A Continuing Flow of Electrons or Protons is Termed

Electrons and electricity

Owen Bishop , in Understand Electronics (Second Edition), 2001

Current through a gas

Gases under low pressure can conduct electric charge. As in the case of a solution, there is two-way conduction.

Electrons flow from the negative plate (negative electrode) to the positive plate (positive electrode). On their way, they strike neon atoms and knock electrons out of them. This creates more electrons to act as negative charge carriers. The neon atoms which have lost electrons become positive ions, and act as positive charge carriers. The energy from the moving carriers excites many of the neon atoms. Excited atoms later lose this energy, which then appears in another form, that of reddish light. An example is the 'flicker flame' lamp shown in the title photograph of this chapter.

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ELECTRON TRANSFER IN PHOTOSYNTHETIC SYSTEMS: ENERGY CONSERVATION IN PHOTOSYNTHETIC ELECTRON TRANSPORT OF CHLOROPLASTS

A. Trebst , in Living Systems As Energy Converters, 1977

B Photophosphorylation

The electron flow system is coupled to ATP formation, that is in addition to NADPH and oxygen a stoichiometric amount of ATP is formed in the light (non cyclic photophosphorylation). Recent evidence led to the recognition of two coupling sites along the electron flow system and a stoichiometry of more than 1 ATP formed per 2 electrons moved through the chain. In Fig. 1, the two coupling sites are indicated as two native energy conserving sites. For the mechanism of ATP formation the orientation of the electron flow is of great importance. According to a chemiosmotic mechanism of coupling an energy conserving site is composed of an electrogenie step in one direction and a proton translocating step in the other direction of a vectorial electron flow system, together comprising a mitchellian loop (3). The sequence of events in a loop will result in a proton transport across the membrane, which will built up a pH gradient. Two such loops, i.e. two energy conserving sites may be recognized in photosynthetic electron flow: the electrogenic photosystem I and the proton translocation via plastoquinol and the electrogenic photosystem II with proton liberation in the water oxidation inside. The proton motive force generated in the light consists of a pH gradient of about 3 pH units across the thylakoid and a (small) contribution of a membrane potential. In an ATP-synthetase, spatially removed from the electron flow system, the proton motive force will form ATP from ADP and inorganic phosphate at the expense of protons moved back out from the inside space of the thylakoid.

Two electrons are required to reduce NADP⁺ at the expense of water oxidation moved via two photosystems, therefore four quanta of red light of 43 kcal each (the long wavelength absorption band of chlorophyll a) are needed. The redox potential energy stored in NADPH is 52 kcal. At the same time 4 protons for 2e are transported from the outside to the inside of the thylakoid. These protons then drive ATP synthesis with a stoichiometry of 2 or 3 protons/ATP used up. Assuming a stoichiometry of 2 ATP/2e, an additional 16 kcal are stored in the ATP of coupled photophosphorylation. The overall biochemical energy yield in coupled photosynthetic electron flow is therefore 68 kcal from the electromagnetic energy input of 4 mol quanta red light (172 kcal).

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The pursuits of solar application for biofuel generation

Sanjay Sahay , in Handbook of Biofuels, 2022

34.1.1.4 Electrobiofuels

Electrobiofuels may be produced either using the inherent capacity of certain microbes to capture electrons from electrodes or applying metabolically engineered (designer) microbes that can use renewable electricity (solar or wind) as an energy source to transform CO₂ into biofuels and other commodity chemicals. The process called microbial electrochemical synthesis (MES) thus can store energy from solar cells into a storable form (preferably liquid) of electrofuels. For example, engineered Clostridium ljungdahlii can synthesize butanol (biofuel) through electrochemical synthesis (Jabeen and Farooq, 2016). Metabolically engineered bacterium Ralstonia eutropha has been shown to utilize energy from a metal catalyst-based water-splitting system to produce isopropanol (Torella et al., 2015). Bacteria, suitable for MES, capture electrons from the cathode via specific membrane-embedded electron conduits.

34.1.1.4.1 Mechanisms of electron transfer

Electron flows from electron donor with a relatively lower redox potential to the electrode with relatively higher redox potential. The microbes with embedded electron carriers in their membrane generally act as electron donors. Only limited examples of microbes with the capacity to accept electrons have been reported ( Thrash et al., 2008). The former are important for electricity generation (biofuel cell), and the latter are useful in electrochemical synthesis. In both processes, however, there is extracellular electron transfer (EET).

