Science in Action Notes: Electrical Principles & Tech. (3.0) | Print |


'The Atoms Family' (A Great Electrical Resource Site)
'Just for Kids' (Fun and Games - About Electricity)
'Theatre of Electricity'

3.1 .

Energy Forms and Transformations




The scientific definition of energy is the ability to do work. The four most common forms of energy are:

  • chemical - this is energy stored in chemicals. It is potential or stored energy. It is released when the chemicals react.
  • electrical - this is energy of charged particles. Electrons are negatively charged and emectrical energy is transferred when these charged particles travel from place to place.
  • mechanical - this is the energy possessed by an object because of its motion or its potential to move.
  • thermal - this is the total kinetic energy of a substance. The faster the particles move the higher the kinetic energy.

Chemical Energy
Chemical energy is the energy found in chemicals, including food. Glucose molecules are used in your body cells to produce thermal energy and mechanical energy. Chemical energy can also be converted into mechanical and sound energy (a CD player). Chemical energy can also be transformed into mechanical energy, with heat and light (dynamite).

Transformations Involving Chemical and Electrical Energy
Examples of Devices that convert Energy from one form to another include:

Input Energy


Output Energy






electrical, then light and thermal





battery-operated clock

electrical, mechanical and sound

Transformations Between Thermal and Electrical Energy
A thermocouple is a device that can convert thermal energy into electerical energy. It consists of two different metals (bimetal) joined together that conduct heat at slightly different rates. When heated, the difference in conduction results in electricity flowing from one metal to the other. Thermocouples are useful for measuring temperatures in areas tyhat are difficult to access or too hot for a regular liquid-filled thermometer. Ovens and heaters do the opposite. They convert electrical energy into thermal energy.

3.2 .

Energy Transformations Involving Electrical and Mechanical Energy


St. Louis

Deflection of a compass needle using electrical current showed that there is a relationship between electricity and magnetism. Oersted found that the current created a magnetic field around the wire.
constructed the first motor.

Electric Motors
A strong
electromagnet can be made by coiling (copper) wire around a (iron) metal core. When attached to an electrical source it will produce a strong magnetic field. To keep this electromagnet spinning in a magnetic field, the direction that the current is travelling through the coil must be switched. This is accomplished by with a gap, which allows the polarity of the electromagnet change just before it alligns with the permanent magnet. Many electric motors use a commutator (a split ring that breaks the flow of electricity for a moment and then reverses the flow in the coil, when the contact is broken, so is the magnetic field) and brushes (contact points with the commutator) to reverse the flow of electricity through the magnetic field. The armature (the rotating shaft with the coil wrapped around it) continues to spin because of momentum, allowing the brushes to come into contact once again with the commutator.

The Steering Analogy
Turning a steering wheel is similar to turning the armature in a motor. At some point you have to release the wheel and start again. This is what the commutator allows the armature to do.

Direct and Alternating Current
Some motors run on direct current (DC). It is 'direct', because the electricity flows in only one direction. Alternating current (AC) flows back and forth 60 times per second.
are used to change the amount of voltage with hardly any energy loss. Voltage change is necessary because the most efficient way to transmitt current over long distances is at high voltage and then reduced when it reaches its destination, where it will be used.

A step-up transformer increases voltage, while a step-down transformer reduces voltage.

Generating Electricity
Electromagnetic induction was discovered in 1831 by Michael Faraday. He demonstrated that electrical current could be generated by moving a conducting wire through a magnetic field. By moving it back and forth through the field, Faraday created the first electricity producing generator.

Massive coils of wire rotating in huge generators can produce enough electricity to power an entire city.

Generating DC and AC
DC generator is much the same as a DC motor. The spinning armature procuces the electricity (if electricity is passed through a DC generator, it will spin like a motor). The central axle of an AC generator has a loop of wire attached to two slip rings. The current is switched as the loops move up and down alternatively thriough the mgnetic field. The slip rings conduct the alternating current to the circuit through the brushes (the brush and ring assembly allows the whole loop to spin freely). In large AC generators many loops of wire are wrapped around an iron core.

3.3 .

Measuring Energy Input and Output

Arc Disch. Light

is the rate at which a device converts energy. The unit of power is the watt (W), which is equal to 1 joule per second. For an electrical device the power is the current multiplied by the voltage.

(P) Power in watts
I) current in amperes
V) voltage in volts

P = I x V
I = P / V
V = P / I


The power rating of a device can be used to determine the amount of energy the device uses. Multiply the power rating by the time the device is operating.

(E) Energy in joules
P) Power in watts (J/s)
t) time in seconds

E = P x t
P = E / t
t = E / P

P t

Kilowatt Hours is used as a unit for energy. The energy calculation is the same, except that hours are substituted for seconds and kilowatts (kW) are substituted for watts.
Electricity meters measure the energy used in kilowatt hours and then bills you for every kilowatt hour used.

Energy Dissipation
Energy is neither created nor destroyed. It doesn't appear and then disappear, but transformed from one form to another. This is known as the
Law of Conservation of Energy. No device is able to be 100% efficient in transforming energy. Most often, the energy is lost, or dissipated as heat. Mechanical systems also dissipate energy to their surroundings, but not as obvious as the heat loss. Much of the dissipated energy is sound.

Understanding Efficiency
efficiency of a device is the ratio of the useful energy that comes out of a device to the total energy that went in. The more input energy converted to output energy, the more efficient the device is.

% Efficiency = Joules of useful output x 100%
Joules of input energy

Most of the energy transformed in a lightbulb is wasted as heat. (5% is light energy, while 95% is heat)

Comparing Efficiencies
Comparing efficiencies of devices the energy cost and their environmental impact can be determined.
  • Florescent lights are about 4x more efficient than incandescent lights.
  • Arc-discharge lights (streetlights) are even more efficient.
  • Hybrid gasoline-electric vehicles are more efficient than gas-powered vehicles.

3.4 .

Reducing the Energy Wasted by Devices


Devices, which have an energy-efficient design, are an important consideration for the consumer, because these devices use less electricity. Energy costs money and it also affects the environment, so reducing energy consumption is a good practice.

Limits to
Electric heater come very close to being 100% efficient, but devices which convert electricity to other forms can never be 100% efficient. Some energy is lost, or dissipated in a form that is not useful output. Friction causes thermal energy to be lost, or dissipated in many devices.

Increasing Efficiency
Increasing the efficiency of a device depends on its purpose. The easiest way to increase efficiency in many devices is to reduce friction, as much as possible. Insulating a device from heat loss is also another practical way to increase efficiency. Using capacitors in electrical circuits is also another way to increase efficiency...