wiki:ElectricalIntroduction

Electricity Overview

Young Frankenstein

Batteries

SLA Battery Robots need power and most robots, including our FRC robots draw their power from energy stored in a battery.

Batteries have two metal electrical terminals each connected to one of two plates called an anode and a cathode, and chemicals between the plates called electrolyte. The plates and electrolyte are chosen so that they can react with each other chemically when they are allowed to exchange electroncs. In order for the reaction to take place, one terminal must lose electrons and the other gain them. When electrons are able to move from one terminal to the other, the chemical reaction proceeds; when no electrons can flow, the chemical reaction stalls. Normally, the battery terminals are separated by air which resists the flow of electrons, but if the terminals are connected together with wire or other electrically conductive material, electrons flow and the chemical reaction proceeds until the chemicals are all fully reacted (consumed). When the chemicals are fully reacted, the battery is considered drained.

  • The battery terminal that supplies electrons is called the negative terminal and is usually marked with a black color or '-' sign.
  • The battery terminal that consumes electrons is called the positive terminal and is usually marked with a red color or '+' sign.

There are two main types of batteries:

  • Primary: disposable batteries that you throw away when their chemicals have fully reacted and can't make electrons flow anymore (battery is fully discharged)
  • Secondary: rechargeable batteries where if you force current to flow through the battery in the reverse direction after the battery is discharged, the chemical reactions reverse themselves (the battery is re-charged) so it can make electricity flow again.

Common commercial batteries include different chemical compositions: carbon-zinc, alkaline, lithium, lead-acid, etc. and different sizes including: AA/penlight, D/flashlight, 9V/radio, 6V/lantern, and a wide range of custom sizes. Batteries vary widely in size, capacity, and characteristics. In general, the larger the battery, the more chemicals it contains and the more electrical energy it can generate through the chemical reactions. The chemistry and construction determine whether the battery is disposable or rechargeable.

Batteries can be made using a wide variety of chemical reactions. You can make a battery at home using a potato or a lemon (see exercise under multimeter below).

FRC competition robots use large re-chargeable lead-acid batteries. Team 2537 also uses smaller lead-acid batteries to power lighter duty robots like peanut bots. All of these batteries are sealed so the acid/electrolyte doesn't get out and are called Sealed Lead Acid (or SLA) batteries; they also store a lot of energy and should be treated with respect.

For more information and details on batteries, see here and here.

Voltage

Dam block diagram

The difference in electrical potential between the two terminals of a battery is measured in units of Volts (named after Alessandro Volta). Voltage is *potential* energy, like the reservoir/lake behind a hydroelectric dam; no work is being done if the water isn't flowing, but you can measure the pressure the water is exerting on the dam. Voltage is a potential, it represents a difference in charge, the strength of attraction between two opposite charges. Different types of batteries use different types of chemical reactions to generate that charge potential difference. For more information about electricity and voltage, see here

Static shock Different battery chemistries generate different voltage potentials. For example, NiMH rechargeable batteries create about 1.2 volts, alkaline disposable batteries create about 1.5 volts, lead-acid rechargeable cells create about 2 volts, and lithium batteries create 3-3.7 volts. You can connect batteries in sequence (series) to increase the difference in charge (voltage) between the battery terminal at the start of the sequence and the battery terminal at the end; this is called placing the batteries in series. By placing 6 lead-acid cells in series you can make a 12V battery (which is what we use for robots). When you buy a 12v SLA battery, it really contains 6 individual 2V cells in one package. The lead-acid battery is the oldest type of battery, and is the one we still use on our robots, lead-acid batteries have lead and lead-oxide terminals and sulphuric acid electrolyte.

  • When you get a static shock, such as touching a doorknob in the winter, you are feeling thousands of volts!
  • Voltages below 48vdc are generally safer to work with because the human body does not conduct them well. Above 48V, the resistance of the human body breaks down and lots of current can flow through you (which can kill you). There's a saying: it's the volts that jolt but the mils (mil = 1/1000 of an ampere of current) that kill.
  • Note that although the 12V battery on your robot generally can't electrocute you, it can still release huge amounts of energy if short circuited which generates a huge amount of heat (enough to melt steel) so robot batteries still need to be treated with great care.
  • Exercise:
    • Materials needed: rectangular 9v battery, tongue, courage
    • Activity: Taste (with your tongue) the terminals of a 9v battery to see how 9v feels (this is not dangerous, it just tingles)
    • What happened: your wet tongue is a fairly good conductor of electricity and formed the conductive element of a circuit allowing electrons to flow from one terminal of the battery to the other.

