# Foundations of Physical Science

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Foundations of Physical Science
Workshop: Electric Circuits Foundations of Physical Science is a new hands-on program developed in-house by Cambridge Physics Outlet Note to presenter: 4 people per game board (or more if they play in teams) Materials: -8 atom building games (plastic version) with all parts and proper periodic tables -atom building game workshop packet (contains all program pieces for 18.3) -brochures and catalogs -pens and pencils and tablets -other giveaways?

Electric Circuits CPO Science

Key Questions What “flow of understanding” provides the necessary foundation for an understanding of electricity? What kinds of electric circuits can you build? How does electricity behave?

What needs to happen to get the bulb to light?
Light the Bulb! What needs to happen to get the bulb to light? Take one wire, one D cell, and one bare light bulb (no socket) from the kit – figure out how to light the bulb

Parts of our Circuits Kit
Wooden Board Wires of various lengths On/Off switches Bulbs and holder Resistors – fixed and variable These are the the parts of our circuit kit. and its helpful to introduce each piece. These pieces help students make an intuitive jump from using part of a circuit to reading or even drawing a circuit diagram.

Build a simple Circuit Place the bulb in a socket Use one D cell
Make the bulb light! Add a switch to conserve D cell energy Use your finger to trace the path of electricity from one terminal of the D cell to the other terminal Now use the actual kit parts to build the simple one D cell, one light bulb (in socket, of course) circuit. Add a switch to turn the light on and off. Tracing the path is very important!

Symbols used for Diagramming
Parts of a Circuit Symbols used for Diagramming Wire Bulb Battery Switch There is a system for diagramming circuits. These are the agreed upon symbols that represent some of the basic elements of circuits that we will be dealing with. These can be arranged in a diagram and anyone, regardless of what language they may speak, will be able to understand how to make the circuit.

Let’s build on this… Add a second D cell to your circuit, right next to the first. Be sure to match up positive terminal with negative terminal Do you notice any difference? Add a second light bulb to the circuit, keeping only one pathway for electricity to follow What do you observe now? Adding a second D cell makes the bulb brighter. Adding a second bulb causes both bulbs to be dimmer than the circuit that had only one bulb.

Series Circuit Probably the most important part of this section is understanding that a circuit must be complete, in other words provide a path that allows electricity to circulate from one pole of the battery, along a wire, through the elements of the circuit, along the wire again and back to the other pole. This is completing the circuit, and without this there is no flow of electricity.

Another way to light two bulbs
Keep two D cells in the circuit Wire up the 2 light bulbs so that there are two branches or pathways for electricity to follow What differences do you observe? Wiring the two light bulbs in parallel will result in two very bright bulbs, as compared to the dimmer bulbs in series circuit.

Parallel Circuit Probably the most important part of this section is understanding that a circuit must be complete, in other words provide a path that allows electricity to circulate from one pole of the battery, along a wire, through the elements of the circuit, along the wire again and back to the other pole. This is completing the circuit, and without this there is no flow of electricity. Here we have two circuit diagrams using the proper circuitry symbols.

Can you explain why the bulbs in a parallel circuit are brighter?

Water Analogy The Water analogy can be useful to explain the different properties of electricity and circuits. The voltage can be described as the height of the water tower, which would be the potential energy, or in other words the capacity of the water to do work. The current would be the flow rate of the water, and in a circuit that is measured in coulombs of charge per second as opposed to the water’s gallon per second rate. The resistance can be equated to the water’s ability to flow. A large hose would allow a lot of water to flow per second, and offer low resistance. A smaller hose would impede the flow of water and cause less water to flow per second, representing a higher resistance.

Resistance and Current
Inverse Relationship The current would be the flow rate of the water, and in a circuit that is measured in coulombs of charge per second as opposed to the water’s gallon per second rate. The resistance can be equated to the water’s ability to flow. A small hose would impede the flow of water and cause a small amount of water to flow per second, representing a high resistance. A large hose would allow a lot of water to flow per second, and represent a low resistance.

The amount of potential energy that each unit of charge has
Voltage The amount of potential energy that each unit of charge has To use the water analogy, we can think of voltage as the potential energy of each unit of charge, which would be the height of the water tower. The higher the elevation, the more the potential energy.

Review V = voltage, measured in volts
I = current, measured in amperes, or amp R = resistance, measured in Ohms, symbol W

Using the Multimeter to measure Voltage
Battery in a circuit Battery by itself The Multimeter has a variety of useful functions, and the key is to make sure the control dial is adjusted to the proper setting, and the positive and negative leads are plugged into the proper jacks. Keeping this straight will make the Investigation move along smoothly. Measuring voltage will tell you the difference in voltage from one point to the other. When measuring the voltage of a battery, connecting the proper lead to the proper terminal of the battery will indicate the voltage difference between the two poles of the battery. In a circuit, it will also measure the voltage difference between two points. If there is an element of the circuit in between the two points that are being measured, like a light bulb, the result will indicate how much voltage that element has used.

