In the circuit, the power supply voltage is constant. R1 is a certain value resistor, R2 is a sliding rheostat. After the switch S is closed, when the slide rheostat picture P slides before a and B, the maximum indication of voltmeter is 4V. The electric power variation range of resistor R1 is 0.8W ~ 7.2W, then when the slide rheostat picture moves back and forth, the indication range of current scale is? Circuit diagram: the power switch a R1 R2 is connected in series V to measure the voltage of R2

In the circuit, the power supply voltage is constant. R1 is a certain value resistor, R2 is a sliding rheostat. After the switch S is closed, when the slide rheostat picture P slides before a and B, the maximum indication of voltmeter is 4V. The electric power variation range of resistor R1 is 0.8W ~ 7.2W, then when the slide rheostat picture moves back and forth, the indication range of current scale is? Circuit diagram: the power switch a R1 R2 is connected in series V to measure the voltage of R2

When the resistance of the sliding rheostat is the minimum, the power of R1 is 7.2W, u-square / r = 7.2 (2) when the resistance of the sliding rheostat is the maximum, the power of R1 is 0.8W, the voltage of the sliding rheostat is 4V, so the voltage of R1 is the square of u-4v

4、 The voltage of normal operation of electrical appliances is called, and the power of electrical appliances under rated voltage is called
The actual applied voltage and the power of electrical appliances under the actual voltage are called. The rated power of each electrical appliance is only one, while the actual power has one. The actual power is different when the voltage is different. The relationship between the actual value and the rated value is as follows:
(1) When u is u and P is p, the appliance is in normal working condition;
(2) When u is u and P is p, the electric appliance can't work normally;
(3) When u is u and P is p, the service life of electrical appliances is shortened and easy to burn out
5、 Joule's Law: the heat generated by the current passing through the conductor is directly proportional to the heat of the conductor, directly proportional to the heat of the conductor, and directly proportional to the power on. This law is called, the mathematical expression is: q =. If the current does work through the conductor, if all the electric energy is converted into internal energy, then: Q w = = =. In the series circuit, the greater I is, the greater R is, the more heat is generated in unit time. In the parallel circuit, the smaller u is, the smaller R is, The larger I is, the more heat is produced per unit time
6、 Electric heating: electric heater is the use of heating equipment, such as electric furnace, electric iron, electric iron, electric cooker, electric oven. The main component of electric heater is. Heating body is made of resistance wire wound on insulating material

4 rated voltage rated power actual voltage actual power 1 innumerable equal to or less than or greater than
5 square resistance time Joule's law of current through conductor q = I ^ 2rt = Pt uitu ^ 2 / RT I ^ 2rt
much
Because of the thermal effect of current, the resistivity and melting point of the heater are high

Lights L1 and L2 are marked with "9V, 3W" and "3V, 3W" respectively. Now connect them in series and connect them to the two ends of the power supply with voltage u, which can make one of the lights light normally. Then the normal light and voltage u are ()
A. L2   9VB. L2   12VC. L1    9VD. L1   10V

∵ P = UI, ∵ rated current of the two bulbs: I1 = p1u1 = 3w9v = 13a, I2 = p2u2 = 3w3v = 1A; ∵ I = ur, ∵ resistance of the two bulbs: R1 = u1i1 = 9v13a = 27 Ω, R2 = u2i2 = 3v1a = 3 Ω; ∵ one of the lamps can light up normally, ∵ bulb L1 can light up normally, and the current in the circuit is I = I1 = 13A

The resistance wire of Xiaogang's home appliance thermal blanket is broken. His father connected the resistance wire and continued to use it. During the use, he found that the joint was burnt. Please explain the cause of the burning with your physics knowledge

A: after connecting the broken part of the resistance line, the resistance at the contact increases due to poor contact (or a large resistance is formed at the contact). In the same time, the current through the point generates more heat, which makes the temperature of the contact very high and scorches the joint