LABORATORY WORK No 2

Performed by:

Zdenek Vodak

Conditions:

t = 22.5 °C; p = 1012.0 hPa; φ = 38.5 %

Topic:

Electric Resistance of Liquid Foods

Tasks:

1.

Determine the electric resistance of liquid foods used in cooking.

2.

Compare the electric resistance of used foods by means of bar chart.

Theory:

Electric resistance is the property of substance to defend itself (resist) the passage of  electric current. Electric resistance R is a quantity that reflects the amount of proportion of direct voltage U between the ends of a body and current I  passing  through the body. By increasing the voltage U between the ends of the conductor, the current I passing the conductor increases so that this proportion of voltage and current is constant and is called electric resistance R = U : I. Electric resistance R of liquids depends on the kind of liquid, on the distance and surface of submerged electrodes and on the temperature.

Aids:

glass vat, 2 pieces of copper sheet (4x8 cm) as two electrodes, source of direct voltage (for example the school stabilized source BK 125, Tesla, 15V), ammeter, voltmeter, liquid foods.

Procedure:

1.

Pour the investigated liquid food into the glass vat. Pour two copper electrodes fixed at the top in the rack into the liquid (electrolyte).

2.

Put the electric circuit together according to the diagram so that the ammeter is connected in series with the source of direct voltage and with the electrodes with electrolyte in glass vat. Connect the voltmeter into the circuit in parallel to electrodes and electrolyte.

3.

Determine the current I [mA] carried by the liquid, determine the voltage U [V] developed between the electrodes on the scale of the voltmeter. Calculate the amount of electric resistance R [Ω] of the food as the share of voltage U [V] and current I [A] according to the formula R = U : I.

4.

Carry out the same measuring also with the other liquid foods. Take into consideration that the distance between the electrodes and the depth of their submersion should be the same.

5.

Draw a   bar chart in which the length of the column is directly proportional to the electric resistance of the food.

Diagram:

Table:

No

Gradations

Constant

Voltage

Gradations

Constant

Current

Resistance

Liquid

/

D

k

U

D

k

I

R

/

/

V/d

V

1

mA/d

mA

mA

W

/

1.

28

30/60

14

43

300/60

215

65.1

Water + table salt

2.

30.5

30/60

15.25

54

12/60

10.8

1412

Raspberry juice 20%

3.

29

30/60

14.5

52

12/60

10.4

1394

Water

4.

30

30/60

15

22

60/60

22

682

Spirit vinegar 8%

5.

29

30/60

14.5

41

60/60

41

354

Orange juice 50%

6.

30

30/60

15

33

60/60

33

455

Skimmed milk 1,5%

7.

29

30/60

14.5

57

3/60

2.85

5088

Water + sugar

8.

30

30/60

15

18

60/60

18

833

Multivitamin nectar 50%

9.

29

30/60

14.5

24

300/60

120

121

Cucumber pickle

10.

30.5

30/60

15.25

4.5

0.12/60

0.009

1694000

Sunflower
oil

Graph:

The resistance of sunflower oil was not included into the graph for its excess dimension.

Conclusion:

The resistance of sunflower oil is clearly the biggest (R = 1694000 Ω = approximately 1694 kΩ) and therefore it is the worst conductor of electricity (belongs to nonconductors). The resistance of salted water, on the contrary, is the least (R = 65.1 Ω) and consequently it is the best conductor. The resistances of raspberry juice and sweet water are bigger than the one of clear water; the resistances of juice, nectar, milk and cucumber pickle are less than resistance of clear water.