même eleve fait fonctionner son televiseur 275 jours par an à raison de 3 heures par jour il le laisse en veillè le reste du temps c'est a dire 21 heures par jour pendant 275 jours et 24 heures par jour pendant les 90 jours restant de l'annee. La puissance du televiseur est de 100W quand il fonctionne et de 20W quand il est en veille. Calculer la quantite d'enrgie transforme par le televiseur en fonctionnent pendant une annee. Calculer la quantite d'energie transformee par le televiseur en veille pendant une annèe

electromagnetic shielding is the practice of reducing the electromagnetic field in a space by blocking the field with barriers made of conductive or magnetic materials. shielding is typically applied to enclosures to isolate electrical devices from their surroundings, and to cables to isolate wires from the environment through which the cable runs. electromagnetic shielding that blocks radio frequency electromagnetic radiation is also known as rf shielding.

the shielding can reduce the coupling of radio waves, electromagnetic fields and electrostatic fields. a conductive enclosure used to block electrostatic fields is also known as a faraday cage. the amount of reduction depends very much upon the material used, its thickness, the size of the shielded volume and the frequency of the fields of interest and the size, shape and orientation of apertures in a shield to an incident electromagnetic

from wikipedia

the answer is the first electromagnet (a). this is because it has more coils around the ferromagnetic material. increasing the number of coils increases the surface area in which the metal interacts with the current in the coil. this enables more atoms in the metal to become aligned hence becoming more magnetized.