Physics
Current Density
Current density refers to the amount of electric current flowing through a given cross-sectional area of a conductor. It is a vector quantity, with both magnitude and direction, and is typically denoted by the symbol J. Current density is an important concept in physics, particularly in the study of electromagnetism and the behavior of conductive materials.
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eBook - ePub- C R Robertson(Author)
- 2008(Publication Date)
- Routledge(Publisher)
analogy . Thus, an electron orbiting the nucleus may be compared to a satellite orbiting the Earth. The satellite remains in orbit due to a balance of forces. The gravitational force of attraction towards the Earth is balanced by the centrifugal force on the satellite due to its high velocity. This high velocity means that the satellite has high kinetic energy. If the satellite is required to move into a higher orbit, then its motor must be fired to speed it up. This will increase its energy. Indeed, if its velocity is increased sufficiently, it can be made to leave Earth orbit and travel out into space. In the case of the electron there is also a balance of forces involved. Since both electrons and protons have mass, there will be a gravitational force of attraction between them. However the masses involved are so minute that the gravitational force is negligible. So, what force of attraction does apply here? Remember that electrons and protons are oppositely charged particles, and oppositely charged bodies experience a force of attraction. Compare this to two simple magnets, whereby opposite polarities attract and like (the same) polarities repel each other. The same rule applies to charged bodies. Thus it is the balance between this force of electrostatic attraction and the kinetic energy of the electron that maintains the orbit. It may now occur to you to wonder why the nucleus remains intact, since the protons within it are all positively charged particles! It is beyond the scope of this book (and of the course of study on which you are now embarked) to give a comprehensive answer. Suffice to say that there is a force within the nucleus far stronger than the electrostatic repulsion between the protons that binds the nucleus together.All materials may be classified into one of three major groups—conductors, insulators and semiconductors. In simple terms, the group into which a material falls depends on how many ‘free’ electrons it has. The term ‘free’ refers to those electrons that have acquired sufficient energy to leave their orbits around their parent atoms. In general we can say that conductors have many free electrons which will be drifting in a random manner within the material. Insulators have very few free electrons (ideally none), and semiconductors fall somewhere between these two extremes.Electric current This is the rate at which free electrons can be made to drift through a material in a particular direction
eBook - ePubElectrical Engineering
Fundamentals
- Viktor Hacker, Christof Sumereder(Authors)
- 2020(Publication Date)
- De Gruyter Oldenbourg(Publisher)
1 The basic physic principles and definitions1.1 The simple circuit
In everyday life, people do not distinguish between technically correct designations for electric quantities but abbreviate and incorrectly name it “electricity”. Colloquially, the expression “electricity bill” is used, when in reality the electrical energy consumption is meant; when an electrical accident happens, it is referred to as “electric shock”.A person with technical knowledge is aware that a flow of an electric charge is designated “electric current” and that the physical quantity of current (intensity) uses the unit ampere. Furthermore, an expert knows that it is the voltage (measured in volts) that drives the current and that resistance (measured in ohm) at constant voltage determines the current (Figure 1.1 ).To better understand the correlation between electric current, voltage and resistance, we look at the water cycle as analogue to the electric circuit.Figure 1.1: Correlation between current, voltage and resistance.Table 1.1: Water cycle as analogue to electric circuit.Water cycle (analogue) Electric circuit Figure 1.2: Closed water cycle.Figure 1.3: Closed circuit.The flow of water is caused by the pressure differenceQ tΔ Pgenerated by pump P.The current flow is caused by the potential difference (= voltage V ) generated by the voltage source. The pressure difference Δ Pdetermines the amount of water pumped via the load per time.The potential difference (voltage V ) determines the electric charge per time (current I) flowing through the load. The pressure loss due to the resistance in the container C is as high as the pressure difference Δ P
eBook - ePub- Adrian Waygood(Author)
- 2018(Publication Date)
- Routledge(Publisher)
specific number of individual electrons per unit time past a given point in a conductor.- The amount of charge transported, per second, by a current of one ampere is one coulomb (symbol: C ).
- The coulomb is equivalent to the amount of charge generally rounded off to 6.24×1018 electrons. This figure will be defined to a greater level of accuracy once the ampere is redefined in 2019.
Misconceptions
The ampere is defined in terms of the rate of drift of electric charge
Current is defined as the quantity of charge transported per unit time . But, at present, its unit, the ampere is defined in terms of the force between current-carrying conductors. Proposed changes will likely mean that, from May 2019, the ampere will be redefined in terms of the rate of flow of elementary charges (individual electrons), not coulombs.The ampere describes the speed of an electric current
The ‘speed’ of an electric current has nothing to do with its unit of measurement. Electric charge drifts v-e-r-y slowly, regardless of the value of current.Review your learning
Now that we’ve completed this chapter, we need to examine the objectives listed at its start. Placing ‘Can I… ’ at the beginning, and a question mark at the end, of each objective turns that objective into a test item . If we can answer each of those test items, then we’ve met the objectives of this chapter.Online resources The companion website to this book contains further resources relating to this chapter. The website can be accessed via the following link: www.routledge.com/cw/waygoodAn Introduction to Electrical Science, Waygood, ISBN 9780815391821, 2019. © Taylor & Francis
- Franklin Bretschneider, Jan R. de Weille(Authors)
- 2018(Publication Date)
- Academic Press(Publisher)
Electrophysiology is the field of research on electrical processes in living creatures. This includes everything from current flow through wires to signals in our instruments to electrochemical processes, in living cells and organs, and in the electrodes we use to study them. So, although most of the vast field of electricity theory is outside the scope of this book, we need to deal with a handful of quantities that play important parts, such as charge, voltage (potential), current, resistance, impedance, and especially their complicated changes in time. We start with the main definitions and the relationships between these quantities.Electric Charge, Current, and Potential
The basic quantity is the electric charge, buried in the atomic nucleus as what we call a positive electric charge and in the electrons surrounding it, which we call negative charge. The unit of electric charge (abbreviated Q) is the coulomb (abbreviated C), defined in (macroscopic) electric circuits in the 18th century.The underlying fundamental constant, found much later (in 1909, by R. Millikan), is the elementary charge, the charge of one electron, which amounts to 1.6021 × 10− 19C. Since this “quantum of electricity” is so small, most electric phenomena we will describe may be considered as continuous rather than discrete quantities.By the nature of atoms, most substances, and indeed most materials in daily life, are neutral. Obviously, this does not mean that they have no charges at all, but that (1) the number of positive charges equals the number of negative charges and (2) the opposite charges are so close together that they are not noticeable on a macroscopic scale. This means that a number of substances can be “teased” to release electricity, e.g., by rubbing them together. This was indeed the way electricity was discovered in antiquity and was examined more systematically from the 18th century on. Many science museums are the proud owners of the large static electricity generators invented by among others van Marum and Wimshurst. These machines generated rather high voltages (around 50 kV), but at very low current strengths (1 μA), and so were not of much practical use.Nowadays, most sources of electric energy are electrodynamic, made by rotating machines such as the generators in our power plants, in cars, and on bicycles.These machines can produce almost any voltage and current needed, usually as alternating current (AC) that can be transformed into lower or higher voltages as desired. In addition, electrochemical processes, found originally by Galvani and Volta, are employed in the arrays of galvanic cells we call batteries and accumulators. Both forms of source deliver the electrical energy at lower voltages (say, 12
