Tiêu đề PHOTOELECTRIC EFFECT.pdf ✅

www.teachersofphysics.com Copyright © 2020 17. The figure below shows ultra violet light striking a polished zinc plate placed on a negatively charged gold leaf electroscope. Explain the following observation i) The leaf of the electroscope falls.  Photoelectrons emitted from negatively charged zinc plate are repelled and electroscope becomes discharged causing the lf divergence to fall. ii) When the same experiment was repeated with a positively charged, the leaf did not fall.  Photoelectrons emitrted from positively charged zinc plate are attracted back i.e they do not escape. Thus leaf divergence remains. 18. A candle is used to heat the cap of a charged electroscope as shown in the figure below. Explain the following observation i) It is observed that the electroscope leaf falls after sometime. Explain why the leaf falls after some time.  Heat from the candle causes emission of electrons from the cap of charged electroscope. Emitted electrons repelled by negative charges discharges the electroscope causing the electroscope leaf to fall. ii) State with a reason the likely charge of the electroscope.  Negatively charged. Negative charges repel emitted electrons discharging the electroscope.