Disc overy of Current Electric ity - PowerPoint PPT Presentation

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  1. DiscoveryofCurrent Electricity

  2. A great debate between two Italian scientists about the nature of current electricity vs Luigi Galvani 1737-1798 (Medical doctor) Alessandro Volta 1745 - 1827 (Physicist)

  3. Luigi Galvani Started…with an unexpected observation by Galvani Observed twitching (抽搐) of frog’s legs when dissecting them Click to see ! What causes the legs of a dead frog to twitch?

  4. Luigi Galvani More observations by Galvani They twitched when I was dissecting the frog near a static electricity machine! They also twitched during thunderstorms!

  5. Luigi Galvani Galvani’s experimental setup 1: Frog’s legs stimulated by electrical storms Galvani’s experimental setup 2: Frog’s legs stimulated by an electrostatic machine The twitching may be connected in some way to electricity……

  6. Further observations Twitching still occurred in good weather conditions away from the static electricity generator when he hung the dead frogs from an iron fence by brass hooks through their spinal cord. Galvani began a series of investigations to test his ideas He transferred the garden setting to his laboratory (with no thunderstorm or electrostatic machine). Twitching still occurred when he laid the frog on an iron plate and pressed the brass hook, which passed through the spinal cord, against the plate.

  7. Luigi Galvani Based on his observations, what was Galvani’s conclusion? Observed the twitching of frog’s legs during dissection. Further observation: Twitching of frog’s legs also occurred near an electrostatic machine and during thunderstorms. Hypothesis: The twitching is connected with electricity. Investigation: Confirmed that the twitching still occurred without an external supply of electricity. (electrostatic machines and thunderstorms) Conclusion: The electricity was inside the animal. (He called it “animal electricity”)

  8. Luigi Galvani “The electricity comes from the animal itself.” However, another Italian scientist named Alessandro Volta was skeptical (懷疑) of Galvani’s explanation…… • Galvani’s followers travelled all over Europe • publicly performing the demonstrations that • made the frogs twitch. • They did the demonstration using different animals. • The number of supporters for Galvani kept growing. Alessandro Volta

  9. Volta’s different viewpoints • He drew attention to the different metals used in Galvani’s experiments. • He suggested that the twitching could only be produced when two different metals were used in dissection or in the setup. “The electricity comes from outside, not from the frog itself.” Alessandro Volta

  10. Think about these questions … • What would have happened if no one had been skeptical of the “animal electricity” theory proposed by Galvani? • Since Galvani was facing the challenge by Volta, he had to further investigate to find more solid evidence for his claim. Volta also had to seek for more evidence to challenge Galvani’s view. Skepticism (懷疑)is important for advances in scientific knowledge. • Suggest why Volta and Galvani had different theories about the same observation(twitching of frog’s legs)? • Let’s play a game …

  11. What does this symbol represent?

  12. Some people may interpret the symbol as a sign pointing to the right.

  13. Musicians may interpret this symbol as “decrescendo”, which means “get gradually softer”

  14. a + b > 12 Mathematicians may interpret this symbol as “greater than”

  15. Volta: Physicist’s view Galvani: Medical doctor’s view Observation is not always objective Observation may be affected by personal background or training. vs

  16. Describe this picture.

  17. < Mother and Son> Describe this picture.

  18. Observation is often affected by our background knowledge and what we expect to see. • This phenomenon is described as “theory-laden observation”

  19. “Animal electricity” vs“Metal electricity” Science is evidence-based.It is always evidence that matters. Evidence can help to establish or refute (駁倒)scientific knowledge. Q: Imagine you are Galvani, what evidence would you give to support your “animal electricity” theory ? Q: Now imagine you are Volta, what evidence would you give to support your theory, that it is the two different metals that cause the twitching of the frog’s body?

  20. Round One Galvani showedthe legs could twitch even when two similar metals were used for the dissection. Luigi Galvani Click to see! Volta arguedthere existed a tiny difference between the metals even though they appeared the same. Alessandro Volta

  21. Round Two Galvani successfully demonstrated thatthe frog’s legs could twitch in the absence of any metals! (by just touching the nerves) Luigi Galvani It appeared to be a piece of evidence that no one can object to!

  22. In fact Galvani stimulated the sciatic nerve of the frog Sciatic nerve ( 坐骨神經 ) Stimulating the sciatic nerve can make the frog’s legs twitch.

  23. Location of the sciatic nerve inside the frog The sciatic nerve is a large nerve that runs down the lower limb. It is the longest single nerve in the body.

  24. Round Two Galvani successfully demonstrated that the frog’s legs could twitch in the absence of any metals! Had Galvani defeated Volta? Not yet… Galvani got rid of the metal but Volta got rid of the whole frog!

  25. Round Two • Volta’s Experiment:Put pieces of zinc metal and pieces of copper metal alternately with cotton pads soaked in a solution of sodium chloride (electrolyte) between each pair. Electricity was generated as shown by repeated sparks.This wasthe first electric battery in history providing continuous current. It was later called a Voltaic Pile. Pictures of Voltaic Pile Repeated sparks

  26. Do you support Galvani or Volta? Q: Now you know all the evidence provided by Volta and Galvani, choose whom to support. What considerations affect your evaluation of the two theories?

  27. Galvani was defeated! (for a while…) Reason 1: Other scientists can reproduce Volta’s experimental work easily. (Just putting two metals into sodium chloride solution) Galvani’s experimental work is relatively more difficult to repeat. (Good surgical skills are required to dissect and stimulate the nerve ends of frog’s legs) The difficulty of reproducing evidence affects the acceptability of the claims. Scientific experiments should be repeatable/reproducible.

  28. Galvani was defeated! (for a while…) Reason 2: The electric current produced by Volta’s work could be easily demonstrated by showing repeated sparks. However, “bioelectricity” (生物電流) in living cells is too small and Galvani did not have the technology (e.g. CRO) to measure the current. Actually both Galvani and Volta were right! Limitation of technology can hinder the development of science. Galvanometer CRO

  29. Significance of Volta’s Findings … The Voltaic Pile was the first device that could produce a steady current. (The first electric battery in history) An Italian banknote showing Volta’s picture and his Voltaic Pile to celebrate his contribution to science. Volta demonstrating his battery to Napoleon in 1801

  30. Significance of Galvani’s Findings … Galvani was the first scientist to discover that muscles and nerve cells produce electricity. Though he could not prove it due to the limitations of technology at that time Galvanism(電療法) describes the contraction of a muscle stimulated by an electric current. (This effect was named by Volta to honour Galvani.) With the help of modern technology, the function of nerve cells and their structures are studied in great detail nowadays.

  31. Scientific knowledge is tentative (暫時性的) Through the debate between Volta and Galvani, we learn that: Scientific knowledge changes because of - New evidence (Volta’s evidence refuted Galvani’s “animal electricity” theory.) - Advances in technology (Galvani’s “animal electricity” was recognized only after the invention of instruments for measuring bioelectricity.) Can you think of more examples?

  32. Scientists’skepticism Through the debate between Volta and Galvani, we also learn thatskepticism can drive the development of scientific research. Q. Can you think of recent examples where skepticism has driven the development of scientific knowledge?

  33. This story also tells us …… Scientific discovery work carried on by others Building up and further development of scientific knowledge Perseverance in investigation Observation by chance Insight + + Not enough for success