Multimeter

You can use a meter called a multimeter to measure the amount of voltage "pressure" a battery exerts. Multimeters are inexpensive ($5 - 25) and incredibly useful; every house should have at least one and every robotics student and mentor should be comfortable using a multimeter. More expensive meters can usually measure more things and require fewer manual steps. For example, some multimeters require that you select a range of voltages to measure such as 2V, 20V, 200V, etc. whereas other multimeters will automatically select the proper measurement range (called auto-ranging meters). To learn how to use a multimeter, see this tutorial. We'll use multimeters in the next few exercises.

Multimeters include two wires called leads which are used to connect the meter to whatever it is measuring. The leads can usually be unplugged from the meter for replacement, easier storage, and sometimes for making special measurements. Leads come in different lengths and terminations to make it easy to take different kinds of measurements. Most leads end in a pencil-style probe with a sharp tip to allow you to probe small electrical contacts precisely. Other leads have spring-loaded toothy clips at the end called alligator clips that make it easy to clip them on to wires or terminals and leave your hands free to do other things.

UT136B Multimeter Cen-Tech Multimeter Measure AA battery voltage Measure SLA voltage

  • Exercise: Use a multimeter to measure the voltage of a battery:
    • Materials needed: multimeter with leads, battery (AA, 9v, any battery will do)
    • Activity:
      • Turn the multimeter on and move the range selector knob to DC Volts (often shown as DCV or a dashed line over a solid line). If you are not using an auto-ranging meter, choose the voltage range that includes the battery you will measure (e.g. the 20V range if you are going to measure a 12V battery)
      • Plug the black probe wire into the hole labeled COM
      • Plug the red probe wire into the hole labeled V/mA/...
      • Touch the other end of the black wire to the negative (-) terminal of the battery
      • Touch the other end of the red wire to the positive (+) terminal of the battery
      • The multimeter will display the battery voltage
    • What happened: the electrical pressure, (the difference in positive vs. negative charge) between the two terminals is measured by the multimeter and displayed. Virtually no energy is consumed from the battery in making this measurement.
    • Bonus Exercise: Measure the voltage of several different types and sizes of batteries: 9v, AA, SLA
  • Exercise: Make a lemon battery for details on how it works see here
    • Materials needed: multimeter, knife, lemon, copper penny (preferably minted before 1982) or copper wire, galvanized (zinc-coated) steel washer or nail
    • Activity:
      • The two metals (copper and zinc) will form the electrodes (terminals) of the battery. If you are using a penny and a washer, use your knife to cut a small slit at each end of the lemon, a little smaller than the round object's diameter to make it easier to insert the electrodes.
      • slip the copper into one end of the lemon, leaving some of it sticking out to serve as a battery terminal
      • slip the zinc into the other end of the lemon, leaving some of it sticking out to serve as the other battery teminal
      • use your multimeter to measure the voltage between the zinc and copper terminals
      • Bonus points for AP chemistry students: calculate the electron volts associated with each of the two reactions in the battery
    • What happened: the citric acid in the lemon juice reacts with the copper and the zinc: the Zinc reaction releases electrons (oxidation) and the Copper reaction consumes electrons (reduction). If you've taken chemistry, you've learned about redox reactions; to see this in more detail watch this 9 minute video. If a conductive path (circuit) exists from the Zinc anode to the Copper cathode, as electrons flow through the circuit, they allow the redox reaction to proceed, and you can use the flowing electrons (electricity) to do work!
    • Bonus activity (ask for help): Use lemon-batteries to provide power for an LED light (you might need a few in series)
    • Double bonus activity (don't do this!): watch how to start a fire with a lemon battery; see: discount crazy russian hacker

Current

When electrons are allowed to flow between the positive and negative battery terminals, we have a *circuit* and the battery's chemical reaction proceeds (until the chemical reaction is complete). The number of electrons flowing between the terminals is called "Current" and is measured in Amperes aka Amps (named after André-Marie Ampère. The number of electrons flowing depends on how many electrons the battery can supply (how fast the chemical reaction can proceed) and the nature of the path carrying the electrons from one terminal to the other. Current Waterfall The electron flow generated by a chemical battery or a solar cell is always from one terminal to the other; this is called Direct Current or DC. Electricity can be generated in other ways such as turning an electromagnetic generator (alternator in your car, steam turbines in a power plant, etc.); when generated in this way, electrons are alternately pushed then pulled by the same terminal of the generator which is called Alternating Current or AC. The electricity from a wall outlet is alternately pulled/pushed 60-times per second.

The amount of current a battery can supply (it's capacity) is often measured in Amp-Hours. This is useful because the battery voltage is fixed by its chemistry; the size and construction of the battery determine how much current it can provide and for how long before it is depleted. For example, the lead acid batteries used on the peanut robots typically are rated for 7 Amp-Hours (often abbreviated Ah). This mean the battery can supply 1A of current for 7 hours before it is used up and needs to be recharged (or 2A of current for 3.5 hours or 7A of current for 1 hour, etc.). If you've taken calculus, you'll recognize the power consumed by a battery as the area under the curve formed by current consumed vs. time; the Ah capacity of the battery is the total area under that curve.

