Physics Paper 2, Nov/Dec. 2011
 Questions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Main
Weakness/Remedies
Strength

Question 6

The table shows the pointer reading of a spring balance when different weights are hung on it.
Determine the value of F.

 Weight/N 5 10 F Pointer reading/cm 8 12 30

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Candidates performance in this question was poor.  Most of them could neither state the
similarities nor the differences between photoeletronic emission and thermionic emission.
(a)     Differences between thermionic and photoelectric emission

 Thermionic emission Photoelectric emission -   Caused by heat/themalenergy -   Electrons are released at a      particular temMany candidates attempted this question and majority of them faired very well in it.   The expected answer is: .         F  =  ke             10 – 5  =  k(12 – 8)                        ……..(A)                                      F – 10  =  k(30 – 12)                      ……..(B)                                     Solving simultaneously equations   (A) and (B)               F  =  32 . 5N perature -    Time rate of emission of       electrons increases with      increase in temperature Caused by lightelectromagnetic wave energy Electrons are released at threshold frequency  Time rate of emission of electrons increases with/is proportional to increase in intensity

(b)    Similarities between thermionic and photoelectric emission

• both are surface phenomena
• electrons are released in both cases
• ejected/emitted electrons have kinetic energy.