A current of 50A is established in a slab of copper 0.5cm thick and 2 cm wide .The slab is placed in a magnetic field B of 1.5 T. The magnetic field is perpendicular to the plane of the slab and to the current .The free electron concentration in copper is 8.4 × 10²⁸ electrons/m³? . What will be the magnitude of the gall voltage across the width of the slab?