How to grasp physical concepts and laws

To understand and master physical concepts and laws, it is necessary to have a certain understanding of the proposal and establishment of concepts and laws, have a clear understanding of various forms of expression (written and digital) of the contents of concepts and laws, understand their exact meaning, understand their establishment conditions and scope of application, and understand their position in the building of physical theory, They can be used to analyze and solve problems. Before reviewing, candidates already have a certain understanding of this. However, they should know that the basic physical concepts and physical laws reveal the essence of objective things. They are the crystallization of human beings through a long and tortuous historical process and have profound and rich significance. Their understanding of their essence and significance is hierarchical, In the first and second grade of senior high school, the understanding is low level, so we should try to improve a level in the review process
For example, the understanding of the concept of force includes the understanding of the concept of specific force (gravity, elastic force, friction force, electric field force, Ampere force, Lorentz force, etc.), the understanding of the concept of general and abstract force, and the understanding of the different effects of force acting on objects, In the motion of charged particles in electric and magnetic fields, there are various forces. Through these problems, we can deepen our understanding of different forces and the concept of abstract universal force
It is a common situation that the supporting force does not do work when the object glides along the inclined plane, but the wrong conclusion that the supporting force always does not do work can not be drawn. The characteristic of supporting force is that the direction is vertical to the inclined plane. If the inclined plane is movable, the supporting force can do either positive work or negative work;
Static friction can accelerate or decelerate an object. It can do positive work, negative work and no work, but a pair of static friction always does no work (algebraic sum of work is zero);
Sliding friction can make the object decelerate or accelerate. It can do positive work and negative work, but a pair of sliding friction always do negative work. The work of the system to overcome a pair of sliding friction is equal to the increase of the internal energy of the system;
The direction of Lorentz force is always perpendicular to velocity, and it does not do work. It only changes the direction of velocity, but does not change the magnitude of velocity. This is the biggest characteristic of Lorentz force, which is not possessed by other forces;
Force produces acceleration, on the contrary, if the acceleration of the object is found, it must be the force
There are many similar problems, we should constantly sum up

Basic concept of physical current direction
It's just that I don't understand the concept very well. If the direction of positive charge is defined as the direction of current, isn't the free electron moving in the circuit? Is the electron negatively charged?
Please make it clear,

In metals, conducting electrons are free and negatively charged. The direction of the current is opposite to that of the electrons
If the conducting ion is a positive ion, such as a proton, the direction of the current is the same as that of the charge
There are both positive and negative ions conducting in solution. The direction of positive ion is the same as that of current, but the direction of negative ion is opposite to that of current

If the object just falls vertically with an acceleration equal to g, then M (a + G) = 0, then the object has no effect on the support and suspension
Force, as if there is no gravity, this state is completely weightless
How can the formula m (a + G) = 0 be equal to 0

Without considering the air resistance, a free falling body is always in a state of complete motion
For example, in bungee jumping, although a person has a rope on his feet during the falling process, there is no tension on the rope, so he is not subject to other forces except gravity, and he is in a complete state all the time
Is that understandable?

Under the action of other forces, the change of electric potential energy of the charge is no longer equal to the work done by the electric field force. The wrong reason is not that under the action of other forces, other forces do not necessarily do work, but that the change of electric potential energy of the charge must be equal to the work done by the electric field force?

Since we are studying the work of electric field force, what's the relationship with other forces?
When analyzing the functional relationship, you should have the ability of "turning a blind eye". That is, you know so and so energy and so and so work, but only look at the one you analyze
For all potential energies are the same, if the force does positive work, the potential energy corresponding to the force decreases, that is, △ EP = - W
The work done by the electric field force is equal to the variable of the electric potential energy, which has been shown in the previous formula;

How to understand this physical concept?
The centripetal force does not change the speed of the moving object around the circle

Here's how you remember:
A force perpendicular to velocity only changes the direction of motion
Since the centripetal force is perpendicular to the direction of velocity, only the direction of velocity is changed

A. The potential difference between two points B = the work done by the electric field force to move the positive charge from point a to point B
When the charge moves from point a to point B, in addition to the electric field force, it is also affected by other forces, and the change of electric potential energy of the charge is no longer equal to the work done by the electric field force
Why are these two sentences wrong?

In addition to the electric field force, it is also affected by other forces,
The action of other forces does not necessarily mean that other forces do work

What is "3500imp / kW · H" on the electric energy meter?

It indicates that the meter runs 3500 cycles or the red indicator light of the electronic energy meter flashes 3500 times, then it is 1 kilowatt hour

Xiaoqiang's electric energy meter is marked with "2500r / kW · H", so how much J of electric energy is consumed when the dial turns 1 turn? (2) if the dial turns 250
How much electricity will be consumed in 4 hours after half an hour? (3) it is known that a car with 1 degree electricity can run for 0.85 km, and how many km can the car run with the electricity calculated in the above question?

1） 1kw.h =3.6*10^6J
The energy of one revolution is 3.6 * 10 ^ 6 * (1 / 2500) = 1440j
2) 250 * 2 * 4 * (1 / 2500) = 0.8 degrees (8.5 hours in total)
3）s=0.8/0.85=0.94km

Two lights three hours a day, the light is 400W
How many kilowatts of electricity are used every day,

0.4 × 2 × 3 = 24 million hours = 2.4 degrees

How much is the monthly electricity charge for a 400W light to be on 24 hours a day

400w=0.4kw
Daily electricity consumption
0.4KW × 24 = 9.6kwh = 9.6 kWh
Monthly electricity charge
9.6 kwh × electricity price × 30 days = electricity charge

How to grasp physical concepts and laws