In the gravity field, gravity does work, kinetic energy and gravity potential energy are transformed into each other. During the transformation process, the sum remains unchanged, which is called conservation of mechanical energy.
In electrostatic field , electrostatic force performs work, and the potential energy and kinetic energy are converted into each other. During the process, the sum remains unchanged. It is called type mechanical energy conservation (the potential energy does not belong to mechanical energy), which is actually energy conservation .
1. If only electrostatic force does work, energy only converts between kinetic energy and potential energy, then the sum of kinetic energy (Ek) and electric potential energy (Ep) remains unchanged.
. If only static force and gravity do work, the sum of gravitational potential energy, potential energy and kinetic energy remains unchanged.
. If only static force and gravity do work, the sum of gravitational potential energy, potential energy and kinetic energy remains unchanged.
3. If only electrostatic force, gravity and spring force do work, the sum of the four energy of gravity potential energy, electric potential energy, elastic potential energy, elastic potential energy,
4. In the electrostatic field, in addition to electrostatic force, there is also gravity and friction to do work, the sum of the system potential energy, gravity potential energy, kinetic energy and internal energy that rub against each other remains unchanged.
5. In the electrostatic field, in addition to electrostatic force, there is also gravity and friction to do work, the sum of the system potential energy, gravity potential energy, kinetic energy and internal energy that rub against each other remains unchanged.
Example: The motion trajectory of a belt of electric oil drops in the uniform electric field E is shown in the dotted line in the figure.
electric field direction vertically downward. If the air resistance is not included, the energy change of this charged oil droplet from a to b is (C)
A. Kinetic energy decreases
B. The potential energy increases
C. The sum of kinetic energy and electric potential energy decreases
D. The sum of gravitational potential energy and electric potential energy increases
Example: As shown in the figure,
dotted lines 1, 2, 3, and 4 respectively represent the equipotential surfaces in the electrostatic field. The potential difference between adjacent equipotential surfaces is equal. Assume that equality is assumed. The potential of potential surface 3 is 0.. Positively charged Point charge only moves from a to b in the figure under the action of electrostatic force. The kinetic energy passing through the equipotential surface a is 50eV, and the kinetic energy passing through the equipotential surface b is 26eV. When the charge at this point moves to a certain position, its kinetic energy is 30eV, and the potential energy should be 4eV.
[Analysis]
Since it is an arithmetic equipotential surface, the work done by the adjacent equipotential surface is equal.
The work done by each adjacent equipotential surface is (26eV-50eV)/3=-8eV, that is, the kinetic energy is reduced by 8eV. The kinetic energy of the equipotential surface 3 is 34eV, and the total energy of the equipotential surface 3 is 34eV+0=34eV. According to energy conservation (mean mechanical energy conservation), at a certain position, its kinetic energy is 30 eV and the potential energy should be 4 eV.
Example: There is a vertical insulated light spring fixed at the lower end. As shown in the figure, a positively charged ball P is connected to the upper end of the spring. It is known that there is a vertically upward uniform electric field in the entire space, and the ball is in equilibrium and the spring is in the original length. If the ball is given an upward initial velocity, the highest point it reaches is M. Then the correct statement about the ball when it moves to the highest point is ()
A. The electric potential energy of the sphere decreases, the gravitational potential energy increases, and the two are numerically equal B. The mechanical energy of the sphere increases, and the increase is numerically equal to the work done by electric field force
C. The reduction of the kinetic energy of the small ball is numerically equal to the algebraic sum of the work done by electric field force and gravity
D. The decrease in the kinetic energy of the small ball is equal to the increase in the spring elastic potential energy
[Analysis] The initial state of this question is that the spring is in the original length and the ball is in force balance, that is, the gravity and electric field force of the small ball are balanced, that is, the combined force of the gravity and electric field force mg=qE, it can be seen that the combined force work of gravity and electric field force is zero, so option A is correct. During the vertical upward movement of the small ball, the kinetic energy of the small ball decreases and the elastic potential energy increases, so option C is wrong, D is correct; the force that changes the mechanical energy of the small ball is other forces other than gravity, that is, the electric field force and elastic force do work, so option B is wrong. The answer to this question is AD.
☞ Change of kinetic energy is based on the combined force to do work; the change of electric potential energy is based on the electrostatic force to do work; the change of elastic potential energy is based on the spring elastic force to do work; the change of mechanical energy is based on the non-gravity to do work.
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Example: As shown in the figure,
In an electrostatic field, a negative charge q is affected by a non-static force and moves from point A to point B. The correct statement is (BCD)
A. The sum of the work done by non-static force and the electrostatic force is equal to the sum of the charge potential energy increment and the kinetic energy increment
B. The work done by non-static force is equal to the sum of the potential energy increment and the kinetic energy increment
C. Charge gram The work done by electrostatic force is equal to the increment of electric potential energy
D. The sum of non-static force work and electrostatic force work is equal to the increment of charge kinetic energy
Example: As shown in the figure,
uniform electric field direction is horizontally left, and the positively charged object moves uniformly along the insulated horizontal plate to the right. When passing point A, the kinetic energy is 100J, the kinetic energy is reduced by 80J when reaching point B, and the potential energy increases by 48J. What is the kinetic energy when the object returns to point A again?
[Answer] 20J
Example: As shown in the figure, a positively charged small block P is fixed at the bottom of the smooth insulating inclined surface. Another charged small block Q is released from static at a certain position on the inclined surface, and it will move upward along the inclined surface. Assuming the slope is long enough, (D)
A during the upward movement of Q (D)
A. The kinetic energy of block Q continues to increase
B. The electric potential energy of block Q continues to increase
C. The sum of gravity potential energy and electric potential energy of block P and Q continues to increase
D. The mechanical energy of block Q continues to increase
Example: As shown in the figure,
0 There is a positive charge at point O below the insulating slope with uniform roughness. The negatively charged small object slides from point M from point M at the initial velocity v₁ , When reaching point N, the velocity is zero, and then slides back to point M. At this time, the velocity is v₂ (v₂＜v₁). If the charge amount of small objects remains unchanged and OM=ON, then (AD)
A. The maximum height of the rise of small objects is (v₁²+v₂²)/4g
B. In the process from N to M, the electric potential energy of small objects gradually decreases
C. In the process from M to N, the electric field force first does negative work on small objects and then positive work on small objects
D. In the process from N to M, the friction force and electric field force that small objects are first increased and then decreased
