Grade 10 Physical Sciences · Electricity and Magnetism

Calculate shared charge and electrons transferred

Find the charge on identical spheres after they touch using Q = (Q₁ + Q₂)/2, or count electrons moved with n = Q/e.

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Electrostatics Grade 10 Physics Exam Question, Number of Electrons Transferred During ContactKhoza Explains

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Question 1.7 · May/June 2023 · Paper 1 · National (DBE) · 2 marks

Various options are provided as possible answers to the following questions. Each question has only ONE correct answer.

1.7 Two identically charged spheres, X and Y, carry charges of +2q and −6q respectively.

Sphere X experiences an electrostatic force F to the right when the distance between their centres is r.

The spheres are brought into contact and are then returned to their original positions.

Which ONE of the following represents the magnitude of the electrostatic force that sphere X experiences now?(2) Diagram for question 1.7

A $\frac{1}{4}$ F
B $\frac{1}{3}$ F
C 4F
D 12F

Work it out on paper first. Then mark yourself with the official memo, one tick at a time.

More past paper questions: calculate shared charge and electrons transferred

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3. Question 7.2.2 · May/June 2024 · Paper 1 · National (DBE) · 6 marks
7.2 Two identical neutral polystyrene balls A and B are suspended from a ceiling by insulated, light inextensible strings of equal length, as shown in the diagram below.

Ball B is then given an initial negative charge, $Q_{B}$ , of unknown magnitude. The balls attract each other, touch and then repel each other. The balls come to rest with their centres $10$ cm apart so that each string makes an angle of $9°$ with the vertical. See the diagrams below.

Answer the questions that follow.
7.2.2 Calculate the magnitude of the initial charge $Q_{B}$ given to ball B if the mass of each ball was $0, 012$ kg.(6)
Show the memo (6 ticks)Memo
1. ✓
  Correct trig expression equated to $F_{E}$ $F_{E} = w tan θ$
  
  Any one: $F_{E} = m g tan θ$ or $F_{E} = \frac{w}{t a n ( 90° - θ )}$
  1 mark · Find resultant of perpendicular vectors
2. ✓
  Substitution of mg with angle $9°$ $F_{E} = (0, 012) (9, 8) (tan 9°)$
  $= 0, 0186$ N
  1 mark · Find resultant of perpendicular vectors
3. ✓
  Coulomb's formula $F_{E} = \frac{k Q _{A} Q _{B}}{r ^{2}}$
  1 mark · Apply Coulomb's law
4. ✓
  Substitution into Coulomb's formula $0, 0186 = \frac{( 9 \times 1 0 ^{9} ) ( Q ^{2} )}{0 , 1 ^{2}}$
  $Q = 1, 44 \times 1 0^{- 7}$ C
  1 mark · Apply Coulomb's law
5. ✓
  Calculating 2Q $Q_{B} = 2 (1, 44 \times 1 0^{- 7})$
  1 mark · Calculate shared charge and electrons transferred
6. ✓
  Final answer $Q_{B} = 2, 88 \times 1 0^{- 7}$ C
  1 mark · Calculate shared charge and electrons transferred
4. Question 6.3.3 · November 2023 · Paper 1 · Provincial · 4 marks
Two small, identical spheres A and B are suspended on long strings, as shown in the diagram below. The spheres carry charges of $+ 5 \times 1 0^{- 9}$ C and $- 2 \times 1 0^{- 9}$ C respectively.
6.3 Calculate the:
6.3.3 Number of electrons transferred during contact(4)
Show the memo (4 ticks)Memo
1. ✓
  Formula $Q = n q_{e}$
  1 mark · Calculate shared charge and electrons transferred
2. ✓
  Charge transferred $3, 5 \times 1 0^{- 9} = \dots$
  1 mark · Calculate shared charge and electrons transferred
3. ✓
  Charge of an electron $3, 5 \times 1 0^{- 9} = n \times 1, 6 \times 1 0^{- 19}$
  1 mark · Calculate shared charge and electrons transferred
4. ✓
  Answer $n = 2, 19 \times 1 0^{10}$ electrons
  1 mark · Calculate shared charge and electrons transferred
5. Question 10.1.3 · November 2023 · Paper 1 · National (DBE) · 6 marks
10.1 Two small spheres, $A$ and $B$ , made of pure zinc are at rest $0, 1$ m apart on a wooden table. Sphere $A$ is negatively charged and is free to move on the table, while sphere $B$ is uncharged and fixed to the table, as shown in the diagram below.

High-intensity ultraviolet light of frequency $2, 8 \times 1 0^{16}$ Hz is now shone continuously onto sphere $B$ .

The work function of zinc is $6, 63 \times 1 0^{- 19}$ J.

Answer the questions that follow.
Sphere $A$ carries a charge of $- 5, 4 \times 1 0^{- 6}$ C and requires a minimum force of $0, 027$ N to move from rest.
10.1.3 Calculate the minimum number of photons of ultraviolet light that must strike sphere $B$ which will cause sphere $A$ to move from its rest position.(6)
Show the memo (6 ticks)Memo
1. ✓
  Formula $F = \frac{k Q _{1} Q _{2}}{r ^{2}}$
  1 mark · Apply Coulomb's law
2. ✓
  Substitution of the force $0, 027 =$
  1 mark · Apply Coulomb's law
3. ✓
  Substitution of the charges and distance $\frac{( 9 \times 1 0 ^{9} ) ( 5 , 4 \times 1 0 ^{- 6} ) Q _{2}}{( 0 , 1 ) ^{2}}$
  $Q_{2} = 5, 56 \times 1 0^{- 9}$ C
  1 mark · Apply Coulomb's law
4. ✓
  Substitution into n = Q/e $n = \frac{Q}{e}$
  $n = \frac{5 , 56 \times 1 0 ^{- 9}}{1 , 6 \times 1 0 ^{- 19}}$
  1 mark · Calculate shared charge and electrons transferred
5. ✓
  Number of electrons $n = 3, 47 \times 1 0^{10}$ (electrons)
  1 mark · Calculate shared charge and electrons transferred
6. ✓
  Number of photonsnumber of photons $= n = 3, 47 \times 1 0^{10}$ ( $3, 475 \times 1 0^{10}$ )
  1 mark · Explain the photoelectric effect
6. Question 7.5 · November 2022 · Paper 1 · Provincial · 3 marks
Two identical insulated spheres, A and B, suspended by a light inextensible string from a ceiling, are held a distance apart, as shown below.

Sphere A carries a charge of $+ 2$ nC, while sphere B is neutral.
7.5 Calculate the charge on each sphere after they have separated.(3)
Show the memo (3 ticks)Memo
1. ✓
  Formula $Q_{n e t} = \frac{Q _{1} + Q _{2}}{2}$
  1 mark · Calculate shared charge and electrons transferred
2. ✓
  Substitution $Q_{n e t} = \frac{( 2 \times 1 0 ^{- 9} ) + ( - 3 , 2 \times 1 0 ^{- 18} )}{2}$
  1 mark · Calculate shared charge and electrons transferred
3. ✓
  Answer with unit $Q_{n e t} = 1, 00 \times 1 0^{- 9}$ C
  1 mark · Calculate shared charge and electrons transferred

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