Both Gram-positive and Gram-negative bacteria exhibit EET, but the latter are usually found to be more effective (Milliken et al., 2007; Rabaey and Rozendal, 2010). At least six mechanisms of EET have been reported (Fig. 34.5), which may be grouped into direct (electron flows directly from microbe to electrodes and vice versa such as via endogenous mediator, nanowire, or direct contact) and indirect (shuttling component mediates electron flows such as exogenous or by-product shuttlers or EPS). Geobacter sulfurreducens (Bond and Lovley, 2003) and Shewanella oneidensis str. Mr-1 (Venkateswaran et al., 1999) are the most studied metal-reducing bacteria showing the direct transfer of electron to metal (hydr)oxides (Fig. 34.6) (Lovely, 2006; 2008). These bacteria transfer electrons to electrodes and typically involve many periplasmic and outer membrane carrier complexes viz., inner membrane-anchored OmcA, periplasmic cytochrome c-type carrier MtrA, an intermediate carrier CymA linking OmcA and MtrA, and terminal carrier MtrC showing broad redox potential located on the outer membrane in S. oneidensis. In the case of G. sulfurreducens, the outer membrane-anchored OmcE and OmcS have been reported to transfer electron to type IV pili, the latter then to solid metal (i.e. electrode). The role of pili or pilus-like appendages (called nanowires) in electron transport between microbes has also been reported (Reguera et al. 2005). Shuttlers of bacteriogenic origin like phenazines (Rabaey et al., 2005) and flavins (von Canstein et al., 2008) or of humic origin have been reported that carry out indirect EET. These are the inbuilt components of EET that carry out electron transport from microbes to electrodes. EET from microbe to electrode is important for electricity generation. A reverse process is required for the electrochemical synthesis of biofuels.

Figure 34.5. Schematic of the mode of EET from cathode to microbes through (1) production of H₂, (2) forming building blocks such as formate, (3) directly, (4) exogenous or endogenous mediators or shuttlers, (5) extracellular polymeric compounds (EPSs), and (6) nanowire.

Figure 34.6. Schematic of EET in (A) Geobacter and (B) Shewanella according to Kumar et al. (2017). In Geobacter, OmcZ is assisted by OmcS and Omc EET from the outer membrane, while type IV pili are involved in direct EET from the inner membrane to the electron acceptor. Omc mediates both of these types of EET. In Shewanella, CymA transports electron from the inner to outer membrane; from there; electron is transported to the extracellular acceptor by Mtrs and OMCs. The nanowires are outer membrane and periplasmic extensions providing another path for electron transport via electron jumping through cytochrome network. Flavin may also be involved in the EET shuttler or cofactor.

Information on the possibility of a reverse flow of electron has been provided by the same two bacteria viz., S. oneidensis Mr-1 (Xafenias et al., 2013) and G. sulfurreducens (Gregory and Lovley, 2005) while studying their ability to reduce hexavalent chromium (Cr(VI)) using electron from the cathode. S. oneidensis could do this by the reversed OmcA-MtrABC pathway (Ross et al., 2011), but G. sulfurreducens utilizes a different pathway involving a periplasmic monoheme cytochrome PccH and not involving PccP, the latter being used for outward electron transfer (Strycharz et al. 2011). The engineered strain of G. sulfurreducens ACL with heterologous ATP-dependent citrate lyase showed remarkably higher electron consumption when grown on a graphite electrode (cathode) (Ueki et al., 2014). Other examples of engineered microbes consuming electrons from cathodes are G. metallireducens (Gregory et al., 2004), C. ljungdahlii (Nevin et al., 2011), and Rhodopseudomonas palustris (Bose et al., 2014).

Although the mechanism of electron consumption has been more studied in Geobacter sp. and Shewanella sp., their applications in electrochemical synthesis are limited because of their heterotrophic nature (Rabaey et al., 2010). Microbes utilizing energy from inorganic chemical reactions (chemotrophs) or light (phototrophs) for CO₂ fixation and electron sources for growth (electrotrophs) are especially useful. Acetogenic bacteria forming a biofilm on cathode such as Sponosa ovate have been shown to reduce CO₂ to acetate utilizing electrons from the cathode (graphite electrode) with a > 85% electron conversion efficiency (Nevin et al., 2010). Likewise, Sponosa (Nevin et al., 2011) and Clostridium (Nevin et al., 2011; Choi et al., 2014) species and Mororella thermoacetica (Nevin et al., 2011) do utilize electrons from the cathode to convert CO₂ to acetate. Their ET pathways, however, are yet to be fully explored.

