Electric charge is the property of particles to attract or not attract others. For example, electrons attract protons, whereas neutrons are neither attracted nor repelled by electrons.
A body will be neutral when it has the same amount of electrons and protons. When it has a greater number of electrons than protons it is negatively electrified. On the other hand, when the number of electrons is less than the number of protons, it will be positively electrified.
Take advantage of the resolved and commented questions to clear your doubts about this subject of electrostatics.
Resolved issues
1) UFRGS - 2018
A negative charge Q is approximated to an insulated, electrically neutral conducting sphere. The sphere is then grounded with a lead wire. Check the alternative that correctly fills in the blanks in the statement below, in the order in which they appear. If charge Q is moved far away while the ball is grounded, and then the ground is ungrounded, the ball will become ________. On the other hand, if the ground is first removed and then the charge Q is removed, the sphere will be ________ .
a) electrically neutral - positively charged
b) electrically neutral - negatively charged
c) positively charged - electrically neutral
d) positively charged - negatively charged
e) negatively charged - positively charged
When a negative charge is approached from a neutral conducting sphere, a repulsive force causes electrons to accumulate in the region of the sphere furthest away from the charge.
In this way, the region closest to the sphere is short of electrons. In the first situation, grounding the ball while the load is being removed causes the load on the ball to return to neutral.
In the second situation, as the load is moved away after the ground is removed, this causes excess loads. negatives accumulated at one end of the sphere are drained to earth, making the sphere positively loaded.
Alternative: a) electrically neutral - positively charged
A metallic object, X, electrically insulated, has a negative charge 5.0 x 10-12 Ç. A second metallic object, Y, neutral, kept in contact with the Earth, is brought closer to the first and there is a spark between them, without them touching. The spark duration is 0.5 s and its intensity is 10-11 THE. At the end of this process, the total electrical charges of objects X and Y are, respectively,
a) zero and zero.
b) zero and – 5.0 x 10-12 Ç.
c) - 2.5 x 10-12 C and - 2.5 x 10-12 Ç.
d) - 2.5 x 10-12 C and + 2.5 x 10-12 Ç.
e) + 5.0 x 10-12 C and zero
The amount of cargo transferred in the situation presented can be calculated using the following formula:
Replacing the indicated values, we have:
Thus, object y received a charge of 5.10-12 C, but since it is grounded, this excess charge will drain away and the object will have a total charge equal to zero.
The excess of negative charges that were on object x, was totally transferred to object y, as we calculated. Therefore, your total charge will also equal zero.
Alternative: a) zero and zero
Two identical metal rods are charged with a charge of 9.0 µC. They are brought into contact with a third stick, also identical to the other two, but whose net charge is zero. After contact is established between them, the three sticks move apart. What is the net charge, in µC, on the third rod?
a) 3.0
b) 4.5
c) 6.0
d) 9.0
e) 18
In contact electrification, when the bodies are identical, the charge will be divided equally between the bodies.
Thus, the charge of 18 µC (9 + 9) will be divided by 3, so each stick will, after contact, have a net charge of 6 µC (18: 3).
Alternative: c) 6.0
In a physics class, two identical metallic spheres were used, X and Y: X is suspended by a insulating wire in the form of a pendulum and Y fixed on an insulating support, as shown in the figure bellow. The spheres are initially spaced apart, with X being positively charged and Y electrically neutral.
Consider the description below of two simple procedures to demonstrate possible electrification processes and then mark the alternative that correctly fills in the blanks in the statements, in the order in which appear.
I - The sphere Y is approximated to X, without them touching. In this case, it is verified experimentally that the sphere X is _____________ by the sphere Y.
II - The sphere Y is approximated to X, without them touching. While held in this position, a connection of the Y-ball to earth is made using a lead wire. Still in this position close to X, Y breaks off contact with the earth and then Y again moves away from X. In this case, sphere Y becomes _____________ .
a) attracted - electrically neutral
b) attracted - positively charged
c) attracted - negatively charged
d) repelled - positively charged
e) repelled - negatively charged
In procedure I, when approaching sphere Y to sphere X, which is positively charged, electrons will be attracted to the region closest to X. Thus, sphere X is attracted to sphere Y.
In the second process, by connecting the Y sphere with a conducting wire, the region that was left with a lack of electrons will receive negative charges. Upon breaking this connection, sphere Y will become negatively charged.
Alternative: c) attracted - negatively charged
To learn more, see electrostatics and Electrostatics: Exercises.
5) Fuvest - 2015
In a physics laboratory class, to study properties of electrical charges, an experiment was carried out in which small electrified spheres are injected into the upper part of a chamber, in a vacuum, where there is a uniform electric field in the same direction and direction as the local acceleration of the gravity. It was observed that, with an electric field of a modulus equal to 2 x 103 V/m, one of the spheres, of mass 3.2 x 10-15 kg, remained at constant speed inside the chamber. This sphere has
a) the same number of electrons and protons.
b) 100 more electrons than protons.
c) 100 electrons less than protons.
d) 2000 more electrons than protons.
e) 2000 electrons less than protons.
Note and adopt: electron charge = - 1.6 x 10-19 Ç; proton charge = + 1.6 x 10+19 Ç; local acceleration of gravity = 10 m/s2
As the charge remained inside the chamber with constant velocity, this means that the net force is equal to zero.
Since the weight force and the electrical force are the forces acting on the load, they must have the same intensity and opposite directions so that the resulting force is equal to zero.
The electrical force is calculated by the formula Felectric = q. E is the force weight given by P = m.g, so we have:
As the charge is given by q = n.e, we have:
For the electric force to have the opposite direction of the weight force, it will be necessary for the charge to be negative, in this way, the force will have the opposite direction to the electric field. So we're going to have excess electrons.
Alternative: b) 100 more electrons than protons.
To learn more about, see also Electric field and Electric Field Exercises.
6) UFLA - 2010
Two equal discharged conductive spheres 1 and 2 are in contact with each other and supported on an insulating surface. A positively electrified baton is approached to one of them, without touching it, as shown in the figure below. Then the balls are removed and the electrified rod is removed.
It is correct to state that
a) the spheres remain unloaded, as there is no transfer of charges between rod and spheres.
b) sphere 1, closest to the stick, becomes positively charged and sphere 2 negatively charged.
c) the spheres are electrified with equal charges and opposite signs.
d) the spheres are charged with charges of equal sign and both with a negative sign, as the rod attracts opposite charges.
The positive charges on the rod will attract the negative charges to sphere 1 and sphere 2 will be short of electrons.
By separating the two spheres, keeping the stick in the same position, sphere 1 will be negatively electrified and sphere 2 will be positively charged.
Alternative: c) the spheres are electrified with equal charges and opposite signs.
