IN CONSTRUCTION
reference: THe physics of every day phenomena, A conceptual introduction to Physics - Mac Graw hill
in construction, please be patient.
PART I: GRAVITATIONAL FORCE / GRAVITATIONAL FIELD
1) letters in bold are vectors.
In 11th grade, we saw that the force between 2 masses is:
Now consider one mass m in space at a place A and a mass m' at a place B.
The distance between A and B is r. u is a unit vector. The mass m attract m' with a force F such as :
F A>B = - G m m' / r2 u (units in N)
The quantity G = - G m / r2 u (units in N/Kg) only depends on the mass m. We define this quantity as the gravitational field G
This quantity can be understood as the force experiences by a mass of 1kg in presence of the mass m.
but the field is here even if no charge is placed around m . The gravitational force becomes F A>B = m G
2) Mars (M=6.4 1023 kg) has 2 small satellites Phobos (Rfrom MArs= 9,000km and M= 9.6 1015 kg) and Deimos
(Rfrom MArs=23,000km and M = 1.9 1015 kg)
A) Find the fields produced by Mars on Phobos and Deimos (or at the location of .. ).
B) Find the field producd by Deimos on Mars( where Mars is)
3) On Earth G= g . weight of a mass m = w = mg
Find the gravitational field g at the surface of the EArth. ( Mearth = 5.98 1024kg and Rearth = 6,370 km)
Is this consistent with what you already know ?
4)
A) Find the weight of a person of mass 70kg on Earth and on the moon. (Rmoon=1,738 km and Mmoon=7.35 1022 kg)
PART 2: ELECTRIC FORCE / ELECTRIC FIELD
1) Do you remember Coulomb's law ? In a vector form it becomes : FA>B= K qA qB /r2 u
2) Like for the gravitational force, The electric force can be written as FA>B = qB E with E = K qA /r2 u
E is the electric field created by qA at a distance r. It is there even if no charge is there to feel it.
The electric field is the electric force per unit positive charge. The unit is newton per coulomb.
The electric field is created by any charge or distribution of charges. The charges altered the space around
and a test charge will " feel" the field and experience a force. An electric field is a storehouse of energy.
you could say that a charge qA creates an electric field around itself, and the electric force is the interaction between
a charge qB and the field.
3) A) Find the electric field created by a charge Q = 1.6 10-19C at a point M located at a distance d = 70pm
B) Using the result fronA, find the electric force applied to a charge q = 3.2 10-19C . Draw the force F.
C) same question for a charge q = - 3.2 10-19C
4) In the late eighteen century the nature of the electrostatic force was studied by the French scientist __________ ________.
He discovered that the force between 2 point charges (designated as q1 and q2), separated by a distance r, experience
a mutual force along the line connecting the charges that varies directly as the product of ____________ and
inversely as the square of the distance between them. This law is known as __________ 's law (after the French guy).
Fe = _________ with Ke = 9 109 N m2 / C2 q1 and q2 are the point charges and r is the distance between q1 and q2.
5) if q1 = 6 10-6C , q2 = -3 10-6C , r = 0.1 m. Calculate the electric force between the charges. Is this an attractive or repulsive force ?
6) Use vector or scalar
The electric field E = Fe/q is a ________ quantity since it is derived from the force (a ______) and the charge (a ______ ).
THe units are _________ per ___________ (N/C).
7) A small positive test charge q is brought in the vicinity of a large positive charge Q, at a distance r.
The small charge q experiences repulsive force Fe = __________. (use Ke, q, Q, r).
The electric field produced by Q at this location is E = Fe/q = __________. (Use Ke, Q, r)
8) F = qE
What is the force acting on an electron placed in an external electric field E = 100N/C
9) 2 point charges experience a force of repulsion equal to 3 10-4N when separated by 0.4 m. If one of the charges
is 5 10-4C, what is the magnitude of the other charge?
PART3: GEOMETRY of the electric field - mapping
An electric field is represented by lines.
Let's take for example a Van Graaf generator. The distribution of charges spread over the sphere alters
the space around and creates an electric field.
