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Sources of Electromotive Force (EMF)

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Current Electricity

Current electricity is defined as an electric charge in motion. Current flow consists of a flow of negative electron charges from atom to atom, as illustrated in Figure 1.

Current electricity.

Figure 1 Current electricity.

The external force that causes the electron flow is called the electromotive force (emf) or voltage which is supplied by the battery. The negative terminal of the battery has an excess of electrons, while the positive terminal has a deficiency of electrons. Since the positive terminal of the battery has a shortage of electrons, it attracts electrons from the conductor. Similarly, the negative terminal, with an excess of electrons, repels electrons into the conductor.

Electric Current Classification

Current electricity is classified as being direct current (DC) or alternating current (AC) according to its voltage source. 

Direct current voltage produces a flow of electrons in one direction only. Alternating current voltage produces a flow of electrons that changes both in direction and in magnitude.

Typical symbols and waveforms for DC and AC voltage sources are shown in Figure 2. A battery is a common DC voltage source, while an electrical receptacle is the most common AC voltage source.

All voltage sources share the characteristic of an excess of electrons at one terminal and a shortage at the other terminal. This results in a difference of electric potential between the two terminals.

DC and AC current electricity.

Figure 2 DC and AC current electricity.

Polarity identification (+ or −) is one way to distinguish a voltage source. Polarity can be identified on direct current circuits, but on alternating current circuits the current continuously reverses direction; therefore, the polarity cannot be identified.

Sources of Electromotive Force

For electrons to flow there must be a source of electromotive force (emf) or voltage. This voltage source can be produced from a variety of different primary energy sources. These primary sources supply energy in one form, which is then converted to electric energy.

Primary sources of electromotive force include friction, light, chemical reaction, heat, pressure, and mechanical-magnetic action.

Light

A solar photovoltaic power system converts sunlight directly into electric energy using solar or photovoltaic (PV) cells. These are made from a semiconducting, light-sensitive material that makes electrons available when struck by the light energy (Figure 3).

Solar cells operate on the photovoltaic effect, which occurs when light falling on a two-layer semiconductor material produces a DC voltage, between the two layers. The output voltage is directly proportional to the amount of light energy striking the surface of the cell. One of the best solar cells is the silicon cell. 

A single cell can produce up to 400 mV (millivolts) with current in the milli-ampere range and can be used in constructing larger solar panels. Small current solar cells are often used as sensing devices in automatic control systems and to power electronic devices such as calculators.

Generating electricity from sunlight.

Figure 3 Generating electricity from sunlight.

A solar module or panel consists of solar cells electrically interconnected and encapsulated as shown in Figure 4. Solar panels typically have a sheet of glass, on the side facing the sun, and a translucent resin barrier, allowing light to pass through while protecting the semiconductor from the rain, snow, and hail. Solar panels can be grouped together to form an array capable of delivering large amounts of electric power.

Solar module or panel.

Figure 4 Solar module or panel.

grid-tie solar system connects your solar power system to the electric power grid. This enables you to send any excess power you produce back to the electric company through a plan known as net metering. 

At night, or on cloudy days, you simply revert to buying power from the utility company. With this type of solar system installed, the electricity that you generate either offsets your usage or, if you are producing more than you are using, is fed back to the electric grid, crediting your utility account.

One key point to remember is that grid-tie PV system must be shut down when the power from the utility company is down. This is primarily a safety requirement to ensure that power is not being fed back into the grid while maintenance workers are restoring power.

Figure 5 shows the parts of a typical grid-tie photovoltaic power system. These systems use solar modules along with a DC to AC power inverter. The inverter changes DC to AC and synchronizes the power produced by solar modules with the electricity that comes from the utility company.

The operation is straightforward. When the sun is shining, the solar array generates DC voltage. The inverter automatically connects to the electric utility grid and delivers AC power to the grid.

Grid-tie PV power system.

Figure 5 Grid-tie PV power system.

Off-grid PV solar systems are used for applications where utility lines are not available, not desired, or just too expensive to bring in. Off-grid solar systems use solar panels to produce DC electricity, which is then stored in a battery bank (Figure 6).

An inverter converts the DC power stored in the batteries to AC power of the kind used in the residence or commercial establishment. Typically, off-grid systems will include a backup power generator to charge the batteries if they get too low and a charge controller to regulate the power flowing from a photovoltaic panel into the rechargeable battery bank.

Off-grid PV power system.

Figure 6 Off-grid PV power system.

Chemical Reaction

The battery or voltaic cell converts chemical energy directly into electric energy (Figure 7). Basically, a battery is made up of two electrodes and an electrolyte solution. One electrode connects to the (+) or positive terminal, and the other to the (−) or negative terminal.

Battery converts chemical energy directly into electric energy.

Figure 7 Battery converts chemical energy directly into electric energy.

