Nicolo Maganzini, Geronimo Fiilippini, Aditya Kuroodi
2015
Relevant Course(s): EE3, EE11L

What are we learning?
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To be able to build something that uses electricity to do something, we need to understand what electricity is and how it works.
Learn about Voltage: The driving force that pushes electricity.
Learn about Current: What electricity actually is.
Learn about Conductors: What materials can be used to make electric circuits.
Start learning about circuit diagrams and breadboards.
 Learn how to measure voltage and current using a multimeter.

Conductors and Insulators
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Electrons of different types of atoms have different degrees of freedom to move around.
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Metals: electrons are loose, and they form a sea of free electrons
Electrical conductivity: the relative mobility of electrons within a material.
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Conductors: high electron mobility (many free electrons)
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Insulators: low electron mobility (few or no free electrons)

Free electrons and Electricity
 Free electrons move around randomly, with no particular direction or speed.
 We need electrons to move uniformly in one direction, like marbles in a tube.
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We can do this for conductive materials to produce electricity or electric current.

Electron Flow
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Electrons can flow only when there is a continuous path of conductive material.
For example, if the tube with marbles was blocked at one end, marbles inserted from the other side would not be able to flow.

Electric Circuits
 An electrical circuit is a closed path, never ending loop that provides a pathway for electrons.

Electric Circuits
 If there is a break in a circuit, there will be no electron flow regardless of where the break is.
 But what is it that makes electrons flow in one direction as opposed to random motion?

Voltage and Current
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We need some means to push these electrons around the circuit.
Need a force to maintain electron flow. This force is produced by an imbalance (difference) of electric charge.
If there are a lot of electrons on one side of the circuit and very few on the other, these electrons will flow from the side with many electrons to the side with few electrons

Comparison to water flow:
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Say we pump some water from a pond to a reservoir high up on a hill.
Due to the force of gravity, water will want to flow down back to the pond. If we put a pipe from reservoir to pond, water will flow down.

Voltage and Current
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If the water is pumped to an even higher level, it will take even more energy to do so, thus more energy will be stored, and more energy released if the water is allowed to flow through a pipe back down again.

Table of analogy:
Water Analogy
Water
No tube, no water flow
Circuits and Electrons
Electrons
No closed path, no current
Pump water to a high place: store energy in water. Potential Energy stored.
Force on water to go back.
Let water flow down: release energy.
Bunch up electrons on one side of the circuit: store energy. Potential energy stored.
Force on electrons to go back.
Electrons going back to normal: release energy.

Voltage and Current
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How strong is the push on electrons to go back to their ‘unbunched’ state?
This is expressed as voltage . This is a number with units of Volts (V).
Higher voltage = greater push for electron flow.

Voltage and Current
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Voltage can be measured between any two points in a circuit.
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If voltage between two points is large
, then the push on electrons in large there.
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If two points in the circuit are at the same potential, no electron flow in that direction.

Voltage and Current
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There are various ways to generate voltage:
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Chemical reactions (batteries)
Magnetism on conductors. (generators, alternators)
These are examples of transducers
 Transducer: something that converts energy from one form to another (i.e. chemical to electrical)

Voltage and Current
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Any source of voltage, including batteries, has two leads:
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+ (high voltage)
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- (low voltage)
The negative end of a battery tries to push electrons out of it.
The positive end is that end which tries to attract electrons.

Voltage and Current

Voltage and Current
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If we break the circuit at any point, the electric current will cease in the entire loop.
The electrons will only reach the edge of the wire but won’t be able to go past it.
Voltage drop is now between the two ends of the wire.
Remember that voltage is always relative between two points.
Electrons are all bunched up but here they can’t make it to the other side!

Using a Breadboard

Learning how to use a multimeter: Voltage.
 Demonstration on using multimeter to measure voltage between two points.
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Task: You have a battery and two cricuits.
 Measure voltage across the leads of the battery.
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Measure voltage across the leads of the battery in the circuits.
Measure voltage across each resistor in each circuit.
Circuit 2 Circuit 1
Copy down on paper and write voltage measurements.

