5.2. Direct method

1. Mount the scheme Fig. 18. As a modelling source of the EMF we choose a source e₁, which is on the left side of a laboratory board. Resistors box R₂ simulate the sources internal resistance.

2. Turn on observable source. Set the value R₂=0, thus . Lock the switche К₁.

3. Write down the voltmeter scale reading U_i into a measurements table.

4. To set sequentially values of resistance of resistors box R₂ equal 10k, 30k, 50k, 70k, 90k, thus .It is equivalent to a changing of sources. Make measurements according to point 3.

5. Calculate a relative error (67) of each direct measurement (for each another sourse).

6. Represent the final result by a draph of dependence error of direct method of measurements β_i^DIR from relative value of internal resistance of a source r_Xi/R_V .

7. Conclude about range of values of internal resistances of source at which measuring it is possible to consider correct (when the value of error of direct method of measurements _i^DIR is less).

7. Test questions

1. What is potential, voltage (potential difference), EMF of the source, voltage drop?

2. How does Ohm's law look for uniform part of circuit, for non-uniform part of circuit, for the closed circuit?

3. When do potentials difference on the terminals of the source is equal to its EMF?

4. What is compensation method of measurement EMF of sources? Draw the scheme.

5. How the voltage between the points of the circuit can be calculated, if all EMFs of all sources and resistances of all parts of the circuit are known?

6. What is compensated in a compensation method of measuring of the EMF? Draw the scheme.

8. Content of the report

Homework to Laboratory work №2-3

(Answers on test questions from p.14)

…

Laboratory work № 2-3 implementation protocol

1) Topic: DETERMINATION OF EMF OF CURRENT SOURCE.

2) Goal: Studying EMF measurement methods: a) compensation method; b) method of direct measurements.

3) Scheme of laboratory research facility:

a) compensation method	b)direct measurements

4) Table of measuring instruments:

№	Name	Type	Serial number	Grid limit	Grid unit	Accuracy class
1.	One-decade resistors box R2
2.	Voltmeter
4.	Microampermeter (zero-indicator)

5) Equations for calculation:

5.1) Compensation method:

U_V = IR_V = ε_x –Ir, when I =0 (compensation condition), then U_V = ε_x.

Hhere r = r_x + R₂.

Statistical absolute error of measurements we estimate by Sdudent’s equation (27):

,

where =0,95 – confidence probability; n=5 – number of measurements;

t _{0,95 ; 5}= 2,77 – Sdudent’s coefficient.

Device error of voltmeter (28):

,

where β_V - is the accuracy class of voltmeter, _max - its grid limit of voltmeter.

Total absolute error (29):

.

Relative error of measurements in compensation method:

.

5.2) Direct measurement method:

U_V = IR_V = ε_x –Ir, where r = r_x + R₂.

We don’t calculate an absolute error of direct measurement method because we represent the final result by a draph.

Relative error of direct measurement method:

,

where <_x> – average value of EMF from point 5.1).

6) Table of measurements: RV=2105 , r1= 2 k.
Compensation method						Direct measurement method
№	r, kΩ	x, V	xi, V	(I)2, V2	U, V		βiDIR , %	r / RV
1	rх + 0
2	rх + 10
3	rх + 30
4	rх + 50
5	rх + 70
6	rх + 90	–	–	–
Average evalue <x>=		…		…