Chapter 16- electric charge and field Flashcards
What is static electricity?
when two unlike materials are rubbed together, they become charged
What is charging?
transfer of electrons from one material to another material
What is an insulator?
materials in which electrons cannot move within the material
What is a conductor?
materials in which free electron can move within the material
What is the law of conservation of electric charge?
the net amount of electric charge produced in any process is zero;
or, said another way,
no net electric charge can be created or destroyed.
What happens if q is positive in F = qE, what direction will F and E point?
If q is positive, F and E point in the same direction.
What happens if q is negative in F = qE, what direction will F and E point?
If q is negative, F and E point in opposite directions.
What is the general property of electric field lines?
the closer together the lines are, the stronger the electric field in that region.
What is an electric dipole?
electric field lines due to two equal charges of opposite sign
What are 3 properties of field lines?
- Electric field lines indicate the direction of the electric field; the field points in the direction tangent to the field line at any point.
- The lines are drawn so that the magnitude of the electric field, E, is proportional to the number of lines crossing unit area perpendicular to the lines. The closer together the lines, the stronger the field.
- Electric field lines start on positive charges and end on negative charges; and the number starting or ending is proportional to the magnitude of the charge.
A neutral hollow metal box is placed between two parallel charged plates as shown in Fig. 16–36a. What is the field like inside the box?
If our metal box had been solid, and not hollow, free electrons in the box would have redistributed themselves along the surface until all their individual fields would have canceled each other inside the box. The net field inside the box would have been zero. For a hollow box, the external field is not changed since the electrons in the metal can move just as freely as before to the surface. Hence the field inside the hollow metal box is also zero, and the field lines are shown in Fig. 16–36b. A conducting box is an effective device for shielding delicate instruments and electronic circuits from unwanted external electric fields. We also can see that a relatively safe place to be during a lightning storm is inside a parked car, surrounded by metal. See also Fig. 16–37, where a person inside a porous “cage” is protected from a strong electric discharge. (It is not safe in a lightning storm to be near a tree which can conduct, or out in the open where you are taller than the surroundings.)
What is the property of static electric fields and conductors?
is that the electric field is always perpendicular to the surface outside of a conductor.
What would happen if component E is parallel to the surface of conductors?
If there were a component of E parallel to the surface ,it would exert a force on free electrons at the surface, causing the electrons to move along the surface until they reached positions where no net force was exerted on them parallel to the surface—that is, until the electric field was perpendicular to the surface.
Why do any net charges on a conductor distribute itself on the surface?
Because there can be no field within the metal, the lines leaving the central positive charge must end on negative charges on the inner surface of the metal. That is, the encircled charge ±Q induces an equal amount of negative charge, –Q, on the inner surface of the spherical shell. Since the shell is neutral, a positive charge of the same magnitude, ±Q, must exist on the outer surface of the shell. Thus, although no field exists in the metal itself, an electric field exists outside of it, as if the metal were not even there.
