28. Magnetic Fields and Forces

Which option best completes the following sentence? If a magnet is carefully balanced, as in a compass, the north end of the magnet points roughly toward…

A wire carries a positive current to the left in the plane of this screen. What is the direction of the magnetic field at a point near the top of the screen, still in the plane of the screen?

At what distance from a wire carrying a 10 A current is the magnetic field strength equal to $50\mu \mathrm{T}$ ? This is the approximate value of the earth's magnetic field.

Two long parallel wires lie in the $xy$-plane, each carrying a $5.0$ A current in opposite directions. Wire 1 lies along the $y=3.0\mathrm{ cm}$ line and carries a current in the positive $x$-direction; wire 2 lies along the $y=0$ line and carries a current in the negative $x$ -direction. What is the magnitude of the magnetic field at the point $\left(0\text{cm,}1\text{.5}\text{cm,}0\mathrm{ cm}\right)$ ?

Singly ionized (one electron removed) atoms are accelerated and then passed through a velocity selector consisting of perpendicular electric and magnetic fields. The electric field is 155 V/m and the magnetic field is 0.0315 T. The ions next enter a uniform magnetic field of magnitude 0.0175 T that is oriented perpendicular to their velocity. (a) How fast are the ions moving when they emerge from the velocity selector? (b) If the radius of the path of the ions in the second magnetic field is 17.5 cm, what is their mass?

A 150-g ball containing 4.00x10^8 excess electrons is dropped into a 125-m vertical shaft. At the bottom of the shaft, the ball suddenly enters a uniform horizontal magnetic field that has magnitude 0.250 T and direction from east to west. If air resistance is negligibly small, find the magnitude and direction of the force that this magnetic field exerts on the ball just as it enters the field.

An open plastic soda bottle with an opening diameter of 2.5 cm is placed on a table. A uniform 1.75-T magnetic field directed upward and oriented 25° from vertical encompasses the bottle. What is the total magnetic flux through the plastic of the soda bottle?

A horizontal rectangular surface has dimensions 2.80 cm by 3.20 cm and is in a uniform magnetic field that is directed at an angle of 30.0° above the horizontal. What must the magnitude of the magnetic field be to produce a flux of 3.10x10^-4 Wb through the surface?