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)
• The unit of charge, q , is called the coloumb.
• 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 negative material to positive material and is essentially the number of electrons per second that are carried through a conductor.

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.

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 voltages in an outlet is 120 Volts.
• Toaster element is 15 ohms
• I = V/R = 120/15 = 8 amps flow through your toaster

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

  Power = V * I

  (but for most appliances/consumer electronics V is fixed at 120 Volts). So the toaster has a power of 120x8 = 960 Watts.

  A better way to understand your electricity bill:

  Since V = R*I then Power = I*I*R = I²R

  Energy = Power * Time (and its energy --> kilowatt hours that you pay for - a 100 watt light bulb left on 10 hours = 1 kilowatt hour. )

  Your stove "burners" are made of some material which has some resistance R. You can't change R. To cook your food you need to have so much power radiated by the stove. Since R in the burner material is quite high, the material resists the flow of electrons and the material heats up. The more electrons per second (e.g. I) that you can put through the burner the more power which is radiated away as heat.

  You pay for I, but its I² that determines the power usage that you pay for. The longer you leave the stove burner on, the more energy you have used. For instance, if you leave the toaster on for one hour, you have used 1 KWH of energy.

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