EP 2: Lecture 3
Properties of a conductor in electrostatic equilibrium?
1. The electric field is zero inside a conductor. 2. Just outside a conductor, the electric field lines are perpendicular to its surface, ending or beginning on charges on the surface. 3. Any excess charge resides entirely on the surface or surfaces of a conductor.
What is a capacitor?
A capacitor (originally known as a condenser) is a passive two-terminal electrical component used to store energy electrostatically in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors (plates) separated by a dielectric (i.e., insulator). The conductors can be thin films of metal, aluminum foil or disks, etc. The 'nonconducting' dielectric acts to increase the capacitor's charge capacity. A dielectric can be glass, ceramic, plastic film, air, paper, mica, etc Unlike a resistor, a capacitor does not dissipate energy. Instead, a capacitor stores energy in the form of an electrostatic field between its plates.
What is an electric dipole?
An electric dipole is two charged objects, with equal but opposite electric charges (say a +q and a -q), that are separated by a small distance. A dipole can be created, e.g. , suppose you place a neutral atom in an electric field, because the positively-charged constituents of the atom will be pulled one way, and the negatively-charged constituents the other way, creating a separation of charge in the direction of the field. Materials placed in a field of Force may deform and such molecules /atoms are called Polar. A charge distribution placed in a region may lead to multipolar fields in the space and dipole happens to be the simplest.
What is the Electric field of a charged rod (solid, infinite length)?
Due to Gauss's law, the field outside looks as it would be concentrated on the rod axis. If the rod would be hollow then there would be no field inside! (Check the lecture notes as ln appears when r>R and a quadratic dependency when r<R.
What is charge separation by electrostatic induction and why grounding is important (and what does it do)?
Electric grounding, also called earthing, provides a path for current to flow into the ground and excess electric charge to disperse instead of building up and creating a potential hazard. This works because Earth, being electrically neutral but also enormous, can both accept and provide large numbers of electrons (by human-industry standards) without noticeable changes to this "zero voltage" state.
What is electrostatic induction?
Electrostatic induction, also known as "electrostatic influence" or simply "influence" in Europe and Latin America, is a redistribution of electric charge in an object, caused by the influence of nearby charges. However, the induction effect can also be used to put a net charge on an object. If, while it is close to the positive charge, the above object is momentarily connected through a conductive path to electrical ground, which is a large reservoir of both positive and negative charges, some of the negative charges in the ground will flow into the object, under the attraction of the nearby positive charge. When the contact with the ground is broken, the object is left with a net negative charge.
What is electrostatic equilibrium?
If an electric field is present inside a conductor, it exerts forces on the free electrons (also called conduction electrons), which are electrons in the material that are not bound to an atom. These free electrons then accelerate. However, moving charges by definition means nonstatic conditions, contrary to our assumption. Therefore, when electrostatic equilibrium is reached, the charge is distributed in such a way that the electric field inside the conductor vanishes.
What is an electric conductor?
In physics and electrical engineering, a conductor is an object or type of material that allows the flow of charge (electrical current) in one or more directions. Materials made of metal are common electrical conductors. Electrical current is generated by the flow of negatively charged electrons, positively charged holes, and positive or negative ions in some cases. In order for current to flow, it is not necessary for one charged particle to travel from the machine producing the current to that consuming it. Instead, the charged particle simply needs to nudge its neighbor a finite amount who will nudge its neighbor and on and on until a particle is nudged into the consumer, thus powering the machine. Essentially what is occurring is a long chain of momentum transfer between mobile charge carriers;
What is a Leyden jar?
Leyden jar is the first type of capacitors that were made. One can even put a nail into a bottel full of salty water, cover in aluminium foil and electrons could be stored in the water through the nail. A Leyden jar (or Leiden jar) is an antique electrical component which stores a high-voltage electric charge (from an external source) between electrical conductors on the inside and outside of a glass jar. It typically consists of a glass jar with metal foil cemented to the inside and the outside surfaces, and a metal terminal projecting vertically through the jar lid to make contact with the inner foil. It was the original form of the capacitor
Experiment: High fields at surfaces with small radius (Example of the importance of the radius)
So basically, the electric field grows when the same charge is present on a smaller radious? I should double check this.
