As to their conductivity for electrical current, the various substances are divided into conductors, non-conductors insulators and semiconductors. The three characteristic effects of current are - generation of heat in conductors through which current passes - the magnetic field associated with the current - transport of substance by ion currents The current intensity is determinded by the quantity of charge flowing through the conductor during a certain time.
From the definition equation of the current intensity, the basic unit for the quantity of electricity is derived; it is the ampere-second A. A frequently used sub-unit is the ampere-hour A. Questions and problems: 1. How many C are 0,5 A. A quantity of electricity of , C is flowing through a line within 5 hours. Find the current intensity. An electric current having the intensity of 2 A flows through a line for a period of 2 hours.
Calculate the transported quantity of electricity in the units C and A. There is no current without an electrical voltage. The original drive phenomenon for current is called primary electromotive force. It is generated in a voltage source. It imparts energy to the charge carriers which thus are driven through the conductor. Since every conductor offers resistance more or less to the passage of current, the charge carriers lose energy when passing through.
This loss can be characterised as voltage drop. A current can only flow through a conductor; therefore, the current path formed by the conductor must be closed. When a charge carrier has received drive energy from a voltage source, it passes through the conductor, completely transferring the energy taken up to this conductor.
After exactly one circulation, the charge carrier differs by nothing from its state bevor it started the circulation, that is to say, it cannot have stored energy.
The primary electromotive force is designated by the formula sign E, the voltage drop by U. In practice, no difference is made between these two terms and they are called voltage in short. Primary electromotive force and voltage drop have the same unit which is called volt - V in honour of the Italian physicist Alessandro Volta - Voltage Values for a Few Applications Lightning up to For the winning of electrical energy, the generation of the primary electromotive force by chemical and magnetic-field actions is of particular importance.
This charge carrier difference externally acts as electrical primary electromotive force. Diluted sulphuric acid H 2 SO 4 is suitable as electrolyte; as conductor rods electrodes , copper Cu and zinc Zn are particulary suitable Fig. Galvanic element also known as galvanic cell Other substances are also suitable especially coal and zinc in a thickened ammonium chloride solution.
In accordance with the general tendency to balance differences in concentration, the basic units of construction of the solid conductors are eager to migrate as ions in the electrolyte. On the other hand, the electrolyte tries to press its ions into the solid conductor. This impetus of motion is different in the different conductor materials so that, as a result, a primary electromotive force acts externally. When current flows, these voltage sources disintegrate due to the transport of substance and become useless; this is also occurring when stored too long.
Rechargeable voltages sources do not show this disadvantage, therefore, they are called accumulators storage batteries. Lead accumulators and nickel-iron or nickel-cadmium accumulators are of particular importance. According to a law of nature law of induction the following happens: When the magnetic flux enclosed by a conductor loop is changed, the charge carriers in the conductor are subjected to an impetus to move.
Then, the entire conductor loop is a primary electromotive force source. The change of the magnetic flux may, for example, be due to the fact that the conductor loop is turned inside the magnetic field or the magnet is approached to are moved away from this loop.
Primary electromotive force 2 generated by induction 1 - Direction of motion 2 - Direction of the primary electromotive force 3 - North pole 4 - South pole As symbol of a voltage source, the graphical symbol shown in Fig.
Graphical symbol of a direct voltage source; the arrow indicating the direction may be omitted The direction of voltage corresponds to the direction of current defined in Section 2. The voltage direction is indicated by an arrow. The electrical drive exerted on the charge carriers is called voltage. The drive phenomenon originally generated in a voltage source is called primary electromotive force E; the loss in voltage caused when current flows through a conductor is called voltage drop U.
For the winning of electrical energy, the generation of the primary electromotive force by chemical action and by the action of the magnetic field is of particular importance. Give reasons for the fact why in a current passage the sum of all voltage drops must be equal to the entire primary electromotive force!
Resistance and Conductance Every conductor and every electrical device electric bulb, heater, electromotor, wireless reciever, etc. This property is called electrical resistance formula sign B , Depending on the material used and the design of the conductor or the device, it has a different magnitude. For a conductor, the geometrical dimensions and the conductor material are decisive for the value of the resistance. The formula for calculating the resistance is called resistance rating formula.
It is easily understood and can be checked by experiment that a long thin wire will offer a higher resistance to the current passage than a short thick one. This dependence on material is covered by a material constant which is termed as specific resistance or resistivity formula sign r 1. Hence, 1 r Greek letter rho 2. The permeability to current of a conductor is called conductance formula sign G and, hence, is inversely proportional to the resistance.
From the equations 2. In English-speaking countries, the unit siemens has not been generally adopted. The correlation between the units siemens and ohm is given by equation 2. For r from equation 2. Example 2. Calculate the resistance and conductance of a copper wire having a length of m and a cross-sectional areas of 2,5 mm 2.
A copper conductor having a cross-sectional area of 6 mm is to be replaced by an aluminium conductor of the same resistance. What is the size of the cross-sectional area of the aluminium conductor?
The most striking influence on the resistance of a conductor or device is exerted by the temperature. This is due to the fact that the more intensively oscillating crystal lattices offer a higher resistance to the electron current; hence, a is positive.
In electrolytes and semiconductors, the resistance diminishes with increasing temperature. This is due to the fact that with rise in temperature more charge carriers are released which then are available as free charge carriers for the transport of electricity; hence, a is negative.
For practice, the following approximate values of the temperature coefficient will suffice see also Table 2. Find the resistance of the coil at this temperature. Resistor is a component for the realisation of a certain resistance value. Subscribe to our Mailing List and get interesting Stuff and updates to your Inbox. It is the reciprocal of the conductance and is measured in volt per ampere or ohm, Conductance : Conductance of a wire is the ease with which current flows in it.
It is the ratio of current and potential difference. Its unit is ampere per volt or Siemens. Spread Knowledge. Related Posts.
0コメント