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Essentials of Electricity
Published in Anthony J. Pansini, Guide to Electrical Power Distribution Systems, 2020
Resistance is essentially the opposition offered by a substance to the flow of electric current. Remember that a conductor is made of a material whose atomic structure has a large number of free electrons. Because of the greater number of free electrons in their atomic structure, some conductors allow electric current to flow more readily than others. Resistance in a conductor is similar to friction in a pipe. A pipe with a smooth inside surface conducts water with little loss of pressure or hindrance to the flow. But if the pipe is rough inside, the pressure and flow are reduced. Similarly, a good conductor allows electricity to flow with little opposition or resistance and with a small loss of electrical pressure or voltage. A poor conductor offers a large resistance to electric current and this causes a large loss of pressure or voltage. Just as the length and cross section of the pipe affect the flow of water so it is with conductors of electric current: the greater the length, the greater the resistance; the greater the cross-sectional area, the smaller the resistance (see Figure 10-3). Additionally the material of which the conductor is made also affects the resistance. The energy used in overcoming resistance is converted into heat that is why some conductors are found to be warm when touched.
DC circuits
Published in Charlie Cullen, Learn Audio Electronics with Arduino, 2020
This circuit has three stages, where each stage is a separate series loop connected to a common voltage source (Arduino) that can be switched on/off by a push-button switch. Therefore, the overall system has three sensor inputs (push-buttons) and multiple LED outputs (1, 2 and 3 respectively). The purpose of the push-buttons is to give you a visual comparison of the effect of adding multiple resistors in series, where each loop will be dimmer than the previous one due to the additional resistors reducing the overall current (from ≈333mA down to ≈11mA). We know that resistances in a series circuit add up, and by using Ohm’s Law in the equations above we can show that more resistors in series will reduce the overall current flowing. If you don’t have six red LEDs then you can use other LED colours if you have them, but blue and white LEDs will be much dimmer than red, orange or green because of their higher forward voltage (around 4V). You can also leave out the last stage of this project if you do not have enough LEDs and/or resistors – or you can simply build stages 1 and 2 then break them down to build stage 3 separately afterwards. If you do so, you will still see a reduction in LED brightness in the second-stage circuit, but it will not be as marked as the difference between stages 1 and 3.
Steady-State Single-Phase Models of Power System Components
Published in Antonio Gómez-Expósito, Antonio J. Conejo, Claudio A. Cañizares, Electric Energy Systems, 2018
Another important factor that affects the resistance of a conductor is the temperature. For conductor materials, the resistivity increases with temperature according to an approximately linear relationship. Therefore, resistances R1 and R2 at two different temperatures θ1 and θ2 are related by the following expression: R2=R1(1+α(θ2−θ1)) where α is a coefficient depending on the conductor material (Table 2.3).
Optimal energy management of a DC power traction system in an urban electric railway network with dogleg method
Published in Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 2021
Xiaojuan Hu, Shan Zhou, Tie Chen, Mohammad Ghiasi
Reference (Tian et al. 2014) provided a 2-substation sample which is displayed in Figure 4. This substation could be considered as a fixed voltage resource where their voltage in many typical systems usually varies between 600 V and 1.5 kV. ‘RSub’ is the internal resistance (Ω) of the substation that is considered as a fixed value. The “internal resistor” is owing to the adjustment specifications of the traction rectifier system. The “internal resistor” also cannot collaborate to the losses of the system within the substation. ‘R1, R2ʹ provide the resistances of the forward transmission line, whereas ‘R3, R3ʹ represent the resistance of the running rail to return the current. The amounts of the resistances relate to the lengths of the sections and resistivity of the conductor. In this form of a two-substation sample, since the position of the train can be anywhere on the line, the resistors (R1, R2, R3, and R4) are related to the position of the moving train.
Self-sensing concrete: from resistance-based sensing to capacitance-based sensing
Published in International Journal of Smart and Nano Materials, 2021
The measured resistance can be DC or AC. The DC method is advantageous for its greater degree of current penetration, while the AC method is advantageous for the lower degree of electric polarization. Polarization refers to the movement of charges (e.g., ions) so that an electric dipole is formed. Polarization occurs during resistance measurement, which involves the application of a small current provided by the measurement meter for the duration of the measurement. The longer is the time of the measurement, the more is the polarization. Depolarization occurs upon reversal of the polarity of the current. The dipole impedes conduction, thus causing the measured resistance to be higher than the true resistance. In order to measure the true resistance, the resistance should be measured within the first few seconds of resistance measurement, before polarization becomes appreciable. Alternatively, the current polarity can be reversed and the average of the measured resistance immediately before the reversal and that immediately after the reversal gives the true resistance [64,65] (Figure 3). In both DC and AC methods, the removal of the contact resistance from the measured resistance is important. The AC resistance is to be distinguished from the AC impedance, which is a complex quantity that reflects both the real part (resistance) and imaginary part (reactance).
Application of electrical resistivity measurement as quality control test for calcareous soil
Published in HBRC Journal, 2018
Marwa Mostafa, Mona B. Anwar, Amr Radwan
When current (I) is applied to a block of material and potential drop of (V) volts is measured between two ends of the block, the resistance (R) of the block is equal to (V/I). The resistance (R) of a conductor is directly proportional to length (L), and is inversely proportional to cross section area (A).where (ρ) is the resistivity in ohm. The resistivity of a material is a fundamental property of that material.