Charges in Electric Fields
An electric charge in a uniform external electric field will feel a force due to that field given by, F = qE, where F is the force, q is the charge and E is the external electric field (i.e. the charge is not affected by its own electric field.). This force points in the direction of the field if the charge is positive and in the opposite direction if negative.
One of the most famous experiments using this force is the Millikan oil-drop experiment. A microscopic drop of oil was placed in an electric field between two plates. From the motion of the drop, the value of q was determined. Millikan discovered this value was always some integer times a constant, e. E was later determined to be the elementary charge of an electron. This experiment proved that charge was quantized and Millikan won the Nobel Prize in 1923 for his work.
Two common applications of charges in electric fields are inkjet printing and a television set. Both work on the same principle; charges are accelerated through and electric field. The field bends the charges off of the straight line path according to its strength. The charges (electrons in a TV or ink droplets in a printer) are then deposited at a specific location.
When a dipole is placed in a uniform external electric field, it will experience a net torque:
where p is the magnetic moment and E is the electric field. One end of the dipole is positive while the other is negative. Thus, the positive end will feel a force in the direction of E, while the negative end will feel a force in the direction of -E. A dipole in an electric field can easily be considered to be oscillatory; this being said, a potential energy dependent on the angle between the magnetic moment, p and the electric field E can be defined (U = -p·E). When the angle, , is zero, we have minimum potential energy; when the angle is 90, we have maximum.
Microwave ovens use this torque, and the energy it can produce, to heat items. Specifically, microwaves heat the water in the items by causing the water molecules to rotate. Due to the orientation of the hydrogen and oxygen atoms in a molecule of water, each molecule is a dipole. These dipoles have a tendency to align themselves positive to negative, much like north and south poles of a magnet. The microwaves in the oven produce an electric field that rotates the molecules in a group, breaking them free. Since they naturally want to be in groups, they will release the energy they just gained as thermal energy and reform groups that can be broken again, creating a cyclical event until the microwaves stop.
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