. Elmo's fire.

Question 3. 
- Even though both phenomena are static discharges, they nature is completely different. Lightning is an electric discharge: a discharge that occurs when opposite charges accumulate until the electric field becomes strong enough to allow a current to flow. St. Elmo's fire, on the other hand, is a coronal discharge: a luminous phenomena, similar to a neon tube, that occurs when a pointed object in a strong electric field creates plasma.
- Even tough their origin an behavior are different, both of them create plasma in the air.
- Lightening is extremely hot, about the same temperature as the surface of the sun, whereas St. Elmo's fire is relatively cold.

Question 4. Since the glow in the neon tube is actually a coronal discharge that creates red plasma -due to the nature of the gas, it is basically a red St. Elmo's fire inside a tube. if we replace the neon gas with plasma, we will basically create a St. Elmo's fire inside the tube; the air inside the tube will be now a mixture of oxygen and nitrogen, so it will emit a bluish glow like St. Elmo's fire.

Question 5. St. Elmo's fire only appear during thunderstorms because it is the only time when electrons accumulate on the bottoms of clouds and induce a positive charge in the ground creating a strong electric field. The coronal discharge that creates the St. Elmo fire can only happen whiting a strong local electric field, so it can only happen during thunderstorms.

Question 6. No, air is not a good conductor of electric charge; otherwise phenomena like Sr Elmo's fire or lightening will occur outside thunder storms, which is not the case. Those phenomena only occur whiting strong electric fields, and those electric fields are only present during thunder storms. If air was a good conductor of electric charge we couldn't use electricity without killing ourselves in the process.