You can map the space by using a metal rod held by a nylon thread and draw the electric field lines. See below.
 |  This is the way the electric field lines are represented. Closer lines means stronger field. qo is the test charge. The generator is like a large positive charge. qo is small enough not to alter the electric field of the generator. |  If the generator collects negative charges, the electric fields is as above. |
You can also map the electric field generated by 2 charges (same magnitude)
by using 2 electrodes and grains floating in oil. the grains aligned themselves along the lines :
 electric field produces by 2 opposite charges: , one negative and one positive.  |  electric field produced by 2 like charges  |
The electric field can be far more complex if the 2 charges don't have the same magnitude or if you have more
than 1 charge. An electric field behaves like a fluid like air. The positive charge is " blowing out" an electric field
and the negative charge is " sucking in" the electric field. The mathematical equations used to describe the motion
of fluids can be used to describe the behavior of an electric or magnetic field. (see chapter about potential to see more complex
electric field).
part4: UNIFORM ELECTRIC FIELD
A very simple and uniform field is the one generated by 2 parallel charged plates.
one positive and one negative. If the plate are large enough compared to the distance between them,
the electric field is constant and uniform every where between the plate. The direction
is perpendicular to the plates. This is called a capacitor.
 The voltage is 10KV. Each plate is connected to a terminal of the generator. The field is uniform between the plate except at the edges. |  |
(see potential and vector analysis chapters) It can be shown that the electric field between the plate is :
E = U / d = (Q/S) / eo = constant Q is the charge on one plate, eo is a constant, Q/S is the density of charge, S the surface area of the plate.
U is the voltage and d the distance between the plates. eo = 8.85 10 -12 SI electric constant.
U is Vp-Vn difference of potential or tension or voltage. Vp is the potential of the positive plate and Vn is the voltage
of the negative plate.
1) Find the electric field produced by a capacitor if d = 12cm and U = 4.5V
(convert cm to m)
2) A tension of 50V is between 2 plates of a capacitor 5cm apart.
A) Find the electric field
B) Draw and indicate the positive and the negative plate.
3) Inside a capacitor the electric field is E = 500 V/m the tension is Upn = 50V
A) Find the distance d between the plates.
B) What will happen if we increase the distance while keeping the tension the same.
4) A capacitor has plates with surface S = 100 cm2 and the electric field between the plates is 1,000 V/m
Find the charge on each plate. eo = 8.85 10 -12 SI
part5: electric field of conductors
see chapter about potential
PART6: lightning / other connections
1) Benjamin Franklin was the first one to realize that lightning is electricity.
How can you explain lighting ?
 |  when you rub a balloon with you hair , you are separating charges. The balloon get negative charges (excess of electrons) and the hair become positively charged. We saw that in 9th grade. The same thing happens in some clouds. Because of friction charges separation occur and a huge voltage can build up between the bottom of the cloud and the ground. The voltage can reach hundred millions of volts. The air between the cloud and the ground becomes conductive (ionizes) and electricity can flows in an instant. The current can reach 10,000 A. It could be bad if the voltage keeps building up. However, there are ions in the atmosphere that balance the situation. The positive ions move the negative side (ground or cloud) and the negative ions move to the positive side. The electric field between clouds and ground is about 130 V m-1. |
See chapter about electric potential for more computations and explanation about lightning.
2) Why electrical appliances are usually grounded ?
3) Elmo fires are strange phenomena (corona discharges) that are due to a discharge in the atmosphere
from a pointy object like the horns of a cow or the pointy parts of a ship.
learn more here.
4) Elecrically conductive truck tires. The manufacturers produce elctrically conductive tires. Such tires are able to drain charge from
trucks that otherwise would acquire static charge as a result of friction with passing air. With conducting tires there is little danger of
electrostatic discharges (sparking), which might prove catastrophic for trucks carrying exposive fuels.
5) air purification: Many heavy industries release a large amount of air pollutants into the atmosphere. Much of the fine dust
can be eliminated by passing exhaust through electrostatically charged plates and grates that attract electrically charge smoke
particles. Some air-conditioning systems use this technique to remove pollen, dust and cigarettes from the air.
6) photocopier. Photocopiers use the photo electric effect and electrostatics.
A charged selenium plate is exposed to light only if there is no writing. The exposed parts lose its extra electrons (photo electric effect, the
eletrons are kicked out by the photons). The unexposed parts are charged and attract ink. The ink is fixed by heating.
see great animation here
7) Faraday cage see unit about electric field and potential
you are relatively safe from the danger of lightning in a metal airplane or a car because the field inside an electrically
conducting container is zero.
7) computer protection : the components of a computer or other electronic devices are sensitive to electrostatic fields
and discharges and are often housed in metal boxes to shield them from electric fields. To avoid damaging
electrostatic discharges, computer technicians ground themselves before working on computers.