Laboratory work No. 10. "Studying Ohm's Law for a Complete Circuit - Method 3." Purpose of work: to study Ohm's law for a complete circuit. Objectives of the work: determination of the EMF and internal resistance of the direct current source by its current-voltage characteristic; study of the graphical dependence of the power released in the external circuit on the magnitude of the electric current P f I . Equipment: DC power supply, ammeter, voltmeter, connecting wires, key, rheostat. Theory and method of doing the work: Ohm's law I Rr for a complete circuit I Rr. We transform I R r I R I r U I r U I r U I r. expression Therefore, the dependence of the voltage at the output of the DC source on the magnitude of the current strength (volt-ampere characteristic) has the form (see Fig. 1): Fig. 1 Analysis of the current-voltage characteristics of a DC source: 1) for point C: I \u003d 0, then U 0 r 2) for point D: U \u003d 0, then 0 I r I r I 3) tg U r II short-circuit I short-circuit r The expression for the power released in the external electrical circuit is P I U I I r I I 2 r. Therefore, the graphical dependence P f I is a parabola with branches directed downward (see Fig. 2). fig. 2 Analysis of the graphical dependence P f I (see Fig. 3): Fig. 3 1) for point B: P \u003d 0, then 0 I I 2 r 0 I r I r I short. , i.e. abscissa point B corresponds to short-circuit current; 2) since the parabola is symmetrical, then the abscissa tA is half the short-circuit current I 3) since in point А I I к.з. , and the ordinate corresponds to the maximum power value; 2 2r Rr and I 2r, then after the transformations we get R \u003d r - the condition under which the power released in the external circuit with a direct current source takes the maximum value; 2 r 4) maximum power value P I 2 R . 4r 2r 2 Operation progress: 1. Connect a voltmeter to the DC power supply terminals (see Fig. 4). The voltage shown by the voltmeter is taken as the value of the EMF of the direct current source and taken as a reference for this laboratory work. Record the result in the form: (U ± U) V. Take the absolute error equal to the division value of the voltmeter. fig. 4 2. Assemble the experimental setup according to the diagram shown in Figure 5: fig. 5 3. Carry out a series of 5-10 experiments, with a smooth movement of the rheostat slider, enter the measurement results in the table: Current strength Voltage I U А В 4. Based on the experimental data obtained, construct the current-voltage characteristic of the direct current source. 5. Determine the possible value of the EMF of the direct current source and short-circuit current. 6. Apply the technique of graphic processing of experimental data and calculations for calculating the internal resistance of a direct current source. 7. The calculation results are presented in the form: EMF of a direct current source: (av ± av) V; • internal resistance of the direct current source: r \u003d (rav ± rav) Ohm. 8. Build a graphical dependence U f I in Microsoft Excel, using the chart wizard with adding a trend line and indicating the equation of a straight line. Using the main parameters of the equation, determine the possible value of the EMF of the DC source, short-circuit current and internal resistance. 9. On the numerical axes, indicate the range of values \u200b\u200bof EMF, internal resistance of the DC source and short-circuit current obtained by different methods of determination. 10. Investigate the power released in the external circuit from the magnitude of the electric current. To do this, fill in the table and build a graphical dependence P f I : Current strength Power I P A W 11. According to the constructed graph, determine the maximum power value, short-circuit current, internal resistance of the current source and EMF. 12. It is possible to construct a graphical dependence P f I in Microsoft Excel, using the chart wizard with the addition of a polynomial trend line with a degree of 2, the intersection of the curve with the OY (P) axis at the origin and specifying the equation on the diagram. Using the main parameters of the equation, determine the maximum power value, short-circuit current, internal resistance of the current source and EMF. 13. Formulate a general conclusion on the work.
Topic: "Study of Ohm's law for a section of the circuit"
Objective: to establish experimentally the dependence of the current strength on voltage and resistance.
Equipment: laboratory ammeter, laboratory voltmeter, power supply, a set of three resistors with resistances of 1 Ohm, 2 Ohm, 4 Ohm, rheostat, current circuit switch, connecting wires.
Working process.
Brief theoretical information
Electricity -ordered movement of charged particles
The quantitative measure of electric current is current strength I
Current strength -– scalar physical quantity, equal ratio charge q, transferred through the cross-section of the conductor during the time interval t, to this time interval:
In SI units, current is measured in amperes [A].
