A cube of a metal is given a positive charge Q. For this system, which of the following statements is true?Electric potential at the surface of the cube is zeroElectric potential within the cube is zeroElectric field is normal to the surface of the cubeElectric field varies within the cube

The correct statement is "Electric field is normal to the surface of the cube."

Here's why:

1. "Electric potential at the surface of the cube is zero": This is not true. The electric potential at the surface of the cube is not zero because the cube is given a positive charge Q. The electric potential is defined as the work done per unit charge in bringing a positive test charge from infinity to that point. Since the cube has a charge, it will require work to bring a test charge from infinity to the surface of the cube.

2. "Electric potential within the cube is zero": This is also not true. The electric potential within the cube is not zero because the cube is given a positive charge Q. The electric potential is defined as the work done per unit charge in bringing a positive test charge from infinity to that point. Since the cube has a charge, it will require work to bring a test charge from infinity to any point within the cube.

3. "Electric field is normal to the surface of the cube": This is true. The electric field lines are always perpendicular (normal) to the surface of a charged conductor in electrostatic equilibrium. This is because the electric field inside a conductor in electrostatic equilibrium is zero, and the electric field just outside the conductor is perpendicular to the surface.

4. "Electric field varies within the cube": This is not true. The electric field within a conductor in electrostatic equilibrium is zero. This is because the charges in a conductor move until they are as far apart as possible, which is on the surface of the conductor. Therefore, there is no electric field within the conductor.