Electrostatics and electromagnetism Flashcards
How can the total amount of charge (Q) of matter be calculated?
Q = ne
n: number of particles
e: charge of each particle (eg. 1.6x10^-19 coulombs for one proton or -1.6x10^-19 coulombs for one electron)
What does the conservation of charge state?
That a net charge cannot be created but that charge can be transferred from one object to another. One way of charging substances is by rubbing them.
For example, glass rubbed on fur becomes positive and rubber rubbed on fur becomes negative. OBjects can also be charged by induction, which occurs when one charged object is brought near to another uncharged object, causing a charge redistribution in the latter to give ner charge regions. COnductors transmit charge readily. Insulators resist the flow of charge.
What is an electric field?
A field is generated by a charged object and it is that region of space around the object that will exert a force on a second object brought into that field. The field exists independently of that second object and is not altered by its presence. The force exerted on the second object depends upon that object and the field.
Charges exert forces upon each other through fields. The direction of a field is the direction a positive charge would move if placed in it. Electric field lines are imaginary lines which are in the same direction as E at that point. The direction is away from positive charges and towards negative charges (ie. the electric field is directed towards the decreasing potentials)
What is the absolute potential (V)?
A scalar that is defined at each distance (r) from a charge (Q) generating an electric field. It represents the negative of the work per unit charge in bringing a +q from infinity to r
V = Ep/q = kQ/r
Where 1 volts = 1 joule/coulomb
V = Ed for a parallel plate capacitor, where d: distance between the plates
What are equipotential lines? When is work done and when is it not done?
Lines (and surfaces) of equal V (absolute potential) and are perpendicular to electric field lines.
Work can only be done when moving between surfaces of equal V and is, therefore, independent of the path taken.
Work is not done when a charge (q) is moved along an equal potential (equipotential) surface (or line), because the component of force is zero along it.
Potential (V) is defined in terms of positive charges such that V is positive when a +Q and negative when a -Q. Potential (V) is added algebraically at a point (because it is scalar)
How do dipoles align in an electric field?
A force is exerted on the positive side (in the direction of the electric field) and a force is exerted on the negative end (in the direction away from the direction of the electric field)
What is an electric dipole?
An electric dipole consists of two charges separated by some finite distance (d). Usually the charges are equal and opposite. The laws of forces, fields, etc. .. apply to dipoles. A dipole is characterized by its dipole moment, which is the product of the charge (q) and d (distance).
dipole moment = (charge)(distance) = qd
Dipoles tend to line up with the electric field they’re in. Motion of dipoles against an electric field requires energy.
What does Coulombs law state about electrostatics?
It describes the nature of the forces acting upon electric charges at rest, and how new forces appear when the charges are moving. They are not of the same nature as the electrostatic forces, and they act differently on the electric charges. They are called:
Electromagnetic forces
What is the unit for the magnetic induction vector (B)?
The tesla
1 T = 1 N/(Axm) = 10^4 gauss
What is Laplace’s Law for the force acting upon a particle in a magnetic induction field (B)?
A a particle with charge dq moving at a velocity (v) in a magnetic induction field (B) is acted upon by a force dF, given by the following formula
dF = dq(v) x B = dq(v)(Bsinα)
α: angle formed by the direction of v with that of B (cross product)
The force dF is perpendicular to the magnetic induction vector and also to the displacement velocity of the charge
When many charges are in motion, they produce an electric current of intensity I (dq/dt).
How is the intensity calculated?
dq/dt
How can you determine the cross product (direction) of the velocity of a charged particle and an electromagnetic field?
The right-hand rule.
Fingers describe the circular motion around a wire, or the direction of the magnetic induction vector (B). The thumb point in the direction of current flow (flow of charged particles)
Which two perpendicular vectors exist at each point of an electromagnetic field?
- The electric field vector (E)
- The magnetic induction field vector (B)
Give Planck’s equation for the relation between energy (E) and the frequency (f) of electromagnetic radiation
E = hf
Where h: Planck’s constant
Thus high frequency or short wave length corresponds to high energy and vice versa
In a vacuuum photodiode the photoelectric effect causes electrons to be ejected from a metal plate when photons of light are absorbed by the metal. The ejected electron will have a kinetic energy equal to the photon’s energy minus the work function (2x10^-19 J).
If an electron is ejected from a cathode by a photon with an energy slightly greater than the work function of the cathode, how will the final kinetic energy of the electron upon reaching the anode compare to its initial potential energy immediately after it has been ejected? Why?
Potential energy immediately after ejection is -1.6x10^-19 C
Final kinetic energy will be approximately equal to when the electron was ejected.
The near equality of the photon energy and work function means that little initial kinetic energy will be left for the electron. This initial kinetic energy is small compared to the 50 eV it will gain from the potential difference between electrodes.
