Electricity - Voltage and Current - Teacher's Notes  

 

Alessandro Volta An Italian professor of physics. In 1791, Volta was experimenting with metals and acids. He touched a silver spoon and a piece of tin to his tongue (saliva is slightly acidic) and connected them with a piece of copper wire. He experienced a "sour" taste, and realized he was experiencing an electrical phenomenon. (Touch the two terminals of a 9 volt transistor radio battery to your tongue to experience this for yourself.)  After more experiments, Volta was able to assemble a pile of cells to form a battery. Each cell was a disk of zinc and a disk of silver, separated by a layer of brine-soaked pasteboard. We now refer to that original battery as a Voltaic Pile. In Volta's honor, we refer to electrical pressure as "Voltage" and we measure the amount of electrical pressure in units of "Volts".  

 

 

Andre Marie Ampere A French mathematician, often referred to as "The Newton of Electricity". Ampere experimented with electricity and magnetism, and the relationship between the two. In the early 1800's he realised that electrons were moving through conductors in a flow. In his honor, we measure the amount of flow in units of "Amperes", often shortened to "Amps".  

 

 

What Causes Voltage? Electrons and Protons are "charged" particles. Electrons have a negative charge, and Protons have a positive charge. Like-charged particles repel. Opposite-charged particles attract. A piece of material with more Protons than Electrons will have a positive charge. Likewise, a piece of material with more Electrons than Protons will have a negative charge.  Each charged particle that is out of balance exerts some electrical pressure as it tries to get back into balance. Electrons push, as they try to get away from each other. Protons pull, as they try to attract electrons towards them. The total amount of pressure between two points is measured as Voltage.  

 

 

How Does A Battery Work? In a battery, a chemical reaction causes electrons to move away from one terminal and towards the other. The terminal that they move away from is becomes positively charged, and the terminal that they move towards becomes negatively charged.  When the battery is on the shelf, or in a circuit that is turned off, the electrical pressure builds up to the point where the chemical reaction essentially stops. When the battery is in a circuit and current is flowing, the chemical reaction is going on, moving new electrons into position as the previous electrons flow out through the circuit. As the chemicals that support the battery action become depleted, the battery gradually looses its ability to generate a charge at its terminals.  

 

 

What Causes Current? Given a path, electrons will flow away from a negative charge and towards a positive charge. In our simple example (the single cell flashlight), current flows through the light bulb on its way from the negative battery terminal towards the positive battery terminal.  

 

  Questions for Students: Q: Consider a short piece of insulated wire. Is there current flowing in this wire?  

 

A: Yes. Not really correct. There might be a few electrons moving about, but no organized flow, thus no current.  A: No. Correct. There is no voltage acting on the electrons, so there is no flow. And there is no complete circuit to provide a return path for the electrons.  Q: If the two ends of the wire are connected together to form a complete circuit, is there now current flowing?  A: Yes. Not really correct. Again, there might be a few electrons moving about, but no organized flow and no current.  A: No. Correct. There is no voltage acting on the electrons, so there is no flow, even though there is a complete circuit. A wire is only a conductor; it is not a voltiac cell.  Additional Note: Actually, current can be caused to move in this loop of wire using a magnetic field (see AC sources). But in the absence of a magnetic field, there is no force acting on the electrons, so there is no current.

 

Electricity - Schematic Diagrams I Teacher's Notes  

 

Schematic Symbols Engineers need to record their circuit ideas so that they can be shared with others. To record and share their circuits, they use schematic diagrams. Schematic diagrams are a form of short-hand. Rather than drawing each item in a pictoral way, each item is represented by a symbol. Once you learn the symbols, you can read any circuit diagram.  

 

 

Connections The first concept of schematic diagrams is "connection". Items in the diagram at the right are connected via wires (just like in the real world). In this picture, item A is connected to item B using a wire.  

 

 

To show multiple items connected together, we use a dot at the point where the wires cross, to show that they are connected. In the diagram at the left items A, B, C, and D, are all connected together.  When wires cross without the connection dot, they are not connected. In the diagram at the right, item A is only connected to item B, and item C is only connected to item D.

 

 

  Components As mentioned above, in a schematic diagram each type of component is represented by a symbol.

 

Here are the symbols for several types of switches, a lamp, a single cell, and a battery. We'll be using these symbols to create schematic diagrams on the next page.  

   

Electrical CircuitsAn electrical circuit is any combination of wires and electrical devices (also called circuit elements) through which current can flow. We will begin with a typical flashlight powered by two 1.5 V AA batteries.

Below is diagram of the actual flashlight next to a schematic diagram. Note the symbols used to denote the batteries and the switch. The general symbol for any resistor is just

, the oval shown below indicates that it is also a light bulb. (Check out the standard electrical symbols listing.) The switch is shown in the "open" position. That is, the flashlight is off and the circuit is broken or not complete. No current can flow until there is a path. Charge will not flow from one side of the battery unless and equal amount can flow into the other side! When the switch is in the "closed" position, the flashlight is on and there is a path for the current to flow. (Although there is no set rule, switches are usually shown in the open position in schematics.)

