The electrostatic force of repulsion between two charges can be calculated using Coulomb’s Law, which states:
F=k⋅∣q1⋅q2∣r2F=r2k⋅∣q1​⋅q2​∣​
Where:

• FF is the electrostatic force between the charges,
• kk is the electrostatic constant (8.988×109 N⋅m2/C28.988×109N⋅m2/C2),
• q1q1​ and q2q2​ are the magnitudes of the charges (+3.1×10−6 C+3.1×10−6C and +1.2×10−6 C+1.2×10−6C),
• rr is the distance between the charges (5 cm=0.05 m5cm=0.05m).

Let’s plug in the values and calculate the force:
F=(8.988×109 N⋅m2/C2)⋅∣(3.1×10−6 C)⋅(1.2×10−6 C)∣(0.05 m)2F=(0.05m)2(8.988×109N⋅m2/C2)⋅∣(3.1×10−6C)⋅(1.2×10−6C)∣​
F=(8.988×109 N⋅m2/C2)⋅(3.72×10−12 C2)0.0025 m2F=0.0025m2(8.988×109N⋅m2/C2)⋅(3.72×10−12C2)​
Calculate the numerator: F=33.456×10−3 N⋅m2/C20.0025 m2F=0.0025m233.456×10−3N⋅m2/C2​
Calculate the force: F=13.3824 NF=13.3824N
Therefore, the electrostatic force of repulsion between the charges is approximately 13.3824 N13.3824N.