这是一道高中物理题及答案，请生成题目讲解视频---**Textual Information:**
**Question Stem:**
As shown in Figure 12, PR is an insulating flat plate of length L = 4 m, fixed on a horizontal ground. The entire space has a uniform electric field E parallel to PR. In the right half of the plate, there is a uniform magnetic field B perpendicular to the paper and pointing outwards. An object with mass m = 0.1 kg and charge q = 0.5 C starts from rest at point P on the plate, moving to the right with uniform acceleration under the action of the electric field force and friction. It enters the magnetic field and then moves at a constant velocity. When the object hits the baffle at the R end of the plate, it bounces back. If the electric field is removed at the instant of collision, the object still moves at a constant velocity while returning through the magnetic field. After leaving the magnetic field, it undergoes uniformly decelerated motion and stops at point C, where PC = L/4. The coefficient of kinetic friction between the object and the plate is μ = 0.4. Take g = 10 m/s². Calculate:
**(1) Determine the nature of the object's charge, is it positive or negative?**
**(2) The velocities v1 and v2 of the object before and after colliding with the baffle.**
**(3) The magnitude of the magnetic induction B.**
**(4) The magnitude and direction of the electric field strength E.**
**Numerical Constants and Given Values:**
* Length of the plate, L = 4 m
* Mass of the object, m = 0.1 kg
* Charge of the object, q = 0.5 C
* Distance PC = L/4
* Coefficient of kinetic friction, μ = 0.4
* Acceleration due to gravity, g = 10 m/s²
**Chart/Diagram Description:**
* **Type:** Schematic diagram / Geometric figure.
* **Main Elements:**
* A horizontal line segment representing an insulating flat plate, labeled 'P' at its left end and 'R' at its right end.
* A point 'C' is marked on the plate between P and R, closer to P.
* A rectangular region indicated by dashed lines is drawn above the right half of the plate (from approximately the midpoint to R). This region is labeled 'B', indicating a uniform magnetic field.
* A horizontal line segment is drawn above the plate, with 'E' labeled above it, indicating a uniform electric field parallel to the plate.
* A vertical line segment at the right end 'R' represents a baffle.
* **Relative Position and Direction:** The magnetic field region is located in the right half of the plate. The electric field is shown parallel to the plate. The object starts at P and moves towards R.
Here is the extracted content from the image:
**(1)**
由于物体返回后在磁场中无电场，且仍做匀速运动，故知摩擦力为0，所以物体带正电荷. 且:
mg = qBv₂ ①
**(2)**
离开电场后，按动能定理，有:
-μmg(L/4) = 0 - (1/2)mv² ②
由①式得: v₂ = 2√2 m/s
**(3)**
代入前式①求得:
B = √2 / 2 T
**(4)**
由于电荷由 P 运动到 C 点做匀加速运动，可知电场强度方向水平向右，且:
(Eq - μmg)(L/2) = (1/2)mv₁² - 0 ③
进入电磁场后做匀速运动，故有:
Eq = μ(qBv₁ + mg) ④
由以上③④两式得:
v₁ = 4√2 m/s
E = 2.4 N/C