The batteries are called DC sources. DC or "Direct Current" means that current flows just one way. Which way? Positive current, also called conventional current, flows out of the positive side of the battery (indicated by the longer line) and into the negative

(indicated by the shorter line). And what do we mean by positive current? It is positive charge flowing as the arrows show.

Foul you cry! Yes, we now know that what actually happens is that electrons (negative charge) flow out of the negative side and into the positive. But in the late 1700's Ben Franklin, a scientist as well as a statesman, had no way of knowing that. He made a guess at what was happening and we're stuck with it. But it doesn't change any of the physics. So, from hence forth, we pretend positive charge flows out of the positive side of the battery.

Series Circuit

The two batteries are connected in series, that is, one after another. The current that flows through one must also flow through the other. What does that mean for the voltage? Think of climbing a 1.5 ft step and then following it with another 1.5 ft step. Your are now 1.5 + 1.5 = 3.0 ft higher. Similarly, the potential from the negative side of the first battery to the positive side of the second is 1.5 V + 1.5 V = 3.0 V. The potential across any two or more elements connected in series is always the sum of the individual voltages.

What potential is across the lightbulb of the flashlight shown above? The resistance in the "wires" that provide the connections between the batteries and bulb should be much lower than that of the bulb itself. Therefore, there is negligible "potential drop" across the wires. Essentially all 3.0 V provided by the batteries will be across the bulb. Later, we will address the role that resistance in the wires play. But for now, let's assume all potential drops are across the circuit elements. Note that the bulb is in series with the batteries. The current that flows throught the batteries is the same that flows through the bulb. The current has no other path.

There is a method for understanding potential drops. The total potential drop around a complete circuit must be zero. (This similar to saying that the sum of all height changes on a round-trip hike must sum to zero.) As we move across the batteries, we gain potential of 1.5 V each (for a total of 3.0 V), followed by the potential across the bulb ... then we are back to where we started. (Don't forget that potential change along the wires is essentially zero.) Therefore, the potential across the bulb must be a 3.0 V drop.

Let's apply the formulas from the previous section.

Problem: If the bulb in the flashlight above has resistance 2 , then what current flows through the bulb and what power does it consume?

Solution: The current flowing through the bulb is I = 3.0 V / 2 = 1.5 A. The power being consumed by the bulb is P = I2 R = (1.5 A)2 x 2 = 4.5 W. The power being supplied by the two batteries is P = I V = (1.5 A) x (3.0 V) = 4.5 W. It should be no surprise that the power consumed by the bulb is equal to that supplied by the batteries.

Parallel Circuit

Elements in a circuit are in parallel when they are connected in such a way that the potential across them is always the same. Essentially, the ends of each element are connected together. Let's consider two important cases.

The Car - Consider the simplified diagram for the electrical system of a car. Notice that all the elements receive a full 12 V from the car battery.

The current that flows out of the battery can split up and flow to each element individually. Each element will receive a different portion of the total current, according to its resistance, but each element receives the same 12 V. The elements are in parallel with one another when the switches are closed. Note that each switch is in series with the element it controls. Do you understand why a switch must be in series with the device it controls? (Actually, this is not absolutely true. In lab, you will have the chance to observe such an exception.)

Look at the diagram and see if you can understand the effect of each switch. Can you identify which one represents the key ignition switch? Is this consistent with your experience of what can be turned on with your car key? (A more proper diagram would show the ignition and starter switches as part of one compound switch.)

Summarizing Series and parallel circuits ...

Elements in series have the same current, but the voltage drops may be different.

Elements in parallel have the same voltage, but the currents may be different.

Here's an important difference to remember.

Elements in parallel can be operated independently, while elements in series cannot. The Home - Are the electrical elements in your house (the fridge, the toaster, the lights, etc) in series or parallel? They better be in parallel, right? The electrical diagram below shows the wiring for a typical room in a home. In this example, a single power line, consisting of a red wire and a green wire, services the six outlets and two overhead light fixtures. You may think of the red wire as if it were connected to the positive terminal of 120 V battery while the green wire is connected to the negative side . This is not correct, but we will deal with the details of the AC power in your home in the next section.

All the outlets and the light fixtures (with their switches) are connected in parallel. However, the switches are in series with the lights they control. If the switch is closed, the red wire coming into the switch from the power line is connected to the red wire going out to the light, and the light will be on. If the switch is opened, there is no

connection and the light is off. The green wire from the other side of the light fixture is connected directly to the green wire of the power line.

In the simplified diagram above, it may not appear that the outlets are in parallel, but they are. Most outlets provide two screwposts on each side of the casing, like that shown in the picture to the right. Both screwposts on a side are connect to the "slot" on that side. That makes it convenient to run a line into the outlet, say at the bottom posts, and then run a line out to the next outlet from the top posts. When an appliance is plugged into the outlet, it will be in parallel with all the other outlets, lights, etc. (Of course, the appliance may contain its own switch.)

The colors for the wires in the diagram above were chosen for clarity. The colors you are most likely to find in household wiring is discussed in AWG and household wiring.

