(1) Describe the process by which mercury atoms become excited in a fluorescent tube (3)

Transcript

1 Q1. (a) A fluorescent tube is filled with mercury vapour at low pressure. In order to emit electromagnetic radiation the mercury atoms must first be excited. (i) What is meant by an excited atom? (1) (ii) Describe the process by which mercury atoms become excited in a fluorescent tube. (iii) What is the purpose of the coating on the inside surface of the glass in a fluorescent tube? (b) The lowest energy levels of a mercury atom are shown in the diagram below. The diagram is not to scale. energy / J n = n = n = ground state n = Page 1 of 9

2 (i) Calculate the frequency of an emitted photon due to the transition level n = 4 to level n = 3. answer =... Hz (ii) Draw an arrow on the diagram above to show a transition which emits a photon of a longer wavelength than that emitted in the transition from level n = 4 to level n = 3. (2) (Total 12 marks) Q2. Figure 1 shows the energy level diagram of a hydrogen atom. Its associated spectrum is shown in Figure 2. The transition labelled A in Figure 1 gives the spectral line labelled B in Figure 2. Figure 1 Page 2 of 9

3 Figure 2 hydrogen spectrum showing some of the main spectral lines (a) (i) Show that the frequency of spectral line B is about Hz. (ii) Calculate the wavelength represented by line B. (b) The hydrogen atom is excited and its electron moves to level 4. (i) How many different wavelengths of electromagnetic radiation may be emitted as the atom returns to its ground state? (ii) Calculate the energy, in ev, of the longest wavelength of electromagnetic radiation emitted during this process. (2) Page 3 of 9

4 (c) In a fluorescent tube, explain how the mercury vapour and the coating of its inner surface contribute to the production of visible light. You may be awarded additional marks to those shown in brackets for the quality of written communication in your answer. (Total 8 marks) Q3. A fluorescent light tube contains mercury vapour at low pressure. The tube is coated on the inside, and contains two electrodes. (a) Explain why the mercury vapour is at a low pressure. (1) (b) Explain the purpose of the coating on the inside of the tube. You may be awarded marks for the quality of written communication in your answer. (Total 4 marks) Page 4 of 9

5 Q4. (a) Explain what happens to electrons in hydrogen atoms when a spectrum, such as that represented below, is produced. You may be awarded marks for the quality of written communication in your answer. (4) Page 5 of 9

6 (b) A fluorescent tube is normally coated on the inside with a powder. The tube is then filled with mercury vapour at low pressure. When the tube is switched on, the mercury vapour emits ultraviolet electromagnetic radiation. Explain how this ultraviolet radiation causes the powder to emit electromagnetic radiation as well. State the difference between the radiations emitted by the mercury vapour and the powder. You may be awarded marks for the quality of written communication in your answer. (4) (Total 8 marks) Page 6 of 9

7 M1. (a) (i) an electron/atom is at a higher level than the ground state (1) or electron jumped/moved up to another/higher level 1 (ii) electrons (or electric current) flow through the tube (1) and collide with orbiting/atomic electrons or mercury atoms (1) raising the electrons to a higher level (in the mercury atoms) (1) 3 (iii) photons emitted from mercury atoms are in the ultra violet (spectrum) or high energy photons (1) these photons are absorbed by the powder or powder changes frequency/wavelength (1) and the powder emits photons in the visible spectrum (1) incident photons have a variety of different wavelengths (1) max 3 (b) (i) (use of E = hf) (1) = f (1) f = /( ) = (Hz) (1) (ii) one arrow between n = 3 and n = 2 (1) in correct direction (1) 3 2 [12] M2. (a) (i) ( = 1.89) ΔE= (J) (1) (= (J)) (= Hz) (1) (ii) (1) (use of f = gives λ = m) 3 Page 7 of 9

8 (b) (i) 6 (wavelengths) (1) (ii) ( ) = 0.66(eV) (1) 2 (c) mercury vapour at low pressure is conducting (1) atoms of mercury are excited by electron impact (1) producing (mainly) ultra violet radiation (1) which is absorbed/ excites the coating (1) which, upon relaxing, produces visible light (1) electrons cascade down energy levels (1) 3 [8] M3. (a) there must be a large distance between collisions to allow electrons to gain enough energy (1) [or the vapour must not completely absorb the electrons] 1 (b) the mercury vapour emits ultra violet (radiation) (1) the coating absorbs electromagnetic radiation/light from the mercury (1) emits longer wavelengths/lower frequencies in the visible region (1) max 3 QWC 2 [4] M4. (a) electrons in energy levels/orbits (1) excited to higher levels/orbits (1) electrons relax/fall down and emit photons/em radiation (1) photon energies/frequencies are discrete (1) hence wavelengths are discrete (1) intensity depends on number of photons per sec max 4 (b) (ultraviolet) radiation (from mercury vapour) excites/absorbs (1) the atoms of the powder in the tube (1) these (atoms) de-excite and produce radiation (1) radiation is visible light (1) 4 [8] Page 8 of 9

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