The acetotrophic methanogen Methanosarcina acetivorans uses reduced ferredoxin to transfer an electron to membrane-anchored multiheme cytochrome c (Wang et al., 2011, Schlegel et al., 2012). It was also found that methanogens use membrane-associated enzymes hydrogenases with their coenzymes ferredoxin or methanophenazine to mediate electron transfer (Thauer et al., 2010). In Methanococcus maripaludis, hydrogenase and formate dehydrogenase secreted from the cells mediate electron transfer between a cathode and the bacterium (Deutzmann et al., 2015). The bacterium R. palustris, a natural Fe(II)-oxidizing prototroph that is found growing on biocathode, can fix CO₂ under both light and dark conditions (Bose et al., 2014). Its ability has been shown to reside in the operon PioABC comprising genes for OM porin, a periplasmic cytochrome and Fe-S cluster protein (Bose et al., 2014).

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Removal and Recovery of Metals and Nutrients From Wastewater Using Bioelectrochemical Systems

Y.V. Nancharaiah , ... P.N.L. Lens , in Microbial Electrochemical Technology, 2019

4.5.3.2 Microbial Electrolysis Cells

The electron flow from the anode to the cathode is thermodynamically not favorable if metal ions with lower redox potentials than the bioanode are used as the electron acceptors in the cathode ( Table 4.5.1). For example, the standard redox potential of the Ni(II)/Ni(0) couple is −0.25   V and Ni(II) cannot be reduced by accepting electrons from the cathode in an MFC. The principle of metal removal using MECs is depicted in Fig. 4.5.2. The electrons released at the anode are conducted to the cathode via an external circuit. These electrons are used for the reduction of metal ions at the cathode for immobilizing metals. However, the reduction of metal ions such as Cd(II), Co(II), Ni(II), Zn(II), and Pb(II) can be made possible by using an external power to drive the electron flow from the high potential anode to the low potential cathode (Table 4.5.3, Fig. 4.5.2). Unlike MFCs, removal of the metal ions in MECs requires an energy input. However, the overall voltage required is still lower than the electrochemical cells because part of the voltage is supplemented by the oxidation of waste organic matter at the anode. Alternatively, MFCs can be coupled to MECs to further decrease the power requirement for driving metal reduction in MECs. Various studies that determined metal removal at the abiotic cathode of MECs are summarized in Table 4.5.3.

Figure 4.5.2. Immobilization and recovery of Co, Ni, and Cd in microbial electrolysis cells.

Table 4.5.3. Metal Recovery Using Microbial Electrolysis Cells With Abiotic Cathodes

Metals Removed	BES Configuration, Electrode Materials	Electron Donor, Concentration	Metal Salt; Metal Concentration; pH	Metal Removal Efficiency	Power Source/Applied Voltage	References
Cadmium	tMEC, carbon brush anode, carbon cloth cathode	Sodium acetate, 1   g/L	CdSO4; 50, 100, 200   mg/L Cd(II); pH 6	93.6% from 50   mg/L Cd(II) in 60   h	Cr(VI) reducing MFC	[44]
Cadmium	sMEC, graphite fiber mesh anode, stainless steel cathode	Sodium acetate, 0.5   g/L	CdCl2; 12.26   mg/L Cd(II)	50%–67% in 24   h	0.4, 0.6, 0.8 and 1.0   V	[45]
Cobalt	tMEC, graphite felt anode, carbon rod cathode	Sodium acetate, 1   g/L	Co(II); 50   mg/L Co(II); pH 6	7   mg/L   h Cu(II)	Co(II) leaching MFC	[22]
Cobalt	tMEC, graphite brush anode, graphite felt cathode	Sodium acetate, 1   g/L	CoCl2; 874   μM Co(II); pH 3.8 to 6.2	92% in 6   h	0.3–0.5   V	[46]
Nickel	tMEC, carbon felt anode, stainless steel cathode	Sodium acetate, 1   g/L	NiSO4; 50–1000   mg/L Ni(II); pH 5	99% of 50   mg/L Ni(II) in 20   h	0.9   V	[33]

sMEC, single-chambered microbial electrolysis cell; tMEC, dual-chambered microbial electrolysis cell.