You can can measure DC and AC current flow using your multimeter in the next section.

Power

When there is electrical pressure (voltage) and electron flow (current), electricity can do work. Like water under pressure flowing through a dam and turning a hydroelectric generator or water wheel, electrons flowing through a circuit can do work in a variety of ways. Examples include lighting a bulb, turning a motor, or powering a computer.

  • Exercise: light a bulb with a battery
    • Materials Needed: 9v or 12v battery, two jumper wires with alligator clip ends, low-voltage light bulb or pre-wired LED
    • Activity:
      • Connect one end of an alligator clip jumper wire to the negative (-) terminal of the battery; this will be called the negative wire.
      • WITHOUT letting it touch the negative wire, connect one end of another alligator clip jumper wire to the positive (+) terminal of the battery; this will be called the positive wire.
      • Connect the other end of the negative wire to one wire of the bulb (use the black wire for LEDs)
      • Connect the other end of the positive wire to the other wire of the bulb (use the red wire for LEDs)
      • Let there be light!
    • What happened: the two jumper wires and the filament in the bulb provide a path (circuit) for the electrons to flow from the negative battery terminal to the positive battery terminal. As the electrons flow through the circuit, the resistance they encounter as they push their way through the bulb's filament cause it to get hot, glow, and emit light. The LED works a little differently, but the concept is the same: some of the electrical energy moving through the LED is converted to light energy.

The power used to light the light or turn a motor is is measured in Watts (named after James Watt) and is a function of voltage (the pressure from the battery) and current (the number of electrons flowing); specifically

Watts = Amps * Volts

  • Exercise: use a multimeter to measure the current flowing through a light bulb circuit
    • Materials Needed: multimeter with leads, battery, two jumper wires with alligator clip ends, low-voltage light bulb
    • Activity:
      • Turn the multimeter on and move the range selector to DC Amps (usually marked with an A)
      • Plug the black wire into the hole labeled COM
      • Plug the red wire into the hole labeled 10A this is IMPORTANT; if you try to measure much current with the red wire in the V,mA,... hole, you will blow the fuse in your multimeter and potentially damage it.
      • disconnect the negative alligator wire from the bulb and connect it to the black multimeter wire
      • connect the red multimeter wire to the bulb terminal that had previously been connected to the black wire
      • observe the amount of current flowing in Amperes on the multimeter display
    • What happened: The multimeter is now part of the circuit; the electrons flowing through the multimeter and the bulb as they proceed from the negative battery terminal to the positive terminal. The multimeter measures the rate of electrons flowing through it.

Resistance

Our circuits have consisted of a battery, wires to carry electrons to and through a load like a bulb or motor. The load presents resistance to the flow of electrons and when the pressure (voltage) of the battery pushes the electrons through the load, some of that work is converted into other forms of work energy (heat, light, rotational force, etc.). The amount of work done by the electrons flowing is proportional to the resistance presented:

   Watts = Amps * Volts
   Volts = Amps * Resistance
   Watts = Amps * Amps * Resistance

Resistance is measured in Ohms (named after Georg Ohm). The relationship between voltage, current, and resistance is fixed (i.e. the amount of current that flows is determined by the pressure being applied and the resistance to that pressure) and is defined as "Ohms Law":

Volts = Amps * Ohms

Ohm's Law You can measure resistance using your multimeter.

  • Exercise: measure the resistance of various loads
    • Materials Needed: multimeter with leads, low voltage bulb, DC motor
    • Activity:
      • Turn your multimeter on and the range selector to Ohms (often shown with the Greek letter Omega)
      • Plug the black wire into the hole labeled COM
      • Plug the red wire into the hole labeled V/mA/Ohms...
      • Touch the black wire to one of the load wires/terminals
      • Touch the red wire to the other load wire/terminal
      • Observe the resistance measurement on the multimeter display
    • What happened: The multimeter contains a small battery; it measures the current that flows from its internal battery through the load and uses Ohms law to determine the resistance.
  • Exercise: Measure the resistance of pencil lead (graphite)
    • Materials needed: multimeter with leads, pencil, paper
    • Activity:
      • Use a pencil to draw a thick dark line on a piece of paper. Go over the same line many times until it is thick and has a lot of graphite.
      • Turn your multimeter on and the range selector to Ohms (often shown with the Greek letter Omega)
      • Plug the black wire into the hole labeled COM
      • Plug the red wire into the hole labeled V/mA/Ohms...
      • Touch the black probe to one end of the black line on the paper
      • Touch the red probe to the other end of the black line on the paper
      • Observe the resistance measurement on the multimeter display
      • Slide the probes closer together and observe how the resistance changes
    • What happened: all substances present some resistance to the flow of electrons; graphite presents a moderate level of resistance. The more graphite the electrons must pass through, the higher the resistance.
  • Exercise: measure the resistance of other things (e.g. your fingers). Try it with your fingers moist and dry and observe the difference.
  • Bonus Exercise: make your own light bulb.
    • Materials needed: pencil leads, alligator jumper wires, battery, tape, cardboard tube (e.g. TP or paper towel)
    • Activity:
      • see the Crazy Russian Hacker. Note: if you wanted the filament to last longer, fill the glass with an inert gas like Argon and seal it.
    • What happened: the resistance of the pencil lead to the electricity flowing through it converts electrical energy to heat and light.