Using the Multimeter to measure Current
Current in a circuit To use the Multimeter to measure current, the meter itself must be used like an element in the circuit. In the picture you can see the meter is completing the circuit- electricity goes in one lead, through the meter, and out the other lead to the bulb. You may also notice that both the setting for the dial, and the places where one of the leads has changed. When the meter is connected like this as part of the circuit, it will measure the current through that point. Multimeter completes the circuit

Analyze Circuits 1 bulb 2 bulbs in series 2 bulbs in parallel
Total voltage available Voltage across each bulb Total current at terminal Current through each bulb Participants should Find the voltage at the terminal of one battery in each circuit, as well as the current at one terminal of each battery. Don’t forget to break the circuit when measuring current! Remember, voltage will tell us how much energy each unit of charge carries, and current will tell us how many units of charge flow past a certain point in a second. Then the participants should find the voltage across each individual bulb, as well as the current at each individual bulb in the circuit. Example values are filled in on the next slide.

Why are parallel bulbs brighter?
2 bulbs in series 2 bulbs in parallel Total voltage available 2.8 V Voltage across each bulb 1.4 V Total current at terminal .12 A .10 A 0.24 A Current through each bulb 0.12 A The one bulb circuit is analyzed to be a comparison between series and parallel. It should be obvious that 2 bulbs in parallel is simply 2 one bulb circuits put together. 2 bulbs in series: Voltage drops because some energy from each coulomb of charge is converted to heat and light in the light bulbs. Current is the same at all points in the series circuit, because there is only one path for the flow of charge. 2 bulbs in parallel: Total voltage should be about 3 volts. Each light bulb should individually see 3 volts. This is because there are two independent pathways that both receive the same “push” from the D Cell. This is often difficult to understand. One way to approach this is to say that each coulomb of charge leaving the D cell has approximately 3 joules of energy. If one coulomb of charge goes down one path, and another coulomb of charge goes down another path, each coulomb of charge still carries 3 joules of energy! Total current is the sum of all branch currents.

Investigating Ohm’s Law
How are they Related? Investigating Ohm’s Law How does changing resistance affect current? How does changing voltage affect current? Investigation Ohm’s Law  By following the steps in Investigation Ohm’s Law, we can measure the values of voltage, current, and resistance and come up with a mathematical relationship.

How does changing resistance affect current?
Current decreases when resistance is increased Follow the Investigation manual for the exact steps involved. What we are going to find in this part of the investigation that current decreases as resistance is increased. We are going to change the resistance in the circuit by adjusting the dial on a potentiometer, a variable resistor. The letter A on the circuit refers to the current being measured at that particular point on the circuit. Current is measured in Amperes, or Amps as it is commonly called, which is where the A comes from.

How does changing voltage affect current?
Voltage/Current = Resistance The large V and A indicate where voltage and current are to be measured respectively on the circuit. Once again we are using a potentiometer to change values in the circuit, but this time we are changing the voltage. This could be confusing, as someone may easily wonder how this potentiometer can be used to change the resistance in the 1st part of the Inv. and the current in the 2nd part. The reason it works is that as the resistance of the potentiometer is increased, the current it draws will decrease, which in turn will allow less voltage to reach the fixed resistor. This process is used to vary the voltage that eventually gets to the fixed resistor in the circuit. What we find is that as the voltage across the second resistor is increased, the amount of current increased as well. Or, as the voltage is decreased, the current is decreased too.

Voltage/Current = Resistance
Rise/Run = Slope Voltage/Current = Resistance This is the kind of graph that can be expected for part 2 of the Investigation. We can see that the current drawn is always proportional to the voltage across the fixed resistor. In fact the constant of proportionality is the slope, V/I which is equal to the resistance of the fixed resistor. This is a great physical/graphical representation of values that themselves can tend to be elusively tactile

Ohm’s Law I = V/R V = IR R = V/I
If two of the quantities involved are known, the third can be determined mathematically These are different forms of the same equation. I = V/R is the working original equation used to determine how much current an element of known resistance will draw. I = V/R V = IR R = V/I

Basic Circuits These are the two basic kinds of circuits, series and parallel. Series allows only one path for the electricity to travel around the circuit. Parallel has more than one path, providing alternative “branches” for the electricity. The picture shows two possible paths, but parallel circuits are not limited to only two, and in fact can have many, many branches. The ones we will use will have only two for now.

Basic Circuit Diagrams
Probably the most important part of this section is understanding that a circuit must be complete, in other words provide a path that allows electricity to circulate from one pole of the battery, along a wire, through the elements of the circuit, along the wire again and back to the other pole. This is completing the circuit, and without this there is no flow of electricity. Here we have two circuit diagrams using the proper circuitry symbols. Parallel Circuit Series Circuit

Are They Related? Can we measure these values in a circuit?
As we measure these values, do they change in relation to one another  We often seek to find ways to relate parts of a system one another. In circuits, we’ll need to use some special measuring equipment in order to do this, since we can’t directly observe all the values we want to measure. Once we can do this, we will be able to observe the changes, if any, and try to discover a relationship.

Using the Multimeter to measure Resistance
Resistance of a bulb A measure of conductivity Once again, notice the dial and terminal connections being made on the meter. The resistance of any element of a circuit should be measured while it is not connected in the circuit and simply on its own. Each element will have a positive connection and a negative connection. On the light bulb, one will be the metal button on the bottom of the bulb, below the screw attachment and the other will be the metal threading of the screw attachment itself. On the multimeter, the red lead is the positive and the black one is the negative. It really does not matter which is positive and negative when measuring resistance. The resistance of the human body can range from about 1000 to 500,000 Ohms. Dry skin has the highest resistance and as moisture levels increase, resistance drops dramatically. The resistance overall of the human body is lowered by burns and cuts on the skin. ( Facts from the incredible illustrated ELECTRICITY BOOK, 1975 Pathmark Books Inc., Boston MA.)