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Theoretical background

Colin A Vincent , in Modern Batteries (Second Edition), 1997

Relationship between current and reaction rate

Within any cell undergoing charge or discharge, one can consider at least three forms of charge transmission:

•

electron flow in the electronic conductors (e.g. electrode materials, current collectors, terminal connectors, load resistors, etc.);

•

ion flow in the electrolyte (which may be an aqueous solution, solid electrolyte, molten salt, etc.);

•

charge transfer reactions at the electrode/electrolyte interfaces (e.g. $\mathrm{Z}\mathrm{n}\left(\mathrm{s}\right)\stackrel{\begin{array}{ll}-\hfill & 2\mathrm{e}\hfill \end{array}}{\to }{\mathrm{Z}\mathrm{n}}^{2+}\left(\mathrm{a}\mathrm{q}\right)$ ).

Since in the steady state, it is necessary to maintain a condition of electroneutrality in any macroscopic part of the system, the total charge flux through all cross-sections of the circuit must be the same. In particular, the rate of electron flow in the external circuit is equal to the rate of charge transfer at each electrode/electrolyte interface.

For the general electrode process

$\mathrm{Ox}+ne\to \mathrm{Red}$

nF coulombs are required to reduce 1 mole of Ox, so that the rate of reduction of Ox at the electrode is given by

$\left(\frac{\mathrm{Rete}\mathrm{of}\mathrm{passage}\mathrm{of}\mathrm{electrons}\mathrm{into}\mathrm{electrode}\mathrm{from}\mathrm{external}\mathrm{circuit}}{\mathit{nF}}\right)$

i.e.

(2.40) $\mathrm{Rate}\mathrm{of}\mathrm{reduction}\mathrm{of}\mathrm{Ox}=i/\mathit{nF}\mathrm{mol}{\mathrm{s}}^{-1}$

At the other electrode, Red′ – n′e → Ox′, and again

(2.40′) $\mathrm{Rate}\mathrm{of}\mathrm{oxidation}\mathrm{of}\mathrm{R}\mathrm{e}{\mathrm{d}}^{\prime }=i/n^{\prime }F\mathrm{mol}{\mathrm{s}}^{-1}$

It should be noted that the rate of charge transfer at the electrodes (i.e. the rates of the electrochemical processes) may be given directly by the reading on an ammeter inserted in the external circuit. If the current is a function of time, it is still possible to apply the above idea of 'flux continuity' over a succession of small time intervals. It may happen that one of the various rate processes involved in charge transport in different components of a cell, is unable to maintain as high a rate as the others. Under these circumstances it becomes the current limiting process for the cell.

The question now arises as to what factors are responsible for determining the rates at which the various cell processes occur. Thermodynamic arguments permit the feasibility of overall cell reactions to be predicted, but give no information on rates. To understand the latter it is necessary to consider the effects on various parts of the cell of forcing the cell voltage to assume a value different from that of the equilibrium emf. It has been shown above that in the Daniell cell at equilibrium, charge transfer across the zinc/solution interface can be described in terms of processes

${\mathrm{Z}\mathrm{n}}^{2+}\left(\mathrm{a}\mathrm{q}\right)\stackrel{2\mathrm{e}}{\to }\mathrm{Z}\mathrm{n}\left(\mathrm{s}\right)$

occurring at equal rates. The first of these may be considered as constituting a cathodic flow of charge across the interface, i.e. as a cathodic current, i _c. Similarly for the oxidation of metallic zinc to zinc ions, there is a charge flow which may be described as an anodic current, i _a. At equilibrium i _c is equal to i _a and this equilibrium current is known as the exchange current, i _o. The net current i, given as (i _c − i _a) by convention, is of course zero at equilibrium. In the same way, at the copper electrode at equilibrium,

$i_{\mathrm{c}}^{\prime }=i_{\mathrm{a}}^{\prime }=i_{\mathrm{o}}^{\prime }$

(Note that it would be most improbable for i _o and i _o ^′ to be equal.) Now let a constant voltage source be placed across the cell so that the zinc is forced to assume a less negative potential and the copper a less positive potential than their equilibrium values. The altered electric field across the zinc/solution interface makes it easier for zinc atoms to be oxidized but hinders the reduction of zinc ions. Hence i _a increases from its equilibrium value while i _c decreases: therefore

