How to calculate electric power in physics? There are two formulas for calculating electric power w=uIt W = I squared RT I can't tell when to use which formula

In pure resistance circuit, all formulas for calculating electric power are common. When there are motors, light-emitting diodes and other electrical appliances in the circuit that are not used to convert electric energy into internal energy (not separately connected in parallel at both ends of the power supply), w = I square RT should be used to calculate electric power of resistance

When the filament of a light bulb is broken, the length of the bulb will be shortened, the resistance will be reduced, and the electric power will be increased,
In family circuits, sometimes the wires are not enough, so it is necessary to connect the wires, and the wiring is often easy to get hot. Accelerated aging causes fire, because the resistance of the wiring becomes larger and the heating is more, the teacher said
But when the wires are connected, the cross-sectional area becomes larger, the length becomes shorter, and the resistance becomes smaller. The teacher said that if they are twisted together, the contact surface becomes smaller, and the resistance becomes larger. In this case, why does the resistance become smaller

This place typical problem: first, the wire problem: the reason why the wire is easy to burn again is: the area of the contact part is small, so the resistance of the contact part is larger than that of the wire at other positions of the same length, because the current is the same, according to p = I ^ 2R, the heating power at the joint is large and easy to burn

Power of non pure resistance circuit
For example, the useless power of the electric fan is to generate heat energy, I & sup2; r = P
But is the total power UI or u & sup2 / R

The total power is p = UI. This is the original formula and the definition formula of power
I & # 178; r = P is the thermal power of resistance, which is also derived from P = UI
So when we calculate the total power, we need to use p = UI

The power formula of pure resistance circuit
In pure resistance circuit, the electric heating can be calculated by w = i2rt, but not by w = uit

In pure resistance circuit, electric heating can be calculated by w = i2rt or w = uit. Because it is a pure resistance circuit, all the work done by the current is converted into electric heating. Therefore, both formulas can be used. For example, electric heating can be calculated by w = i2rt in a non pure resistance circuit connected with a motor, but not by w = uit. W = uit calculates the total work

How to calculate electric power in physics? There are two formulas for calculating electric power w=uIt W = I squared RT I can't tell when to use which formula