Why Joule s law can only calculate electric heat

You should be wondering about the microscopic explanation of Joule's law. Let me give you an example of a mechanical model.

When an object is placed on a horizontal plane, it will be subjected to a drag force f, which is very special and is proportional to the velocity v of the object: f=kv. Apart from the drag force, this object is subjected to a constant force f so that the object can move in a horizontal plane.

When the motion is long enough, the object will be infinitely close to doing a uniform linear motion, and the stable velocity at this time is v. At this time (the derivation process is very simple if you think about it, I won't push it):

f=kv is true.

The power done is fv

The power of frictional heat generation (the power to do work to overcome resistance) is (v 2)k and fv = (v 2)k

But if the object is subjected to other forces in addition to the drag f, constant force f, after reaching a stable velocity v:

f≠kv f work power is fv

The power of frictional heat generation (the power to do work to overcome resistance) is (v 2)k and fv = (v 2)k

You can review the knowledge of electrical work again.

For purely resistive elements:

u=ri is established.

The component consumes electrical power ui

The Joule heating power of the element is (i 2)r

And ui=(i 2)r

But if the element is not a pure resistor:

U≠ri components consume electrical power ui

The Joule heating power of the element is (i 2)r

And ui=(i 2)r

The question you are asking is not in itself capable of high school physics, so I am not going to give you a very detailed and rigorous explanation. If you are very concerned about this problem and have enough understanding, you should be able to see the similarity and connection between the above two models.

Because the unit joule is a unit of thermal energy, only electric heat can be calculated.

Joule's law is the square of the current multiplied by the resistance you say.

Joule's law: The heat generated by an electric current passing through a conductor is proportional to the quadratic of the current, proportional to the resistance of the conductor, and proportional to the energizing time. This law is called Joule's law, and it was first discovered by the British scientist Joule.

Joule's law can be expressed by the following formula:

q=i^2rt

There is another way to express the electrothermal formula:

q=w=pt=uit=（u^2/r）t

But this formula is only applicable to pure resistance circuits, that is, when the current passes through the conductor, if all the electrical energy is converted into heat, but not into other forms of energy at the same time, then it is a pure resistance circuit, and the heat generated by the current q is equal to the electrical energy consumed w, that is, q=w=uit.

Joule's law, the same resistance is the same electric current.

Joule's law refers to the heat produced by the current flowing through the conductor doing work, r is the conductor resistance, in fact, it is the DC resistance of the conductor, that is, the resistance that we can measure through a multimeter and other tools, the current flowing through the DC resistance must convert the electrical energy into heat, that is, Q = IIRT, in a DC or pure resistance circuit, it is useful work. However, in AC circuits, the so-called resistors will contain reactance components, and at this time, they can no longer be called resistance, but should be called impedance. The impedance will contain two parts: DC resistance and reactance, the value of UI will be the ability of AC current to do work (i.e., power), it contains two parts: useful work and useless work, where useful work is IIRT, which can convert electrical energy into heat, and useless work is IIRT, Z is reactance, which converts electrical energy into other forms of energy, such as electric field energy or magnetic field energy, which are not thermal energy.

Obviously, the current i=u z at this time will no longer be equal to u r, because the voltage and reactance at this time are complex numbers, that is, there is a certain phase difference between u and i, and only the components of u and i in phase can convert electrical energy into heat energy. The ratio of the voltage component to the current component in the same phase happens to be the DC resistance r (i.e., the DC component of the impedance), so the heat in the AC circuit is also the same as the law of the DC circuit, so Q=IIRT is applicable to any circuit, but UU R T is not applicable in the AC circuit. In fact, it is also wrong to write the formula q=uu r t itself, it is only valid for DC circuits.

For electric heatersFor example, resistors, electric blankets, rice cookers, electric soldering irons, etc., how much work is done by the current, how much electrical energy is converted into heat, that is to say: the amount of hot sales leakage released by electrical appliances is equal to the work done by the current.

At this time,w＝uit＝i^2rt, the formula is universal.

But for non-electric heatersFor example, the electric deficit machine mainly converts electrical energy into mechanical energy, but because the coil has a small resistance, it will emit a small amount of heat.

At this time, the power consumption is used: w uit

Heat is released with: Q i 2rt

w＞q

I don't know what to ask.