$\left|i_{\mathrm{Z}\mathrm{n}}\right|=i_{\mathrm{c}}-i_{\mathrm{a}}\ne 0$

At the copper electrode, reduction of Cu^{2   +} is favoured and oxidation of Cu atoms is restricted, so that net cathodic flow occurs. Finally, to prevent a build up of Zn^{2   +} ions near the zinc/electrolyte interface and of ${\mathrm{SO}}_4^{2-}$ counter ions near the copper, a flux of ions must take place in the electrolytic phase to balance the charge transfer processes at the interfaces. To maintain the flux continuity condition, the applied voltage difference becomes distributed in such a way that*

(2.41) $\left|i_{\mathrm{Z}\mathrm{n}}\right|=\left|i_{\mathrm{C}\mathrm{u}}\right|=\left|i_{\mathrm{ion}\mathrm{flow}}\right|$

For spontaneous discharge, the overall cell voltage must be reduced from its equilibrium value, as would happen if a load resistor were connected to the terminals. If a potential difference greater than the emf were applied (i.e. one making the cathode more positive and the anode more negative), the net result would be a current flow in the reverse direction, causing a net charging of the cell.

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Fundamentals of energy storage devices

Nihal Kularatna , Kosala Gunawardane , in Energy Storage Devices for Renewable Energy-Based Systems (Second Edition), 2021

2.3.1 Basic electrical components as in-circuit energy storage

When electrons flow through a resistor, energy is dissipated in the resistance and gets converted as heat and no ideal resistor could store energy. However, in an inductor within a closed circuit carrying a current of I, stored energy E _L is given by

(2.18) $E_{\mathrm{L}}=\frac{1}{2}L{I}^2$

This energy storage occurs due to the magnetic flux generated by the current within the core of the inductor. However, since the energy is stored in the inductor due to the flux lines created by the current, this energy is not physically transportable with the device unless the rest of the circuit responsible for generating this current is kept undisturbed.

In contrast, if we connect a voltage source to a capacitor, its voltage exponentially rises to the value of the voltage source. If the DC source used to charge the device has an open-circuit voltage of V, stored energy in the capacitor of value C is given by

(2.19) $E_C=\frac{1}{2}C{V}^2$

Compared to the case of an inductor, this energy storage occurs in the form of energy associated with an electrostatic field based on electric-charge storage. For this reason, if the capacitor has no significant leakage (which represents an infinite leakage resistance), the device with the energy stored can be transported after disconnecting the voltage source. However, in practical capacitors, the leakage resistance is finite, and this creates a leakage current through this equivalent resistance, and energy is wasted in the form of heat dissipated in this large resistance. Practical devices such as electrolytic capacitors with capacitance values ranging from nanofarads to few thousand microfarads do retain their charge from microsecond order durations to few minutes, but they cannot be treated as useful ESDs similar to electrochemical devices such as batteries. In modern SC families, capacitance values vary from fractional Farads to few thousands of Farads, and their leakage currents are in the order of few microamperes to milliamperes only. This allows them to be used as short-term energy storage and delivery devices in power electronic systems.

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Maintenance and Troubleshooting

George Patrick Shultz , in Transformers and Motors, 1989

Staggering Brushes

Current flow is electron flow. Polarity of current is from negative to positive for the external circuit. Wear on the commutator differs for current flow from brush to commutator as opposed to current flow from commutator to brush for a DC machine. Brushes are staggered on the commutator to provide for even wear under the brushes.

Figure 10-32A illustrates the correct method for staggering the brushes. Brushes of opposite polarity track each other. This procedure helps prevent grooving of the commutator.

FIGURE 10-32. Commutator brush positions.

The method of staggering brushes in Figure 10-32B is incorrect. In this case, brushes with the same polarity follow each other. This will aggravate the pitting and wear on the commutator.

It is not possible to stagger all the brushes in accordance with Figure 10-32A when the machine has a number of poles equal to two times an odd number. Under these conditions, stagger all but one set of positive and negative brushes according to the correct method. This set can then be aligned with any two adjacent rows.