Learning how to use a multimeter: Current
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Demonstration on using multimeter to measure current in a part of a circuit
Use only one circuit, modify it slightly!
 Copy down circuit and record current value

Resistance
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Electrons encounter opposition to motion in the filament.
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This is RESISTANCE.
Resistance limits the amount of current in a circuit with a given voltage.
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If there is little resistance, then the current is very large.
 Short Circuit has very little resistance.
Marble running down slope with pegs
 Many pegs = large resistance. Slow average speed.
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Few pegs = little resistance. Faster average speed.
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No pegs = short circuit  free fall.

Water analogy to resistance.
 In the water analogy, we can think of resistance as a pipe with a smaller diameter, that restricts water flow.

Voltage and Current in a practical circuit
 If there is any resistance between two points in a circuit, then there is a voltage difference between them.
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What is voltage difference between: (wire has no resistance, battery is 5V)
 Red-Purple
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5V
 Red-Blue
 0V
 Blue-Green
 5V

Ohm’s Law: Relating Current, Voltage,
Resistance
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V = IR
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How do we interpret this?
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How is this used?
I = V/R
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How do we interpret this?
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How is this used?
R = V/I
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How do we interpret this?
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How is this used?
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What do actual resistors look like?
 In Circuits:
 In Real Life:
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How a potentiometer works:
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Like two resistors one after the other.
When you turn the knob you are making
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One resistor smaller
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The other resistor larger
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Sum of both resistances is the same.
A C
B
 Excercise: use multimeter for resistance measurement
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Turn knob all to one side: measure resistance between A and B , between B and C , and between A and C . Write the values down. Is Rab + Rbc = Rac ?
 Repeat after turning knob to the other end.

What are we learning?
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Learning about new components called capacitors.
Learn about how they are different from resistors.
Learn about how capacitors are used in circuits with signals to modify and shape the signal as we want.
 Signal filtering with capacitors.
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Used a lot in the pulse meter of the medical station.
 Learn how to use a function generator and an oscilloscope
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Very important tools for an Electrical Engineer!!
Water analogies

Capacitors store charge
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Capacitors in circuits are like water baloons attached to water circuits.
Pump
Flow res. reduces flow water baloon starts filling.
 As pump pushes water, baloon fills up and starts pushing backwards, opposing the flow of water more and more.
 Water wheel slows down.
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Charged capacitor
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At some point, force of baloon pushing water backwards is equal to force of pump pushing water forward
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Assuming weak pump and very strong rubber
No more water flow. Water wheel doesnt turn.
Force of pump = Force of baloon
Water is still.
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What if we turn off the pump?
 Now the pump stops pushing. There is nothing to oppose baloon force, so water flows out of baloon and it starts emptying. The water wheel spins again.
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When baloon is empty, water wheel stops and no more water flow.
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Now with capacitors.
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Circuit analog is RC (resistance-capacitance circuit)
 Water wheel is resistor, capacitor is water baloon.
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Switch in position 1: current flows from battery, through resistor to capacitor, charges capacitor.
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When capacitor is full, force pushing back is equal to force pushing forward, i.e. capacitor and battery are at the same voltage.
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Capacitor charging
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When capacitor is empty,
 Force pushing current back is weak: Low voltage
 Becomes greater and greater until reaches same voltage as battery.
 Amount of current that makes it through is large! (because nothing stops it)
 But as capacitor fills up, no more current makes it through.
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Capacitor Discharging
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When battery is disconnected,
 Capacitor starts emptying, pushing electrons back out and creating a current.
 Initially force is the same as the old battery, but as capacitor is becoming empty, the strength goes down.
 Same with current becomes weaker.
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Large vs. Small capacitors
 Capacitance value of capacitor (like resistance for resistors) tells us how large the capacitor is.
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What does this mean? Like the size of the baloon.
Large or small?
Large or small?
Charging
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Discharging

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We also need to take into account the resistance.
Speed at which capacitor charges depends also on how much resistance in the circuit
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Small resistance = more current = faster charging.
Large resistance = less current = slower charging.
 Time constant 𝜏 = 𝑅𝐶 has both capacitance and resistance
 𝜏 is the time required for voltage or current to change by 63.2%
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