A direct current $I$ is flowing through a plane in the direction designated by an arrow. The plate is placed in a transverse magnetic field $B$. As a result of the Hall effect there appears a transverse potential difference. What is the sign of the potential at point $a$ if the plate is made of metal and if the plate is an $n$-type or $p$-type semiconductor?

           

Two contours are positioned in such a manner that their planes are parallel to each other. Contour $1$ carries a current whose direction is designated by an arrow. The contours move in relation to one another, but their planes remain parallel in the process. What is the direction of the current induced in contour $2$ when the contours are moved toward each other or away from each other?

A spiral made from elastic wire is connected to a DC source. The spiral is stretched. Will the current flowing in the spiral become greater or smaller in the stretching process than the initial current or will it remain unchanged?

A solenoid carrying a current supplied by a DC source with a constant emf contains an iron core inside it. How will the current change when the core is pulled out of the solenoid: will it increase, decrease, or remain the same?

Two identical inductances carry currents that vary with time according to linear laws. In which of the two inductances is the self-induction emf greater? Will the values or signs of the self-induction emf's change if the currents begin to increase in the opposite direction after they pass through zero (with the linear laws retained in the process)?

A current that varies with time according to a law depicted graphically in the figure passes through an induction coil. In which of the moments denoted in the figure will the self-induction emf be maximal (the inductance of the coil remains unchanged in the process)?

Various circuits are used to observe the phenomenon of self-induction. Among these are the circuits shown in Figures. In Figure, key $K$ is initially opened and the current flows through the induction coil $L$ and resistor $R$ connected in series. In Figure, key $K$ is initially closed and the current branches off to $R$ and $L$. In both circuits the resistance of the coil $L$ is much lower than $R$. Can an induction emf be generated in either one of these circuits that is higher than the emf of the DC source?

When a certain circuit consisting of a constant emf, an inductance, and resistance is closed, the current in it increases with time according to curve $1$ (see the figure accompanying the problem). After one parameter ($E$, $L$, or $R$) is changed, the increase in current follows curve, $2$ when the circuit is closed a second time. Which parameter was changed and in what direction?

A current is flowing in a circular contour $1$ whose radius is $R$. A second contour, $2$, whose radius is much smaller than that of the first, is moving with a constant velocity, $v$ along the $r$ axis in such a manner that the planes of the contours remain parallel to each other in the course of the motion. At what distance from contour $1$ will the EMF induced in contour $2$ be maximal?