6. Work execution order and experimental data analysis

Researching useful power, total power and efficiency of the source dependence from external resistance of a circuit

1. Mount scheme Fig 22. Set three-decade resistors box R₃ as changeable resistor.

2. Gradually increase resistance of the three-decade resistor-box R₃ and achieve the maximum scale reading of the voltmeter U_V^max. Miliampermeter scale reading thus should be zero I_mA^min=0. Write down directly ahead of the table of measuring obtained thus the value of the EMF of a source =U_V^max= … V.

3. Find a value of a short-circuit current I_mA^max, when R₃=0. This value of current divide on 10 parts and obtain a step of measuring of a current for gridding of a current axis I= I_mA^max/10.

4. Since from I_mA^max up to I for values:

I₁= I_mA^max, I₂= I_mA^max–I, I₃= I_mA^max–2I, …, I₁₀= I_mA^max–9I=I

make an ten measurings of values of external resistance R_і corresponding to ten values of a of the current. Results of measurements write down into the table of measurements.

5. Individually to make a measuring of resistance R₁₂ and current I₁₂ at U_V=U_V^max/2=/2. Results write down into the table of measurements.

5. Calculate total, useful power and efficiency. Results of calculations write down into the table of measurements.

8. Plot the dependence of P_T, P_U and η versus load resistance on one graph. Place the powers axis on a left-hand side, and the efficiency axis on the right side of the graph.

9. Find from graph a value of internal resistance of a sourse r.

10. Conclude, when the circuit works with maximal power saving, with minimal efficiency. What is the optimal mode of circuit work? Conclude about the condition of matching of source to a circuit loading.

7. Test questions

1. What are total power, useful power, and efficiency of a source?

2. Derive a formula of total power dependence on external resistance. Analyze the obtained formula. Build the graph. What values of external resistance do they equal zero with? Are there maximal values?

3. Derive a formula of useful power dependence on external resistance. Analyze an obtained formula. Build a graph.

4. Derive what value of external resistance useful power is maximal with?

5. Derive a formula of efficiency dependence on external resistance. Analyze an obtained formula. Build a graph.

6. Write formulas of total power, useful power and efficiency dependencies on a current. What value of a current useful power is maximal with?

7. What are unuseful mode, power-saving mode and optimal mode with greatest delivery to external resistance?

8. Content of the report

Homework to Laboratory work №2-4

(Answers on test questions from p. 41)

…

Laboratory work № 2-4 implementation protocol

1) Topic: POWER IN A DIRECT CURRENT CIRCUIT

2) Goal:

2.1) Study total power, useful power and electrical efficiency dependence on external load resistance.

2.2) Finding the condition of matching of source to a circuit loading.

3) Scheme of laboratory research facility

4) Table of measuring instruments:

№	Name	Type	Serial number	Grid limit	Grid unit	Accuracy class
1.	Four-decade resistors box R3
2.	Voltmeter
4.	Miliampermeter

5) Equations for calculations:

1. Total power:

P_T [mW]= I [mA] 

where I – current;  – EMF of the source.

2. Useful power:

P_U [mW]=I²·R ·10 ³,

where R – resistance of load resistor.

3. Efficiency:

η = (Р_U / Р_T)·100 %.

1. Table of measurements:

R=R₃; =U_V= … V, when I =I_min ( or R₃=9 999 ).

№	I, mА	R, Ω	PT, mW	PU, mW	η, %
1	Imax	0
2
3
4
5
6
7
8
9
10
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