Question

1. A _________________ _________ causes light rays from a distant source to converge to a _____________ ______________

2. Charging by induction, a charged rod is brought near a neutral conductor: the electrons in the conductor redistribute such that there is a net positive charge away from the rod (T/F)

3. Electrical resistivity doesn’t change when temperature changes (T/F)

4. With a magnetic core, the total magnetic field is increased by a factor of 10^3 to 10^4 relative to the field without a core. It is described by saying that the magnetic materials have a large _______________ _________________

Answer 1

1. A converging lens causes light rays from a distant source to converge to a point called focal point or principal focus.

The light rays from a distance source are made to fall on a converging lens, they get refracted and the rays converge at a point on the axis. This point is called as focal point.

2. The answer depends on the charge of the rod.

True if the charge of the rod which is brought near is positive. As the like charges repel each other.

False if the charge of the rod which is brought near is negative. Then, the negative charges move away from the rod as like charges repel each other.

3. False. The electrical resistivity changes with temperature. The electrical resistivity depends on the material type, if the material is conductor, the electrical resistivity increases with increase in temperature as the atoms vibrate as the temperature increases in case of conductors making the carriers responsible for conduction to collide the vibrating atoms and hence decreasing electrical conductivity and increasing electrical resistivity. In case of semiconductors or insulators, as the temperature increases, the electrical resistivity decreases as there will be more carriers in conduction band resulting in increasing electrical conductivity and decrease in electrical resistivity (conductivity and resistivity are inversely proportional) .

4. With a magnetic core, the total magnetic field is increased by a factor of 10³ to 10⁴ relative to the field without a core. It is described by saying that magnetic materials have a large magnetic permeability.

Materials with large magnetic permeability can concentrate the magnetic lines of force passing through them which leads to increase in total magnetic field.