A few notes:

The longer slot is connected to the O V wire. This is the convention for the somewhat new "polarized" plug. The extra copper ground wire shown the the right is discussed in the "AWG and household wiring" link above.

The outlet in the upper left corner of the room electrical diagram is at the "end of the line". One of the screwposts on either side will not be used.

Two or more lines may service a room if the electrical needs are particularly high. The kitchen usually has separate lines to the refrigerator, the stove, and the outlets.

Problem: Let's put a few numbers to the car circuit example. Assume we have a 10 W dome light, a 50 W radio, and two headlights at 300 W each. What current do they draw?

Solution: For the dome light, P = I V, so I = P/V = 10 W / 12V = .83 A. The radio consumes 50 W, so I = 50 W / 12 V = 4.2 A. The headlights are rated at 300 W each. So they draw 300 W / 12V = 25 A each! If all these devices are on at the same time, the total current will be about 55 A. And what power is the battery supplying? P = IV = (55A) x (12V) = 660 W, exactly equal to the total (10 + 50 + 300 + 300 = 660 W) of what is being consumed by the devices.

Problem: Let's put a few numbers to the household circuit example. Assume we have a 100 W light fixture, and a 300 W television plugged into one of the outlets. What current do they draw?

Solution: For the light fixture, I = P/V = 100 W / 120 V = .83 A. Note that the 100 W light has the same current as the 10 W dome light in the previous example. Although the current is the same, the light fixture consumes 10 times the power of the domelight since it operates at 10 times the voltage.

The television consumes 300 W, so I = 300 W / 120 V = 2.5 A. Again, notice how the 300 W car headlight draws 10 times the current of the 300 W television. It must, since it operates at 1/10 the voltage.

Why do the Lights Dim?

If the dome light is on when you start the car, you'll probably notice it dims just a bit. The main reason is that the battery has internal resistance. The voltage may be 12 V when no current is being drawn. But when current flows through the battery, there is a potential drop across the internal resistance and the voltage at the battery terminals will be less than 12V. But you must be draw quite a bit of extra current to notice the change. A typical internal resistance might be only .01. Let's say you turn on the dome light, drawing .83 A. The internal potential drop in the battery would be V = I R = .83A x .01 = .0083V. You turn on the radio, creating an additional drop of 4.2A x .01 = .042 V. You won't notice it. But when you start your car, the starter motor pulls 50 A. Now the potential drop is 50 A x .01 = .5 V. So the voltage across the battery and all the devices has dropped to about 11.5 V and you will probably notice the change.

Here's another example with a slightly different cause. The fridge or the air conditioner kicks on and you notice the house lights dim. It is not the internal resistance of WAPA! You may have inadequate wiring. Both these devices draw quite a bit of current in the first second or so when they first start up. If the wiring in your house is not "large" enough (i.e. the cross sectional area is too small), it will have too much resistance. When a large current is present in the wire, there will be a significant voltage drop, leaving less voltage available to your lights. You will notice that they dim.

Before moving to the next section, try a few electricity PROBLEMS to reinforce these new concepts.

If you are comfortable with what you have learned so far, then you are ready to move on to AC Power Systems.

The Service Panel

The drawing above shows the service wires for 240 VAC service coming from the transformer and entering the meter box. The meter box contains the meter which "records" the total kWhr of electrical energy used. In addition, the box contains a cutoff switch and two main fuses, one for each "phase". Coming into and out of the meter box are the white neutral (center tap of the transformer) and two 120 VAC phases represented by the red and black wires. Each phase is 120 VAC relative to the neutral and 240 VAC relative to each other.

Each phase passes through a main breaker (typically 100 A to 150 A). Household wiring for 120 VAC branch circuits enter the service panel from the sides and are connected to one of the two phases. Each of these contain a copper grounding wire, a white "neutral" wire and either a black or red "hot" wire. (Most commercial "three-wire" household wiring has black for the hot wire, but red is available and is shown for clarity.) Note that each branch circuit is connected through a breaker that will trip if the current limit for that branch is exceeded.

Two branch circuits for 240 VAC service are shown entering through the right-bottom opening in the service panel. For this service, standard "four-wire" household wiring is used and contains a copper grounding wire, a white wire, and both a black and red wire.

Each ciruit is connected to both phases. The breakers are typically double width so they can spanned across the connections to both phases of the backplane.

The copper grounding wire for each branch is usually attached to a long metal bar with screwposts. The bar is electrically connected to the ground nearby.

Circuit Breakers

A circuit breaker is a clever device that "breaks" or opens a circuit once the current flowing through it exceeds the limit for which it is designed. Current flows into and out of the breaker through the hot wire (the black wire at the bottom) and the snap connection into the backplane (shown at the top). The current flows through a bi-metallic strip (hidden within the long silver rectangle just above where the wire connects) that is heated by the current. The strip bends as it heats up. If the current is too high, the strip heats up to a point where it pushes on the spring-loaded trip switch. This allows the spring to quickly pull the contact arm (in the upper left) down. To re-set the breaker, you push down on the switch to "cock" the mechanism and then push up. Clever, eh?