Electric Current Unlike static electricity, electric current is a continuous flow of charged particles (electricity). For current to flow, there must

Transcription

1 CURRENT ELECTRICITY

2 Electric Current Unlike static electricity, electric current is a continuous flow of charged particles (electricity). For current to flow, there must be a power source and there must be some type of closed loop for electricity to flow through. So the question is why does electricity flow? When objects have charge, they have electrical force. The amount of electrical force an object has is referred to as electric potential or more commonly as voltage.

3 Voltage Imagine two water towers that are connected together by a valve. Each water tower has a different level of water. If the valve is opened between the two towers, what will happen? As you know, gravity will cause the water at a higher level to flow into the other tank until the water volumes equal out. The same happens with voltage.

4 Voltage (cont.) When something has a high voltage (let s say a wall outlet at 120V) the electric pressure will cause the electricity to flow into any object that is plugged into that outlet (usually a very small voltage or 0V). So, this force (often called the electromotive force or emf) is what provides for the movement of charged particles.

5 Current In electrical terms, current is the amount of energy that can flow through an area in a given time. Current is measured in a unit called the Ampere (usually referred to as Amps, A) that is equal to Coulombs per second. A=C/s Contrary to popular belief, high voltage is not what kills someone in an electrical accident, Amperage is the culprit. Did you ever wonder how Tesla could routinely dose himself with millions of Volts of electricity and live to tell about it? As little as 15 ma is fatal to humans! (Yes, I am aware that the book says 100 ma but it s a range that depends on a lot of factors)

6 Watts Now I m sure that you have seen a label on a hairdryer that tell how many Watts the device uses. Watts (W) are a measure of electrical power. That is, it tells how much voltage and current a device will use per seconds time. Power (Watts) = Current (Amps) x Voltage difference (Volts) OR P = I x V So, if your outlet is 120V and your hairdryer is rated at 1200W, this means that your hairdryer is pulling 10 Amps of electrical current. That might not mean too much to you until this plus other things you have running at the time cause your home s circuit to overload (more about that later).

7 Resistance In a perfect world, electricity would flow unimpeded. Unfortunately, we don t live in a perfect world. The opposition a material has to the flow of electricity is called Resistance. Resistance is measured in a unit called the Ohm which is symbolized by the Greek letter omega ( ). Even good conducting materials like copper have some electrical resistance.

8 Resistance (cont.) So why is resistance so important? When something is resistant to the flow of electricity, some of the electrical energy is converted to thermal energy and even possibly light. A high resistance can prevent a major problem in wiring as this can lead to fires. However some resistance is necessary to control the current that is used in a device. Often times, devices called resistors are used in wiring to cause the draw of a certain amount of current.

9 Ohm s Law OK, now the major formula called Ohm s Law I = V/R OR Current (A) = Voltage diff (V) / Resistance ( ) This is a widely used formula that is almost always true in metals at certain voltages. Not everything obeys Ohm s law (transistors, diodes, etc.) but we are going to mainly focus on those that do.

10 Electrical Schematics Schematics are ways to show how things are wired. We will look at the basic of schematics to understand how to recognize some basic features like power source, lamps, switches, and resistors. In Chapter 23 this becomes even more important as we discuss series and parallel circuits.