On the closed circuit in the power and power efficiency

Correct understanding of concepts and flexible use of problem-solving approach is to study the basis of a constant circuit. Closed electrical circuit is the basic content of high school physics, is divided into a pure resistance circuit and non-resistance circuit on the circuit in this power of knowledge and power efficiency for a law of induction and guidance of learning, the learning of students want to help.

A closed circuit of the four power
1 power supply: a description of a closed circuit in the power supply to other forms of energy into electrical energy speed of the physical quantity which is equal to the number of total current I and the electromotive force E of the product, that is P = IE
2 Power output power: is the foreign way of electric power, which is equal to the number of total current I and the way the product of the voltage U. P out = IU
3 internal loss of power supply: internal resistance refers to the thermal power, that is, P in = I  2r
4 power supply external power loss: external resistance refers to thermal power, ie P = I  2R outside
Twenty-four power relationship between
1 According to the law of conservation of energy can get P = P + P within a
2 For a purely resistive circuit P = P external
3 for the P non-purely resistive circuit outside of ≥ P
See whether it is purely resistive circuit is not purely resistive circuit all the output power and thermal power to meet the basic definition, but the difference lies in the power of the output power and thermal power resistors outside the relationship for purely resistive circuits, the two are equal, for non- ranging in terms of both pure resistance circuit.

Here's the output power of pure resistance circuit for further discussion:
It can be seen from the above equation, when the external resistance is equal to the power of resistance (R = r), the maximum output power, the maximum output power of E  24r 
When R> r, the P out decreases with increasing R, with R decreases, P increases the
When R <r, the P out decreases with increasing R, as R decreases, P out decreases, as shown below:

(Because of Ohm's law P = UR is only applicable to pure resistance circuit, so the formula P = U  2R is only applicable to pure resistance circuit, both can be used to seek the power of the output power can also be used to seek external resistance of thermal power.

Two power supply efficiency
Power efficiency is the power output and the ratio of the power supply, that is
(Purely resistive and non-purely resistive circuit are applicable to the purely resistive circuit, power supply efficiency
Seen from the above equation, the greater the external resistance, the higher the power efficiency.

When R = r, P out = P  out m4x  time, η = 50%.

External short circuit resistance is R = 0, η = 0,
External circuit is disconnected, the power does not work, η = 0.

Analysis of three typical examples
A circuit as shown below, known electromotive force E = 3V, internal resistance r = 1Ω, R1 = 2Ω, Slip-line continuous rheostat resistance R, increased demand:
(1) how much time when R, R maximum power consumption?
(2) how much time when R, R1 maximum power consumption?
Analysis and Solution: In seeking the maximum power consumption when R, R1 included in the internal resistance, when R = R1 + r when, R maximum power consumption, but in seeking the maximum power consumption of R1, because R1 is the resistance value, can not apply the above method, the application of another way of thinking to solve, by P1 = I  2R, we can see, as long as the current maximum, P1 on the biggest, so transferred to zero when the R, R1 has the maximum power.

Solution: (a R1 classified as resistance, when R = r + R1 = 3Ω time, R the maximum power consumption,

(2 from the P1 = I2R we know that when I maximum, P1 maximum, to make the maximum I should make R = 0. When R = 0, the maximum power consumption.

Maximum power output of power is not necessarily the maximum external resistance. Internal resistance when the external resistance is equal to the power of output power, when the external resistance is not equal to the internal resistance within the resistance when the power closer to the maximum output power. The power to determine the maximum external resistance flexible use of time to "go within the law" and "maximum current law."
2 There is a crane using a DC motor, as shown below, its internal resistance, line resistance, supply voltage, source resistance is negligible. Table showing the number of volts 110V, find (a current through the motor, (2 input to motor power, (3 motor heating power, mechanical power output of the motor.

Analysis and answers: the power of resistance is neglected, excluding internal resistance of Joule heating, P in = I  2r = 0.

(1 non pure motor resistor, so the direct application of Ohm's law can not seek its current, but we see the relationship between R and the motor is connected in series between the two currents are equal. You can find the current flowing through R to find the first to ask this question .

(2 input to the motor power is the power we have discussed above the output power: P entry = UI = 110 × 4W = 440W
(3 motor thermal power consumed by the motor resistance thermal power: Pm = P into-Pr = (440-12.8W = 427.2W
CONCLUSION: ① electric power W = IUt and Joule heat Q = I  2Rt, the two are not necessarily equal.

(1 of pure resistance circuit (only with incandescent lamps, electric heater circuits such as current acting completely used to generate heat energy into internal energy, so the electric heating power W equal to Q, then W = Q = UIt = I  2Rt.

(2 for non-pure resistance circuit in the energy conversion into electrical energy in addition to the internal energy, but also into mechanical energy, chemical energy, etc. that is, W = Q + E other. Links to free download http://www. hi138.com
Exercise: 1, a lamp directly connected to the constant power supply, its power is 100W, if this light be connected to a long wire, followed the same power, in the loss of electrical power wire is 9W , then the lights at this time to the actual consumption of electric power (
Less than 91W more than 91W B. A.

C. equal to 91W D. inadequate, unable to determine
② hair using hair dryer in the motor and the electric wire, electric motor driven blades rotating, heated air heating wire to get hot hair dryer. Set the motor coil resistance of R1, with R2 resistance heating wire connected in series DC power supply, the voltage across the hair dryer is U, the current power consumption of I P, then (
A. P = UIB. P = I  2 (R1 + R2
C. P> UID. P> I  2 (R1 + R2
Answer: 1B 2AD
Students to do the right thing? Links to free download http://www.hi138.com