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:

• Electrical charges are either negative (electrons) or positive (protons)

• 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.

• Electrical force is created when electrons are transferred from one material to another (e.g. rubbing a wool cloth with a plastic comb).

• Electrical charge is conserved; charge is neither created nor destroyed

Properties of Electricity:

CURRENT: denoted by I and measured in amperes. Current flows from negatively charged material to positively charged material and is essentially the number of electrons per second that are carried through a conductor. Current is measured in units of amps. 1 amp = 6.2 x 10¹⁸ electrons per second!

VOLTAGE: Potential difference 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 from that difference.

Imagine that you have two opposite charges that you want to separate. It takes work to seperate the charge and thus the separated charges store energy.

If the seperated charges get back together, work/energy can be extracted from the system. If there is some pathway for the charges to flow then we get a current.

High voltage is therefore only dangerous when you complete the circuit!

RESISTANCE: Property of material that helps prevent the flow of electrons in it. Metals are good conductors due to low resistance. Wood is a poor conductor due to high resistance. Resistance, 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. R is also temperature dependent.

OHM's LAW

Is there a relation between I, V, and R ? Let's do an experiment:

In the above circuit there is a battery (V), some resistors (R), and a light bulb that can only be activated if the right number of amps reach it. We can control this buy putting the right resistance in the circuit for a given battery Voltage.

Experimental results then lead to Ohms law:

V = R * I

This is a linear relation. If you double the voltage (V) then for the same value of R you get twice the current. If you want to keep the current the same value after doubling V, you would have to double the resistance (R).

Example:

• Standard US household voltage is 120 Volts.
• The heating element in your toaster has R = 15 ohms.
• What is the current flowing through your toaster?
  I = V/R = 120/15 = 8 amps

Your electricity bill essentially measures the amount of current that you use but you use this current to Power the toaster.

Power = V * I

So the toaster has a power of 120x8 = 960 Watts.

If you leave your toaster on for one hour, than that would also be approximately 1 KWH (960 watt-hours if you want to be exact).