An electron moves through a region of crossed electric and magnetic fields. The

Question

An electron moves through a region of crossed electric and magnetic fields. The electric field E = 2000 V/m and is directed straight down. The magnetic field B = 0.50 T and is directed to the left. For what velocity v of the electron into the paper will the electric force exactly cancel the magnetic force? The magnitude of the charge on the plates of an isolated parallel plate capacitor is doubled. Which one of the following statements is true concerning the capacitance of this parallel-plate system? A sheet of copper is pulled at constant velocity v from a region that contain, a uniform field. At the instant shown in the figure, the sheet is partially in and partially out of the field. The induced emf in the sheet lead, to the eddy current shown. Which one of the following statements concerning the direction of the magnetic field is true? When an RLC series circuit, is in resonance, its impedance is:

Explanation / Answer

4.solution

The electric force = qE = eE for an electron. The force is upwards as the electron is negative.

The magnetic force = Bqv = Bev for an electron. Using Fleming's left t hand rule and remembering electron flow is opposite to conventional current flow (as electrons are negative), the direction of the force is downwards (so it will cancel the electric force).

When the two forces are equal eE = Bev, so v = E/B

v = 2000V/m / 0.50T

v = 4000m/s

5.(a) capacitance remains unchanged

Capacitance depends on permittivty constant, Area of and distance between plates, only.
If the charge is changed, the e-field and thus, voltage changes

6.(c) the magnetic field points out the paper

IB decreases as B decreases

eddy current = Iind

therefore Bind out of paper and Bext is out of paper since the flux due to Bext is decreasing

7.option (d) equal to its resistance

The RLC series circuit is a very important example of a resonant circuit. It has a minimum of impedance Z=R at the resonant frequency, and thephase angle is equal to zero at resonance

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