Series connection: I=I 1=I2.
U=U 1+U2
R=R 1+R2
Parallel connection: I=I 1+I2
U=U 1=U2
One of R = one of R1+one of R2.
Secondly, the values of R, I, U I and U should be clearly defined when reviewing the questions, and then brought in.
Finally, to unify units, international standard units must be used.
1. ohm's law is a very important electrical experimental law, and the experimental research method is the control variable method.
Ohm's law reveals the relationship among current, voltage and resistance. The research steps given in the textbook are: first, keep the resistance constant and study the change of current with voltage; Then keep the voltage constant and study the change of current with resistance. This is a common method in physics to study the relationship between multiple physical quantities. The general steps are as follows: (This is an important research method)
(1) Find out which physical quantities are involved in the study, which changes are independent (in ohm's law, U and R are independent variables, which are called independent variables in mathematics), and which physical quantities are involved in the change (I in ohm's law, which is called dependent variable in mathematics), and take this quantity as the research object;
(2) Study the single relationship between this physical quantity (research object) and other independent variables one by one, and keep the other independent variables unchanged in the research process;
(3) Then integrate these single relationships.
This research method is usually called variable control method. This method is often used in the later learning process.
2. Mathematical expression of Ohm's law
It is a transformation from the literal description of laws to mathematical expressions that is often encountered in physics. In ohm's law, the proportional physical quantity u is located in the numerator part of the formula, and the inversely proportional physical quantity r is located in the denominator part of the formula. When both u and r are expressed in international units, the formula of ohm's law can be written as I = u/r.
Attention should be paid to the word "this paragraph" in the text narration of Ohm's Law, which emphasizes that I, U and R belong to the same paragraph of the circuit and have one-to-one correspondence at the same time. (the so-called "trinity")
3.I = u/r and r = u/i.
I=U/R is the basic formula of ohm's law, which means that u and r are "causes", I is "effects" and I is determined by u and r. As for the ohm's law transformations mentioned at the end of this section, U=IR and R=U/I (this is the physical meaning of this formula), they do not reflect the physical laws, but are only used for formula transformation in mathematical calculation. For example, R=U/I does not mean that "the conductor resistance R is directly proportional to the voltage across the conductor U and inversely proportional to the current I passing through the conductor". In fact, when the voltage increases several times, the current through the conductor also increases several times, and their ratio remains unchanged. When I studied resistance in the last chapter, I already knew that the resistance of a conductor is the nature of the conductor itself, and it has nothing to do with whether there is voltage or current. However, the formula variation R=U/I can provide us with a method to measure resistance, which will be studied in the next section.
4. In the research experiment of Ohm's law, the function of sliding rheostat was used twice.
Ohm's law experiment is to study the quantitative relationship between current, voltage and resistance by controlling variables. In the experiment, the resistance or voltage is controlled separately, and the data relationship between the other two quantities is measured, from which the law is summarized. Therefore, in order to achieve the purpose of the experiment, that is, control variables and multi-measurement, we must first understand the relationship among current, voltage and resistance in series and parallel circuits.
Series connection: I=I 1=I2.
U=U 1+U2
R=R 1+R2
Parallel connection: I=I 1+I2
U=U 1=U2
One of R = one of R1+one of R2.
Secondly, the values of R, I, U I and U should be clearly defined when reviewing the questions, and then brought in.
Finally, to unify units, international standard units must be used.
1. ohm's law is a very important electrical experimental law, and the experimental research method is the control variable method.
Ohm's law reveals the relationship among current, voltage and resistance. The research steps given in the textbook are: first, keep the resistance constant and study the change of current with voltage; Then keep the voltage constant and study the change of current with resistance. This is a common method in physics to study the relationship between multiple physical quantities. The general steps are as follows: (This is an important research method)
(1) Find out which physical quantities are involved in the study, which changes are independent (in ohm's law, U and R are independent variables, which are called independent variables in mathematics), and which physical quantities are involved in the change (I in ohm's law, which is called dependent variable in mathematics), and take this quantity as the research object;
(2) Study the single relationship between this physical quantity (research object) and other independent variables one by one, and keep the other independent variables unchanged in the research process;
(3) Then integrate these single relationships.
This research method is usually called variable control method. This method is often used in the later learning process.
2. Mathematical expression of Ohm's law
It is a transformation from the literal description of laws to mathematical expressions that is often encountered in physics. In ohm's law, the proportional physical quantity u is located in the numerator part of the formula, and the inversely proportional physical quantity r is located in the denominator part of the formula. When both u and r are expressed in international units, the formula of ohm's law can be written as I = u/r.
Attention should be paid to the word "this paragraph" in the text narration of Ohm's Law, which emphasizes that I, U and R belong to the same paragraph of the circuit and have one-to-one correspondence at the same time. (the so-called "trinity")
3.I = u/r and r = u/i.