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Biochemistry and Electrochemistry at the Electrodes of Microbial Fuel Cells

Prasenjit Bhunia , Kingshuk Dutta , in Progress and Recent Trends in Microbial Fuel Cells, 2018

16.3.1 Ohmic Losses

The resistance of the electron flow through the electrodes and interconnections, and the resistance of the ions flow through the CEM (if present) as well as the resistance of the anodic and cathodic electrolytes in an MFC is collectively referred to as ohmic losses (also called ohmic polarization) (region B in Fig. 16.4) [9,20]. This potential drop can be easily ascertained by Ohms law as given in Eq. (16.10).

(16.10) $\mathrm{\Delta }V={\mathit{IR}}_{\mathit{int}}$

where, R _int is the total internal resistance and I is the circuit current. Ohmic losses can be diminished by minimizing electrode spacing and total travel distance of electrons within the anode as well as using a low resistive membrane. In this connection, use of highly conductive anode materials with a 3D architecture (e.g., 3D graphite felt electrode) is beneficial to produce a higher current by overcoming ohmic losses [9]. Moreover, the 3D architecture facilitates a higher electron transport, not only due to a high surface to volume ratio, but also increases the anode-microbe interactions [9]. Apart from this, when graphite electrodes are used, the anodic (electrical) resistance is reported to be negligible [9] and contact resistance can also be significantly low as compared to ionic resistance.

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Introduction to Electrical Units and Circuits

K.M. SMITH C.ENG., M.I.E.E. , P. HOLROYD C.ENG., M.I.MECH.E., A.M.I.STRUCT.E. , in Engineering Principles for Electrical Technicians, 1968

10.1 Coulombs and amperes

As previously stated, an electron flow in a conductor constitutes an electric current. A unit of quantity of electricity is the charge of one coulomb, being 6·242 × 10¹⁸ times as great as the charge of a single electron.

If a charge of one coulomb of electricity passes a point in an electrical circuit in one second, the current is said to be one ampere. Thus

$\frac{\text{coulombs}}{\text{second}}=\text{amperes}.$

There are standard symbols for all electrical quantities. For example, if a quantity or charge (Q) of twelve coulombs (12 C) passes a point in a circuit in 4 seconds (4 sec), the current (I) flowing in amperes (A) is

$I=\frac{12}{4}\text{C/sec}=3\text{A}\text{.}$

Smaller units of current occur in various branches of electrical engineering. Examples of these are:

$\begin{array}{l}\text{1 milliampere (1 mA)}=\frac{1}{1000}\text{ampere}({10}^{-3}\text{A}),\\ \text{1 milliampere (1}\mu \text{A)}=\frac{1}{1,000,000}\text{ampere}({10}^{-6}\text{A}).\end{array}$

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Electronic Concepts: More Interesting Than You Think

Louis E. FrenzelJr., in Electronics Explained (Second Edition), 2018

Electromagnetism

Magnetism is also produced by electricity. Whenever electrons flow in a conductor, they produce a magnetic field. This effect is called electromagnetism. Fig. 2.4A shows how the magnetic lines of force encircle a wire through which current is flowing. Note that the direction of the lines of force depends on the direction of the current flow.

Figure 2.4. (A) Current in a wire produces a magnetic field around it. (B) Coiling the wire increases the strength of the magnetic field.

The strength of the magnetic field around a wire depends on the magnitude of the current flowing. High current (composed of many electrons) produces a strong magnetic field. Despite the current amplitude, however, the magnetic field weakens because it spreads out quickly, even at a short distance from the wire.

If you make a coil out of the wire, as shown in Fig. 2.4B, the lines of force around each turn are added together. The result is that a more powerful, highly concentrated magnetic field is produced. In fact, the coil simulates a bar magnet with north and south poles.

As indicated earlier, the strength of the magnetic field depends on the current amplitude in the wire. High current produces a strong field. The magnetic field is also increased by coiling the wire, which helps concentrate the lines of force. The closer the turns of the wire, the stronger the field. You can also increase field strength for a given current level by simply adding more turns to the coil. Strong electromagnets are made by tightly coiling together many turns of fine wire in several layers.

Another way to increase the strength of the magnetic field is to insert a bar or core of magnetic material inside the coil. Because flux lines flow easier in iron or steel than in air, an iron or steel bar inside the coil helps concentrate the lines of force. Most of the flux lines flow through the core because it represents a lower path of resistance than the surrounding air for the lines of force. The result is a very strong field.

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