When a battery is connected to a closed electric circuit, chemical energy is transformed into electric energy. The chemical action within the cell causes the electrolyte solution to react with the two electrodes. As a result, electrons are transferred from one electrode to the other. This produces a positive charge at the electrode that loses electrons and a negative charge at the electrode that gains electrons. Although the battery is a popular low-voltage, portable DC source, its relatively high energy cost limits its applications.

Heat

Heat energy can be directly converted into electric energy by a device called a thermocouple. Thermocouples operate on the principle that when two dissimilar metals are joined, a predictable DC voltage will be generated that relates to the difference in temperature between the hot junction and the cold junction (Figure 8).

When heat is applied to the hot junction, electrons move from one metal to the other creating a negative charge on one and a positive charge on the other. A thermocouple is often used as a temperature probe for temperature-measuring devices. A voltmeter, calibrated in degrees, is connected across the external thermocouple leads to indicate the temperature.

Thermocouple converts heat energy into electric energy.

Figure 8 Thermocouple converts heat energy into electric energy.

Piezoelectric Effect

A piezoelectric substance is one that produces an electric charge when a mechanical pressure is applied. Certain crystals such as quartz are piezoelectric. That means when they are compressed or struck, they generate an electric charge.

One common application of piezoelectricity is the piezo gas igniter shown in Figure 9. When you push the button, it makes a small, spring-powered hammer rise off the surface of the piezo crystal. When the hammer reaches the top, it releases and strikes the crystal, creating a high voltage. This voltage is high enough to make a spark which ignites the gas. Piezoelectric igniters are used on most gas furnaces and stoves.

Piezo gas igniter.

Figure 9 Piezo gas igniter.

Mechanical-Magnetic

Most of the electricity we use is produced using an electric generator that converts mechanical-magnetic energy into electric energy. The basic components and operation of an AC generator are shown in Figure 10.

As the armature rotates through the magnetic field, a voltage is induced in the armature winding. Slip rings are attached to the armature and rotate with it. Carbon brushes ride against the slip rings to conduct current from the armature.

An armature is any number of conductive wires wound in loops which rotate through the magnetic field. For simplicity, one loop is shown. Although this generator produces AC electricity, it may be designed to produce AC or DC electricity.

AC generator.

Figure 10 AC generator.

Every generator must be driven by a turbine, a diesel engine, or some other machine that produces mechanical energy. Prime mover is a term used to identify the mechanical device that drives the generator.

To obtain more electric energy from a generator, the prime mover must supply more mechanical energy. For example, wind generators are installed in locations with strong sustained winds (Figure 11). The wind pushes against the fan blades of the wind turbine, rotating the fan and a shaft that drives a generator to produce electricity. The electricity is either used or stored in batteries.

Wind generator.

Figure 11 Wind generator.

Review Questions

  1. Define electric current.
  2. What is the external force that causes electron flow?
  3. In which direction do electrons flow relative to the polarity of the applied voltage?
  4. Compare electric current flow in a DC and an AC circuit.
  5. Why is polarity normally identified on DC but not AC voltage sources?
  6. How does a photovoltaic cell produce electricity?
  7. Compare the operation of grid-tie and off-grid PV solar systems.
  8. What is the function of an inverter as part of a solar energy system?
  9. Name the three basic components of a battery.
  10. How does a thermocouple produce electricity?
  11. How does a piezoelectric substance produce electricity?
  12. How does an electric generator produce electricity?
  13. What type of prime mover is used as part of wind generator?

Answers

  1. Electric current is the flow of electrons.
  2. The external force that causes electon flow is called the electromotive force or voltage.
  3. Electrons flow from negative polarity to positive polarity.
  4. With a DC circuit, current flows in only one direction. In an AC circuit, current changes direction.
  5. Polarity is normally identified on a DC circuit because it does not change. Polarity is not usually identified on an AC circuit because it is constantly changing.
  6. A photovoltaic cell produces electricity by converting light energy into a DC voltage.
  7. A grid-tie PV system is connected to the electric power grid and allows you to sell energy back to the electric utility. An off-grid PV system operates separately from the electric utility lines.
  8. The inverter converts the DC voltage of the system to AC power of the kind required to operate the system or to tie into the utility company lines.
  9. A battery is made up of two electrodes and an electrolyte solution.
  10. A thermocouple produces DC electricity from thermal energy when there is a difference in temperature between the two thermocouples (which are the junction of two dissimilar metals.)
  11. A piezoelectric substance produces DC electricity by converting mechanical force into electricity.
  12. An electric generator produces electricity from the mechanical energy that causes the rotation of coils of wires through a magnetic field which produces a flow of current in the coils.
  13. The prime mover for a wind generator is the wind.
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About Ahmed Faizan

Mr. Ahmed Faizan Sheikh, M.Sc. (USA), Research Fellow (USA), a member of IEEE & CIGRE, is a Fulbright Alumnus and earned his Master’s Degree in Electrical and Power Engineering from Kansas State University, USA.

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