Current measuring device Ammeter.Included in the chain consistently
VoltageIs a physical quantity that characterizes the action electric field on charged particles, is numerically equal to the work of the electric field in moving the charge from a point with a potentialφ 1 to a point with potentialφ 2
U 12 \u003d φ 1 - φ 2
U - voltage
A – work current
q – electric charge
Voltage unit - Volt [V]
Voltage measuring instrument - Voltmeter.It is connected to the circuit in parallel to the section of the circuit where the potential difference is measured.
On the electric circuit diagrams, the ammeter is indicated.
The value characterizing the reaction electric current in the conductor, which is due internal structure conductor and the chaotic movement of its particles is calledelectrical resistance of the conductor.
The electrical resistance of the conductor depends onsizes and conductor shapes and from material, from which the conductor is made.
S - conductor cross-sectional area
l – conductor length
ρ - the specific resistance of the conductor
In SI, the unit of electrical resistance of conductors is ohm [Ohm].
Graphical dependencycurrent strength I from stress U - volt-ampere characteristics
Ohm's law for a homogeneous section of a chain: the current in a conductor is directly proportional to the applied voltage and inversely proportional to the resistance of the conductor.
Named after its discoverer Georg Ohm.
Practical part
1. To do the work, assemble an electrical circuit from a current source, an ammeter, a rheostat, a 2 ohm wire resistor and a key. Connect a voltmeter in parallel with the wire resistor (see diagram).
2. Experience 1.
Table 1. Section resistance 2 Ohm
3.
4. Experiment 2.
Table 2.
5.
6. Answer security questions.
Control questions
1. What is electric current?
2. Give the definition of the current. How is it indicated? What is the formula for?
3. What is the unit for measuring current strength?
4. What instrument is used to measure the current? How does it connect to the electrical circuit?
5. Give the definition of voltage. How is it indicated? What is the formula for?
6. What is the unit of measure for voltage?
7. What instrument is used to measure voltage? How does it get involved in an electrical circuit?
8. Give the definition of resistance. How is it indicated? What is the formula for?
9. What is the unit of measurement for resistance?
10. Formulate Ohm's law for the circuit section.
Measurement option.
Experience 1. Study of the dependence of the current on the voltage at a given section of the circuit... Switch on the current. Using a rheostat, bring the voltage at the terminals of the wire resistor to 1 V, then to 2 V and to 3 V. Each time, measure the current and record the results in table. one.
Table 1. Section resistance 2 Ohm
Based on the experiments, build a graph of the dependence of the current strength on the voltage. Make a conclusion.
Experience 2. Study of the dependence of the current strength on the resistance of a section of the circuit at a constant voltage at its ends... Connect a wire resistor to the circuit in the same way, first with a resistance of 1 ohm, then 2 ohms and 4 ohms. Using a rheostat, set the same voltage at the ends of the section each time, for example, 2 V. Measure the current strength, record the results in Table 2.
Table 2.DC voltage at 2 V section
Based on the experiments, build a graph of the dependence of the current strength on the resistance. Make a conclusion.
Presentation: "Laboratory work:" Study of Ohm's law for a section of a circuit. "
(edocs) fizpr / lr7f.pptx, 800,600 (/ edocs)
When designing and repairing circuits for various purposes, Ohm's law for a complete circuit must be taken into account. Therefore, those who are going to do this need to know this law for a better understanding of the processes. Ohm's laws are divided into two categories:
- for a separate section of the electrical circuit;
- for a complete closed circuit.
In both cases, the internal resistance in the power supply structure is taken into account. In computational calculations, Ohm's law for a closed circuit and other definitions are used.
The simplest circuit with an EMF source
To understand Ohm's law for a complete circuit, for clarity of study, the simplest circuit is considered with a minimum number of elements, EMF and an active resistive load. Connecting wires can be added to the kit. A 12V car battery is ideal for power supply; it is considered as an EMF source with its own resistance in the structural elements.
The role of the load is played by an ordinary incandescent lamp with a tungsten coil, which has a resistance of several tens of ohms. This load converts electrical energy into heat. Only a few percent are spent on emitting a stream of light. When calculating such circuits, Ohm's law is applied for a closed circuit.