What is corona discharge?
Strong electric fields (usually around 245kV) causing the air to ionize.
What is the dipole moment?
The electric dipole moment is a measure of the separation of positive and negative electrical charges within a system, that is, a measure of the system's overall polarity. The SI units for electric dipole moment are coulomb-meter (C⋅m); however, a commonly used unit in atomic physics and chemistry is the debye (D).
What is the image theory or method of images??
The image theory states that given a configuration above an infinite grounded perfect conducting plane may be replaced by the charge configuration itself, its image, and an equipotential surface in place of the conducting plane. Or: The fields above a perfect ground plane from a primary source acting in the presence of the perfect ground plane are found by summing the contributions of the primary source and its image, each acting in free space. The validity of the method of image charges rests upon a corollary of the uniqueness theorem, which states that the electric potential in volume V is uniquely determined if both the charge density throughout the region and the value of the electric potential on all boundaries are specified. Alternatively, application of this corollary to the differential form of Gauss' Law shows that in a volume V surrounded by conductors and containing a specified charge density ρ, the electric field is uniquely determined if the total charge on each conductor is given. Possessing knowledge of either the electric potential or the electric field and the corresponding boundary conditions we can swap the charge distribution we are considering for one with a configuration that is easier to analyze, so long as it satisfies Poisson's equation in the region of interest and assumes the correct values at the boundaries.
What is an image charge?
The method of image charges (also known as the method of images and method of mirror charges) is a basic problem-solving tool in electrostatics. The name originates from the replacement of certain elements in the original layout with imaginary charges, which replicates the boundary conditions of the problem The validity of the method of image charges rests upon a corollary of the uniqueness theorem, which states that the electric potential in a volume V is uniquely determined if both the charge density throughout the region and the value of the electric potential on all boundaries are specified. Alternatively, application of this corollary to the differential form of Gauss' Law shows that in a volume V surrounded by conductors and containing a specified charge density ρ, the electric field is uniquely determined if the total charge on each conductor is given. Possessing knowledge of either the electric potential or the electric field and the corresponding boundary conditions we can swap the charge distribution we are considering for one with a configuration that is easier to analyze, so long as it satisfies Poisson's equation in the region of interest and assumes the correct values at the boundaries.
The Millikan oil drop experiment and the discovery of the elementary charge (it's magnitude) and that the charges are quantized.
The oil drop experiment was performed by Robert A. Millikan and Harvey Fletcher in 1909 to measure the elementary electric charge. The experiment entailed observing tiny electrically charged droplets of oil located between two parallel metal surfaces, forming the plates of a capacitor. The plates were oriented horizontally, with one plate above the other. A mist of atomized oil drops was introduced through a small hole in the top plate and was ionized by an x-ray, making them negatively charged. First, with zero applied electric field, the velocity of a falling droplet was measured. At terminal velocity, the drag force equals the gravitational force. As both forces depend on the radius in different ways, the radius of the droplet, and therefore the mass and gravitational force, could be determined (using the known density of the oil). Next, a voltage inducing an electric field was applied between the plates and adjusted until the drops were suspended in mechanical equilibrium, indicating that the electrical force and the gravitational force were in balance. Using the known electric field, Millikan and Fletcher could determine the charge on the oil droplet.
What is the torque of an exeternal field on a dipole and what does it tell us?
The total torque is given by the sum of the two individual torques. For the torque calculation we can conveniently take the dipole center but also any other point in space, since we know from mechanics that for a zero net force, the torque about one axis equals the torque about any axis. In general, one can say that an external field tends to align a dipole along its direction.
What is a faraday cage and does it have a electric Field inside? IF not, why not?
to be answered
What is the charged density on a metal surface, how to describe it, and what is it its dependency to r and why?
to be answered
Are there any field lines or charge in the cavity of a conductor?
to be answered (no)
