Energy, Mechanisms and Electronic Systems

You never get something for nothing

This topic looks like three unrelated subjects bolted together. Where energy comes from and how it is stored. Levers, gears, cams and pulleys. And electronic circuits with inputs and outputs. They are usually taught as three, and the connection is never made. ⭐⭐ A MECHANISM AND AN ELECTRONIC CIRCUIT ARE THE SAME THREE-PART IDEA BUILT OUT OF DIFFERENT MATERIALS. Something goes IN. Something in the middle CHANGES it. Something comes OUT. A push on a handle goes in; the lever changes it; a larger force comes out at the other end. A small voltage from a sensor goes in; a circuit changes it; enough current to run a motor comes out. ⚠️ THOSE ARE NOT TWO DIFFERENT KINDS OF THING. THEY ARE THE SAME DIAGRAM MADE OF METAL IN ONE CASE AND OF SILICON IN THE OTHER. And once you see that, the most important rule in the whole topic becomes obvious, because it applies to both. ⭐⭐ WHATEVER THE MIDDLE PART GIVES YOU, IT TAKES SOMEWHERE ELSE. A lever that doubles your force makes the load move half as far. A gear train that doubles the turning force halves the speed. A pulley system that lets you lift something heavy makes you pull far more rope than the load rises. ⚠️ THE TRADE IS NOT A FLAW IN THE DESIGN. IT IS THE DESIGN. Electronics is no different. A component that lets a tiny signal control a large current has not created the large current; it is switching a supply that was already there. ⭐ AND THAT SUPPLY IS WHERE THE ENERGY PART OF THIS TOPIC COMES IN: GENERATION AND STORAGE ARE WHAT FEED THE INPUT SIDE OF EVERY SYSTEM YOU WILL EVER DESIGN. A battery is itself a trade, between how much it can hold and how much it weighs to carry. ⚠️ WHICH GIVES YOU THE ONE QUESTION TO ASK OF ANY SYSTEM IN THIS TOPIC, MECHANICAL OR ELECTRONIC: WHAT GOES IN, WHAT COMES OUT, AND WHAT WAS GIVEN UP TO GET IT? Most weak answers in this topic name a component and stop. The name is not the answer. The trade is.

The same three parts, twice

One diagram, built out of two different materials, and the supply that feeds both.

Tap what the three-part idea claims

Tap the TWO statements that follow from a mechanism and a circuit being the same three parts.

• A mechanism that increases force must reduce something else, and saying what is part of the answer
• The same input, process and output diagram describes a lever and an electronic circuit equally well
• A well-designed mechanism can increase force without any cost elsewhere
• A component that lets a small signal control a large current is generating that current itself

The handle that got easier

A tool is redesigned with a longer handle, and it becomes noticeably easier to operate. You may already know how to calculate the advantage this gives. What has been given up in exchange?

• Distance: the handle now has to be moved much further to produce the same movement at the working end
• Strength: the longer handle will be weaker and more likely to bend
• Nothing: a longer handle is simply a better design with no cost attached
• Energy: the tool now uses more energy overall to do the same job

Five terms the systems run on

Five terms, each defined by what it is. What each one trades, and for what, is the work of the steps that follow.

Match each system to what it gives up

Two that follow from the trade

Select the TWO statements that follow from the middle part always taking what it gives.

• Naming the mechanism is only half an answer; saying what it trades is the other half
• An electronic output can never be larger than what the power supply can provide, whatever the input signal does
• The best mechanism is the one that gives the greatest mechanical advantage
• A programmable component removes the need to think about inputs and outputs

Answering a systems question properly

Questions on this topic come in two shapes, and the same habit answers both. One shows you a system and asks what it does. The other gives you a job and asks you to choose a system for it. ⭐ IN BOTH CASES, START BY SPLITTING IT INTO THREE. WHAT IS THE INPUT? A hand, a motor, a sensor, a switch. Be specific about what physically arrives. WHAT DOES THE MIDDLE PART DO TO IT? ⚠️ AND NOT JUST ITS NAME. Say whether it changes the SIZE of something, its DIRECTION, or its KIND - rotation into a rise and fall, a small signal into a switched supply. WHAT COMES OUT, AND IN WHAT FORM? ⭐⭐ THEN THE SENTENCE THAT EARNS THE MARKS MOST ANSWERS LEAVE UNWRITTEN: SAY WHAT WAS GIVEN UP. More force for less distance. More speed for less turning force. A smaller supply for less weight to carry. ⚠️ AND IF YOU ARE CHOOSING RATHER THAN DESCRIBING, THAT TRADE IS THE WHOLE ARGUMENT. The right mechanism is not the one with the largest advantage; it is the one whose trade suits the job. A jack that lifts a car needs force and does not care about speed. A bicycle needs to give up force for speed on the flat and the reverse going uphill, which is precisely why it has gears at all. ⚠️ THREE HABITS THAT COST MARKS ON THIS TOPIC. ⚠️ THE FIRST IS NAMING COMPONENTS WITHOUT FUNCTION. "It uses a cam and follower" is identification. What the cam converts, and what that makes possible, is the answer. ⚠️ THE SECOND IS TREATING ELECTRONICS AS DIFFERENT IN KIND. It is the same three parts, and the same rule about supply, so the same structure works. ⚠️ AND THE THIRD IS REACHING FOR A FIGURE YOU DO NOT HAVE. An efficiency, a power rating, a tooth count: if the question did not give it to you, describe the trade in words instead. ⭐ YOU MAY ALREADY KNOW HOW TO CALCULATE MECHANICAL ADVANTAGE AND GEAR RATIOS; USE THOSE WHERE THE NUMBERS ARE SUPPLIED, AND ARGUE IN WORDS WHERE THEY ARE NOT. Which energy sources are better for the environment is a real question, and it belongs to another part of this course rather than here.