Proportionality principle
Experimental studies in the process of measuring quantities at different values \u200b\u200bof the parameters of the complete circuit:
- Current strength - I A;
- The sums of the resistances of the battery and the load - R + r are measured in ohms;
- EMF - current source, denoted as E. is measured in volts
it was noticed that the current strength has a directly proportional relationship with respect to the EMF and an inverse proportional relationship with respect to the sum of resistances that are closed in series in the circuit. Let us state this algebraically as follows:
The considered example of a circuit with a closed circuit circuit - with one power source and one external load resistance element in the form of a lamp with an incandescent spiral. When calculating complex circuits with several circuits and many load elements, Ohm's law for the entire circuit and other rules are applied. In particular, it is necessary to know Kirgoff's laws, to understand what two-terminal networks, four-terminal networks, branching nodes and individual branches are. This requires detailed consideration in a separate article, earlier this course of TERTS (theory of electrical radio engineering circuits) was taught at institutes for at least two years. Therefore, we restrict ourselves simple definition for complete electrical circuit only.
Features of resistances in power supplies
Important! If we see the resistance of the spiral on the lamp in the diagram and in a real design, then the internal resistance in the design of a galvanic battery, or accumulator, is not visible. In real life, even if you disassemble the battery, it is impossible to find the resistance, it does not exist as a separate part, sometimes it is displayed on the diagrams.
Internal resistance is created on molecular level... Conductive materials in a battery or other power source for a rectifier generator are not 100% conductive. Elements with particles of dielectric or metals of other conductivity are always present, this creates current and voltage losses in the battery. On accumulators and batteries, the effect of the resistance of structural elements on the magnitude of the voltage and current at the output is most clearly displayed. The ability of the source to deliver the maximum current is determined by the purity of the composition of conductive elements and electrolyte. The purer the materials, the lower the r value, the EMF source produces a higher current. Conversely, in the presence of impurities, the current is less, r increases.
In our example, the battery has an EMF of 12 V, a light bulb capable of consuming a power of 21 W is connected to it, in this mode the lamp spiral is heated to the maximum permissible glow. The formulation of the current passing through it is written as:
I \u003d P \\ U \u003d 21 W / 12V \u003d 1.75 A.
In this case, the spiral of the lamp burns at half the incandescence, we will find out the reason for this phenomenon. To calculate the total load resistance (R + r) apply Ohm's laws for individual sections of circuits and the principles of proportionality:
(R + r) \u003d 12 \\ 1.75 \u003d 6.85 ohms.
The question arises of how to separate the value of r from the sum of resistances. An option is allowed - to measure the resistance of the lamp spiral with a multimeter, subtract it from the total and obtain the value of r - EMF. This method will not be accurate - when the coil is heated, the resistance significantly changes its value. Obviously, the lamp does not consume the power declared in its characteristics. It is clear that the voltage and current for glowing the coil are small. To find out the reason, let's measure the voltage drop across the battery when the load is connected, for example, it will be 8 Volts. Suppose the coil resistance is calculated using proportionality principles:
U / I \u003d 12V / 1.75A \u003d 6.85 Ohm.
When the voltage drops, the resistance of the lamp remains constant, in this case:
- I \u003d U / R \u003d 8V / 6.85 Ohm \u003d 1.16 A at the required 1.75A;
- Current loss \u003d (1.75 -1.16) \u003d 0.59A;
- By voltage \u003d 12V - 8V \u003d 4V.
The power consumption will be P \u003d UxI \u003d 8V x 1.16A \u003d 9.28 W instead of the required 21 W. Finding out where the energy goes. It cannot go beyond the closed loop, only wires and the structure of the EMF source remain.
EMF resistance -r can be calculated using the lost values \u200b\u200bof voltage and current:
r \u003d 4V / 0.59A \u003d 6.7 ohms.
It turns out that the internal resistance of the power source "consumes" half of the released energy on itself, and this, of course, is not normal.
This happens in old spent or defective batteries. Now manufacturers are trying to monitor the quality and purity of the current-carrying materials used in order to reduce losses. In order for the maximum power to be given to the load, the technologies for manufacturing EMF sources are controlled so that the value does not exceed 0.25 Ohm.
Knowing Ohm's law for a closed circuit, using the postulates of proportionality, one can easily calculate the necessary parameters for electrical circuits to identify faulty elements or design new circuits for various purposes.
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