ELECTRICITY FUNDAMENTALS

Electricity is an apparent force in nature that exists whenever there is a net electrical charge between any two objects.

Basics of Electrostatics: (Charged Particle that are fixed in space: i.e. not moving.)

• Electrical charges are either negative (electrons) or positive (protons)
• Charges of opposite sign have an attracting force and like sign charges have a repelling force.
• The unit of charge, q , is called the coulomb.
• When there are equal numbers of positive and negative charges there is no electrical force as there is no net charge. This is the case for a neutral atom.
• Charged objects are created when electrons are transferred from one material to another (e.g. rubbing a wool cloth with a plastic comb, a chemical reaction...). Where the electrons are transferred becomes negatively charged. Where the electrons came from becomes positively charged.
• Electrical charge is conserved; charge is neither created nor destroyed

Properties of Electricity and Electrical Circuits:

CURRENT:

• Denoted by the letter I and measured in amperes. Current flow is consisted of negative electrons which flow from negative material to positive material and is essentially the number of electrons per second that pass a point usually through a conductive material.
• 1 Ampere = 1 Coulomb of charge flowing past a point in one second. (1A= 1Coul/sec)
• An electron has a charge of -1.6 * 10^-19 Coul. Therefore approximately 1 * 10¹⁹ charges must flow past a point for 1 ampere of current.

VOLTAGE:

• A Potential difference exists between a negatively charged object and a positively charged one (like two terminals on a battery).
• Potential difference is measured in units of Volts (V)which represents the work done per unit charge to move electrons between the positive and negative terminals.
• If a potential difference exists, then energy can be extracted.
• 1 Volt = 1Joule/Coulomb
• Voltage is analogous to a source of pressure which causes electrons to move.

RESISTANCE:

• Property of material that restricts the flow of electrons in it.
• The lower the conductivity, the higher the resistance to current flow. Hence the name resistance.
• Metals are good conductors due to low resistance.
• Wood is a poor conductor due to high resistance.
• Resistance, dented by the letter R , is measured in ohms and depends upon both the type of material and its size.
• Long wires have more resistance than short wires; thin wires have more resistance than thick wires.
• Resistance is also temperature dependent.

OHM's LAW

The amount of current that will flow through a circuit depends directly upon the voltage difference ( The amount of pressure on the charges to move) and inversely to the resistance (restriction to the charges to move). I=V/R or V=I*R

• The greater the voltage the higher the current
• The greater the resistance the lower the current

Example: Ohm's Law and an Electrical Circuit

Standard US household voltages in an outlet is 120 Volts.

Toaster element is 15 ohms

I = V/R = 120V/15(Ohms) = 8 amps of electrical current flows through your toaster.

The current is moving charges. The resistance of the heating element inside your toaster is greater than the wires that brought the current to the toaster. The moving charges are forced through the resistive heating element which causes the element to get hot, and ...well, you get toast!

The amount of current used is directly related to the amount of power delivered to an appliance such as the toaster. Look at the units for Voltage and Current:

• Voltage (V) = Joules /Coulomb and Current (I) = Coul /sec
• Multiply V*I and you get (Joules/Coul)(Coul/sec) = Joules/sec
• Recall that 1 Joule/sec = 1 Watt, A unit of Power

Power = V*I ( Watt's Law)

The toaster from the example above has a power of 120V x 8A = 960 Watts.

Energy = Power*time = P*t

In your electrical bill you are charged for the energy consumed. The energy unit used by the power company is the kW-hour. In the case of the toaster example, which uses 0.96kW, you would have to make toast for a little more than an hour to use 1kW-hour of energy.

Since V = R*I and Power = I*V. Substituting I*R for V in the power equation we get, I(I*R) = I*I*R= I²R. Notice that power delivered is related to the current squared. Although the resistance in wires is relatively small, there is still a power transmission loss from current flowing in the transmission wires. The lower the current the lower the power loss by a squared factor. In other words, lower the current by a factor of 2 and your power loss is decreased by a factor of 4. For this reason power transmission lines operate at a high voltage (high tension lines) and the voltages is "stepped down" to a safer level at the transformer located on the pole close to your house.