A bicycle, taken apart as a system

A bicycle is worth walking through because it contains both halves of this topic and makes the trade impossible to miss. THE INPUT IS A PERSON PUSHING DOWN ON A PEDAL. That is all the energy the machine will ever get, and every part downstream is spending it. THE MIDDLE PART IS A CHAIN RUNNING BETWEEN TWO TOOTHED WHEELS OF DIFFERENT SIZES, and this is where the whole argument lives. Put the chain on a large wheel at the pedals and a small one at the back, and one turn of the pedals spins the rear wheel several times. ⭐ THE BICYCLE GOES FAST FOR EACH TURN OF YOUR LEGS - AND EACH PUSH IS HARDER, BECAUSE THE TURNING FORCE HAS BEEN REDUCED IN EXACTLY THE PROPORTION THE SPEED WAS INCREASED. Now swap to a small wheel at the pedals and a large one at the back. Every push is easy, and you get much less forward movement for it. ⚠️ NOTHING ABOUT THE RIDER OR THE BICYCLE HAS CHANGED. THE SAME EFFORT HAS SIMPLY BEEN DIVIDED DIFFERENTLY BETWEEN FORCE AND SPEED. ⭐⭐ AND THAT IS WHY A BICYCLE HAS GEARS AT ALL. NOT BECAUSE ONE ARRANGEMENT IS BETTER, BUT BECAUSE THE BEST TRADE ON A FLAT ROAD IS THE WORST TRADE ON A HILL. Choosing between them IS the design. THE OUTPUT IS THE REAR WHEEL TURNING, AND THE BRAKES SHOW THE SAME IDEA RUNNING BACKWARDS. A light squeeze at the handlebar becomes a firm grip at the wheel, through a cable and a set of pivoted arms that change both the direction of the movement and its size. ⭐ YOUR FINGERS MOVE FURTHER THAN THE BRAKE BLOCKS DO. THE TRADE IS THE SAME TRADE. Now add the electronic half, because modern bicycles often have one. A small magnet passes a sensor once per wheel revolution, and a display shows a speed. The input is a magnetic pulse, and the process is a small chip counting pulses and timing them; the output is a number on a screen. ⭐ AND IF THE RIDER WANTS MILES INSTEAD OF KILOMETRES, NOTHING IS REWIRED. THE INSTRUCTIONS CHANGE. ⚠️ THAT IS THE WHOLE POINT OF PUTTING A COMPONENT LIKE THAT IN THE MIDDLE OF A SYSTEM. And none of it runs without a supply. A small battery on the handlebars, or a dynamo turned by the wheel, which takes a little of the rider's effort to make its electricity. ⚠️ NOT EVEN THE LIGHTS ARE FREE.

Order how to take a system apart

Put these six moves into the order that turns naming a mechanism into explaining it.

• Identify what physically arrives at the input, and from where
• Identify what sits in the middle and what it is called
• Say whether it changes the size, the direction or the kind of what went in
• Say what comes out, and in what form
• Say what was given up in exchange
• Judge whether that trade suits the job the system has to do

Work out what the trade costs

A lever is arranged so that the output force is 4 times the input force. The load needs to be raised by 6 cm. Work out how far the input end of the lever must be moved, in centimetres, remembering that whatever the mechanism gives in force it takes in distance.

The systems run

Five questions on inputs, outputs and what every system gives up. Three lives.

Complete the systems facts

How many times larger the output force of a mechanism is than the input force is its _____. A set of connected bars that changes the direction or type of a movement is a _____. A shaped rotating part producing a repeating up-and-down movement in a follower is a _____. A component whose behaviour is set by instructions rather than by wiring is a _____.

Three answers to complete

Three student answers about systems. In each case the identification is correct and the reasoning is what needs work.

• An answer identifies a mechanism as a cam and follower, correctly, and moves on to the next question. What has it left on the table?
• Asked to choose a gear arrangement for a winch that lifts heavy loads slowly, a student picks the one giving the greatest speed at the output. How would you correct it?
• A student writes that the small signal from a sensor is amplified by the circuit into enough current to drive a motor. What needs changing?

Explain a system you can hold

Choose a product you own that contains a mechanism, an electronic system, or both. Explain it as a system, and say what it gives up in order to do its job.

• Say what physically arrives at the input, and where the energy for it comes from
• Name what sits in the middle, and say whether it changes the size, direction or kind of what went in
• Say what comes out, and in what form
• Say what was given up in exchange, in words rather than figures unless you are certain of them
• Explain why that trade suits the job this product has to do, and what would be wrong with the opposite trade
• If it contains anything programmable, explain what could be changed about it without rewiring anything