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Title of the Experience

Basoc notions from electricity II.

Name of the teacher

Mária Fabianová

Country where it took place

Slovakia

School typology

Lower Secondary School

Thematic Area

Physics

Experience typology

Teaching in class

Type of contact

Direct

Description of the Experience

When we finally got through the essence of electrical conductivity we can get to basic units whose relationships express basic connections between units of electrostatics.
Although the guys already have some elementary experience they can see them only from practical point of view. After some time we get to the fact that those units are: electrical current (I) in amperes (unit is named after A.M.Amper), electrical voltage (U) in volts (unit is named after A.Volt), electrical resistance (R) in ohms (unit is named after S.Ohm). The only way how to learn this is to memorise it.
The simplest electrical circuit has to contain voltage source and conductors. Usually a bulb is included so we can ensure that there is electrical current going through circuit. This can be done also in a classroom, it is enough to have a battery, two conductors and a little bulb. In order to have electrical current going through the circuit there has to be voltage and the circuit has to be closed.
The basic relationship between I, R, U is stated by Ohms law I=U/R. Mathematically said – the current proportionally increase with voltage and decrease with increasing resistance.

The form of Ohms law can be mathematically change according to what we want to express. The form R=U/I does not mean that the resistance is dependent on voltage. The students have to remember this. The resistance of conductor is based only on the material from which it is made, how long it is and how big is its cross-section. The resistivity is indicated in the table.
The resistance of conductor is based on temperature, the higher the temperature the bigger the resistance it results from the nature of electrical current. With increasing temperature increases also the speed of electron movement and their collision is more frequent what prevents their flow. At this point students can connect their knowledge about temperature and thermometers because on the basis of this principle are constructed resistance thermometers.
The opposite of electrical resistance is conductivity (G).
In the vicinity of each electrically charged body or electrical charge there is electrical field which can be shown using electric field lines. They are in the shape of rays which in case of positive charge come from the charge and in the case of negative charge they come to it. The only thing students can remember is that the same charges repel each other and the different charges attract each other. This is the second physical field that is shown to students, the first one was gravitational field
Passing of electrical current is associated with production of heat Q=U.I.t – this is called Joule-Lenz law. Product U.I represents performance of electrical current. Electrical current can be unidirectional or alternating, so far we spoke only about unidirectional electrical current.
It is difficult to say how much students will learn because they have no devices available. I have to repeat to them basic fact many times in order to make them remember it, for example associating of units to variables.
I think of that experience as of partially positive.

Comments on this Teachers Experience

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Date: 2014.09.30

Posted by Paola Falsini (Italy)

I am interested in this experience because it is on the same subject of the one I uploaded (“Introduction of Electrostatics”). I carried out my experience within the first class of upper secondary school. I am interested in the current experience because it deals with the electric phenomena in an original way. Its approach is different from the one I chose: I usually work on the concept of electric charge while here the departing subject is the electric current.
The “Guida all’insegnamento della Fisica” di A.B. Arons (Zanichelli, 1992), al cap. VII, deals with the better way for the students: starting from experiences about the charges or the currents.
Nevertheless even if the experience declares to work with an electrical network (in a easily replicable way), it is not described which kinds of observations and measurements are proposed and, moreover, which questions are done to the students in order to stimulate them.
Is the Ohm’s law introduced before the experimental part of the work or is it “built” together with the students (helping them)?
I faced the same students’ difficulties. I think, for example, that the idea that the electrical resistance depends on voltage comes from the presentation of the mathematic law earlier then the handling of the circuits. On the contrary, the concepts of resistance must be built starting from the experimental work.
This experimental approach is moreover necessary when the target is the low secondary school.

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