Electromagnets

http://cdn.hitfix.com/photos/4366059/Magneto-floats-on.jpg When electricity moves through a conductor, it generates a magnetic field around the conductor. If you shape the conductor into a coil (like a spring), the field is concentrated and becomes stronger. The more current, the stronger the magnetic field. Industrial electromagnets can lift 25-30,000lbs (think truck); electromagnets hold the door shut at the bank. 2537 has used electromagnets to hold and release heavy mechanisms. Electromagnets often use ferromagnetic materials (like iron cores) to concentrate the magnetic flux.

  • Exercise:
    • Materials: battery, electromagnet, alligator jumper wires
    • Activity:
      • Connect the electromagnet to the battery terminals
      • Pick stuff up with the magnet
  • Bonus Exercise: Make an electromagnet
    • Materials: battery, insulated wire, large nail, paperclips or small nails
    • Activity: (see here)
      • Strip the end of the wire and, leaving ~4" free, wrap the rest of the wire around the nail in a tight coil.
      • Strip the other end of the wire and leave ~4" free
      • Connect the two ends of the coil of wire to a battery and see how many nails/paperclips your electromagnet can hold
    • What happened: When you passed an electric current through your coil, it generated a magnetic field that was concentrated by the nail.

DC Motors

DC Motors are what make robots move. A DC motor converts electrical DC current to magnetic force using two or more electromagnets. The electromagnets are arranged so that their magnetic fields alternately attract and repel to turn the motor shaft, converting magnetic energy to rotational force. How a motor does this is explained in a short video here that you should watch.

  • Exercise: make a motor spin
    • Materials needed: two jumper wires with alligator clips, SLA or 9v battery, small DC motor
    • Activity:
      • Use an alligator jumper wire to connect one terminal of the battery to one of the motor terminals or wires
      • Use another alligator jumper wire to connect the other battery terminal to the other motor terminal or wire
      • Watch the motor spin!
    • What happened: Just as water turning a hydroelectric turbine in a dam converts some of the mechanical turning energy into electrical energy (flowing electrons), the DC motor converts some of the energy from electrons flowing through it into mechanical rotational energy. The DC motor contains coils of wire that allow electricity to flow from one terminal to the other (and cause the motor to spin in the process). The work the motor can do is proportional to the power (voltage * current) of the electrons flowing through the circuit.
  • Exercise: make the motor change directions by reversing the direction of current flow
    • Materials needed: two jumper wires with alligator clips, SLA or 9v battery, small DC motor
    • Activity:
      • Set up materials as in the exercise above, note the direction the motor spins (clockwise or counterclockwise)
      • Disconnect the original wire from the positive battery terminal and connect it to the negative battery terminal
      • Disconnect the original wire from the negative battery terminal and connect it to the positive battery terminal
      • The motor spins...observe the direction!
    • What happened: The direction the DC motor spins is determined by the direction of the flow of electrons through it.
  • Exercise: use a multimeter to measure the current flowing through a DC motor
    • Materials needed: multimeter with leads, two jumper wires with alligator clips, SLA or 9v battery, small DC motor
    • Activity:
      • Connect the positive wire (alligator jumper wire connected to the positive battery terminal) to one of the motor wires or terminals
      • Connect the multimeter red wire to the other motor wire or terminal
      • The motor spins!
      • observe the amount of current flowing in Amperes on the multimeter display (idle current)
      • use your fingers to slow down the motor (but don't stop it) and observe the change in current (load current)
    • What happened: as the load on the DC motor increases, so does the amount of current it draws. Note that if you stall the motor completely, the energy from the battery will continuously be going through one electromagnet which will heat up and eventually fail (often with smoke and sometimes flame involved). When the motor is spinning, the energy (and heat) are divided among all of the electromagnets in the motor, giving each a chance to cool down. On heavy duty motors, the shaft often includes internal fan blades that push air across the hot electromagnet coils to keep them cool.
Last modified 7 months ago Last modified on Oct 12, 2020, 4:32:32 PM

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