**Introduction:**

When two insulated charged copper spheres, A and B, of identical size have charges qa and qb respectively, and a third uncharged sphere C is brought in contact with A and then with B, the charges on A and B will redistribute. This redistribution occurs due to the principle of charge conservation, which states that the total charge in a system remains constant.

**Charge redistribution process:**

1. **Initial charges:**
- Sphere A: Charge qa
- Sphere B: Charge qb
- Sphere C: Uncharged

2. **Contact between A and C:**
- When sphere C is brought in contact with sphere A, some charge will transfer between them.
- Since sphere C is uncharged initially, it will acquire a charge equal in magnitude but opposite in sign to the charge on sphere A.
- Sphere A will lose some charge, and its new charge will be qa' = qa - qc, where qc is the charge acquired by sphere C.

3. **Contact between B and C:**
- After the first contact, sphere C carries a charge equal in magnitude but opposite in sign to the charge on sphere A.
- When sphere C is brought in contact with sphere B, charge will transfer between them.
- Sphere B will transfer some charge to sphere C, which will now have a charge equal to qb - qc.
- Sphere B's new charge will be qb' = qb - (qb - qc) = qc.

4. **Final charges:**
- Sphere A: qa' = qa - qc
- Sphere B: qb' = qc
- Sphere C: qc

**Explanation:**

When sphere C is brought in contact with sphere A, charge redistribution occurs due to the principle of charge conservation. The total charge in the system remains constant, but the charges on spheres A and C change. Sphere C acquires a charge equal in magnitude but opposite in sign to sphere A. This occurs because charge flows from a region of higher potential to a region of lower potential until equilibrium is reached.

Similarly, when sphere C is brought in contact with sphere B, charge transfer occurs again. Sphere B transfers some charge to sphere C, resulting in a redistribution of charges. Sphere B's new charge is equal to the charge acquired by sphere C, while sphere C retains the charge it acquired from sphere A.

The final charges on spheres A, B, and C are qa' = qa - qc, qb' = qc, and qc respectively. The magnitude of the charges on spheres A and B will depend on the initial charges qa and qb and the charge acquired by sphere C, qc.

**Conclusion:**

When an uncharged sphere is brought in contact with two charged spheres, the charges redistribute due to the principle of charge conservation. The charge acquired by the uncharged sphere is equal in magnitude but opposite in sign to the charge on the first sphere. This charge is then transferred to the second sphere when the uncharged sphere is brought in contact with it. The final charges on the spheres can be determined using the equations qa' = qa - qc and qb' = qc, where qa' and qb' represent the new charges on spheres A and B, and qc represents the charge acquired by the uncharged sphere.