Two parallel straight current carrying wires attract each other if the direction of current in both wires is ………… .

Answer

To determine the condition under which two parallel straight current-carrying wires attract each other, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Concept of Magnetic Fields**:
Each current-carrying wire generates a magnetic field around it. The direction of this magnetic field can be determined using the right-hand rule.

2. **Apply the Right-Hand Rule**:
For a straight wire carrying current, if you point your right thumb in the direction of the current, your fingers will curl in the direction of the magnetic field lines surrounding the wire.

3. **Consider Two Parallel Wires**:
Let’s denote the two wires as Wire 1 and Wire 2. Assume that both wires are parallel to each other and carry currents I1 and I2 respectively.

4. **Determine the Direction of Currents**:
If the current in both wires is in the same direction (let’s say both currents are flowing upwards), we can analyze the magnetic fields produced by each wire.

5. **Magnetic Field due to Wire 1 at Wire 2**:
The magnetic field (B1) created by Wire 1 at the location of Wire 2 will be directed inwards towards Wire 1 (using the right-hand rule).

6. **Force on Wire 2 due to Wire 1**:
The force (F) experienced by Wire 2 due to the magnetic field created by Wire 1 can be calculated using the formula:
\[ F = I_2 \cdot L \cdot B_1 \]
where L is the length of Wire 2 and B1 is the magnetic field at Wire 2 due to Wire 1.

7. **Direction of Force on Wire 2**:
Since the magnetic field B1 is directed inwards and the current I2 is also directed upwards, using the right-hand rule again, the force on Wire 2 will be directed towards Wire 1.

8. **Repeat for Wire 1**:
Similarly, we can analyze the force on Wire 1 due to the magnetic field created by Wire 2. The same logic applies, and we find that Wire 1 will also experience a force directed towards Wire 2.

9. **Conclusion**:
Since both wires experience attractive forces towards each other when the currents are in the same direction, we conclude that two parallel straight current-carrying wires attract each other if the direction of the current in both wires is the same.

### Final Answer:
Two parallel straight current-carrying wires attract each other if the direction of the current in both wires is **the same**.