I=U/R is the basic formula of ohm's law, which means that u and r are "causes", I is "effects" and I is determined by u and r. As for the ohm's law transformations mentioned at the end of this section, U=IR and R=U/I (this is the physical meaning of this formula), they do not reflect the physical laws, but are only used for formula transformation in mathematical calculation. For example, R=U/I does not mean that "the conductor resistance R is directly proportional to the voltage across the conductor U and inversely proportional to the current I passing through the conductor". In fact, when the voltage increases several times, the current through the conductor also increases several times, and their ratio remains unchanged. When I studied resistance in the last chapter, I already knew that the resistance of a conductor is the nature of the conductor itself, and it has nothing to do with whether there is voltage or current. However, the formula variation R=U/I can provide us with a method to measure resistance, which will be studied in the next section.
4. In the research experiment of Ohm's law, the function of sliding rheostat was used twice.
Ohm's law experiment is to study the quantitative relationship between current, voltage and resistance by controlling variables. In the experiment, the resistance or voltage is controlled separately, and the data relationship between the other two quantities is measured, from which the law is summarized. Therefore, in order to achieve the purpose of the experiment, that is, to control variables and to measure multiple groups of data for analysis, the above purpose can be achieved by connecting the sliding rheostat in series in the circuit. When the resistance remains the same (taking a constant value resistor as the measured object), the function of the rheostat in series in the circuit is to change the voltage at both ends of the constant value resistor, so as to increase the voltage by integer multiples (here, to control the voltage to change by multiples), thus recording multiple sets of current and voltage data and finding out the quantitative relationship between current and voltage. When measuring the current through different fixed-value resistors with ammeter, when studying the relationship between current and resistance, the control condition is to keep the voltage at both ends of the fixed-value resistor unchanged (here, to keep the voltage at both ends of R unchanged). However, every time the constant resistance is changed in the experiment, the voltage at both ends will change. In order to keep its voltage constant, the voltage at both ends of the fixed resistor is controlled at a certain value by changing the resistance value of the sliding rheostat in the experiment.
Ohm's law involves three physical quantities. Of the three physical quantities, two are known and the other physical quantity can be found. When applying ohm's law and its transformation, we should understand the meaning of ohm's law and the meaning of two transformations. I=U/R reflects the factors that determine the current; R=U/I indicates that the resistance can be calculated by the ratio of voltage to current; U=IR means that the voltage across the conductor is equal to the product of the current through the conductor and the conductor resistance.
6. Pay attention to the following two points when solving problems with Ohm's Law: (1) Draw the schematic diagram of equivalent circuit according to the meaning of the problem, and mark the symbols of known quantity, numerical value and quantity to be solved on the diagram. If there is a circuit diagram, according to the opening and closing situation of the switch and the position of the sliding rheostat, draw the equivalent circuit diagram under various conditions, and mark the symbols of the known quantity and the quantity to be solved; (2) Be sure to use formulas to solve problems. When calculating with a formula, write down the corresponding unit when substituting the numerical value.
Group data provides analysis, and the above purpose can be achieved by connecting sliding rheostat in series in the circuit. When the resistance remains constant (taking a constant value resistor as the measured object), the function of the series connection of the rheostat in the circuit is to change the voltage at both ends of the constant value resistor, so that the voltage is increased by integer multiples (here, the regulated voltage is changed by multiples), so that multiple sets of current and voltage data can be recorded and the quantitative relationship between current and voltage can be found. When measuring the current through different fixed-value resistors with ammeter, when studying the relationship between current and resistance, the control condition is to keep the voltage at both ends of the fixed-value resistor unchanged (here, to keep the voltage at both ends of R unchanged). However, every time the constant resistance is changed in the experiment, the voltage at both ends will change. In order to keep its voltage constant, the voltage at both ends of the fixed resistor is controlled at a certain value by changing the resistance value of the sliding rheostat in the experiment.
Ohm's law involves three physical quantities. Of the three physical quantities, two are known and the other physical quantity can be found. When applying ohm's law and its transformation, we should understand the meaning of ohm's law and the meaning of two transformations. I=U/R reflects the factors that determine the current; R=U/I indicates that the resistance can be calculated by the ratio of voltage to current; U=IR means that the voltage across the conductor is equal to the product of the current through the conductor and the conductor resistance.
6. Pay attention to the following two points when solving problems with Ohm's Law: (1) Draw the schematic diagram of equivalent circuit according to the meaning of the problem, and mark the symbols of known quantity, numerical value and quantity to be solved on the diagram. If there is a circuit diagram, according to the opening and closing situation of the switch and the position of the sliding rheostat, draw the equivalent circuit diagram under various conditions, and mark the symbols of the known quantity and the quantity to be solved; (2) Be sure to use formulas to solve problems. When calculating with a formula, write down the corresponding unit when substituting the numerical value.