The diagram shows the gamma rays produced when positrons and electrons collide and annihilate - Edexcel - GCSE Physics - Question 5 - 2015 - Paper 1
Question 5
The diagram shows the gamma rays produced when positrons and electrons collide and annihilate.
(i) The total momentum of the electron and positron before the annihi... show full transcript
Worked Solution & Example Answer:The diagram shows the gamma rays produced when positrons and electrons collide and annihilate - Edexcel - GCSE Physics - Question 5 - 2015 - Paper 1
Step 1
The total momentum of the gamma rays after the annihilation
96%
114 rated
Only available for registered users.
Sign up now to view full answer, or log in if you already have an account!
Answer
The total momentum of the gamma rays after annihilation is also zero. This is because momentum is conserved in the annihilation process. Initially, the electron and positron have a total momentum of zero. After annihilation, the gamma rays produced must also have zero total momentum.
Step 2
Calculate the total mass of the positron and electron before annihilation
99%
104 rated
Only available for registered users.
Sign up now to view full answer, or log in if you already have an account!
Answer
To find the total mass of the positron and electron before annihilation, we can use the energy-mass equivalence formula:
E=mc2
Given that the total energy of the gamma rays is 1.6×10−13J and using the speed of light c=3.0×108m/s:
Substituting into the equation:
1.6×10−13=m(3.0×108)2
Solving for mass m:
m=(3.0×108)21.6×10−13
This calculation gives:
m≈1.77×10−30kg.
Step 3
Explanation of charges before and after annihilation
96%
101 rated
Only available for registered users.
Sign up now to view full answer, or log in if you already have an account!
Answer
Before annihilation, the positron has a charge of +1 and the electron has a charge of -1. Therefore, the total charge is:
+1−1=0
After annihilation, gamma rays have no charge, so:
- gamma rays have charge = 0.
