Science OlympiadDivision B

Shock Value

Activities involving basic understanding of electricity, magnetism and simple electrical devices.

Hopefully this will involve test type questions and hands-on activities.

Could be:•stations.•building specific circuits.•analyzing circuits.•analyzing graphs.•mapping fields.•there are many possibilities.

Simple Circuits

Shock Value

 Use a single “D” battery, a single bare wire and a light bulb. Find four different ways to light the light bulb using only a battery, one wire and the bulb.

Quite often one thinks that they must run electricity from the battery to the bulb. They try from either end of the battery. The bulb won’t light!

If the wire is HOT the bulb won’t light. Disconnect the wire from the battery and try something else.

So, what will work?

 Use a single “D” battery, a single bare wire and a light bulb. Find four different ways to light the light bulb using only a battery, one wire and the bulb.

There are two positions of the bulb at each end of the battery. The wire must go to a different part of the bulb to the other end of the battery.

Obviously, there are two important parts of the battery. The positive terminal and the negative terminal. These terminals are at opposite ends of the battery.

+-

If you look closely at the bulb, there is a filament that glows when it is properly wired. Many filaments are coiled.

You will also notice that one end of the filament connects to the tip of the bulb and the other to the threads.

In order to light the bulb with two wires and the bulb away from the battery, one wire must go from one terminal of the battery (+) to the tip of the bulb.

+-

The other wire must go from the other terminal of the battery (-) to the threads of the bulb.

It does not matter which wire goes to the tip and which goes to the thread as long as one end of the battery is connected to one part of the bulb and the other terminal is connected to the other part of the bulb.

Now we can say that we have a complete circuit.

How would one define a complete circuit, which we will call just a circuit?

A pathway for charge to flow from one terminal of the battery, through the light bulb and back to the other terminal of the battery.

A battery is an example of a “SOURCE.”A source is a device that converts some form of energy into electrical energy.

+-

A generator converts mechanical energy into EE.

A battery converts chemical energy into electrical energy (EE).

A solar (photovoltaic) cell converts light (radiant) energy into EE.

A light bulb is an example of a “LOAD.”A load is a device that converts electrical energy into some other form of energy.

+-

Other types of bulbs, CFL’s, LED’s, etc. are more effective in converting EE into light with little or no heat. Which do you think require less electricity?

A light bulb converts EE into thermal (heat) energy and light (radiant) energy. Incandescent bulbs need a lot of heat to have the filament glow and give off a lot of light.

A motor converts EE into mechanical energy.

A heater (toaster) converts EE into thermal energy (heat).

Look at the picture of the battery, bulb and wire(s) shown above. Which bulb(s) is lit?

In which case(s) is current flowing through the wire?

SCHEMATIC SYMBOLS

Battery

Light Bulb

Switch (open)

Resistor

Ammeter

Voltmeter

Wire

Junction (wires connected)

A

V

Switch (closed)

Switch (closed)

Switch (open)

Photo of set-up. My sketches are terrible.

Schematic of the same circuit.

What does a switch do? How does it do it?

A closed switch completes the circuit or turns the bulb on and an open switch breaks the circuit or turns the bulb off.

Using an AmmeterAn ammeter measures the current flowing through the circuit.

Current is the flow of charge through the wires. In physics the unit for charge is a coulomb. The symbol for current is an “I” and the unit is an ampere which is abbreviated amp or A. An amp is a coulomb per second.

An ammeter is wired in series in a circuit. That means that you have to break the circuit to insert the ammeter.

I would recommend an analog ammeter. When my students used the ammeter on the multimeter, they usually made a mistake and blew the fuse. It is easy to blow the fuse and not too easy to replace it.

Using an AmmeterA series circuit means that there is only one pathway for the current to flow. If you look at the circuit, the current must pass through the ammeter and then the bulb and then the switch.

A

This leads to an interesting question. What is the direction of the current?

We know now that electrons move in a metal wire. So electrons flow out of the negative terminal of the battery, go through the ammeter, the light bulb, and the switch and return to the positive terminal of the battery.

e-

But, years ago we did not know about electrons! So what then?

Benjamin Franklin is credited with the idea of positive and negative charge. Positive meant an abundance of electrical charge. So it made sense to him that positive charge flowed from an abundance of charge (+) to and absence of charge (-).

A

If you look in a physics book it will say that current is the flow of positive charge from the positive terminal of the battery, through the switch, the light bulb, and the ammeter and return to the negative terminal of the battery. This is called the conventional flow of current.

+

Many electronics books use the flow of electrons for current. Make sure that you look to see which way current is defined. Also, you may want to clarify this with the event supervisor.

For the rest of this presentation I will use electron flow for current.

A

e-

Using an AmmeterDoes it matter where we put an ammeter in a circuit?What happens to the current as it passes through the light bulb?

A

Let’s put the ammeter on the opposite side of the circuit. What will be different about the current?a) The current will be less.b) The current will be the same.c) The current will be greater.Make your prediction. Now wire it and see for

yourself.

Hopefully, by now it is obvious that the current into the bulb is the same as the current out of the bulb.

A

So, it does not matter where you put the ammeter or the switch in a series circuit. The current everywhere is the same!

A

If the values for the current are not the same, make sure that you have good connections in the circuit.To convince you I put two ammeters in this circuit; one before the bulb and one after the bulb.

Multi-scale ammeter.Here you have to look at where the wires are connected. There is always one wire connected to the black (-) terminal.

-

If the other wire is connected to the “50” terminal, you read the 50 mA scale.

50

If the other wire is connected to the “500” terminal, you read the 500 mA scale.

500 If the other wire is connected to the “5”

terminal, you read the 5 A scale.

5

50

500

5

Only one scale can be used at a time!

Voltage or Potential DifferenceSo, the current does not change when it passes through a light bulb, then what does cause the bulb to light? To answer this there is another property that we need to measure.

A voltmeter measures the voltage gain or potential gain at the battery (source) or the potential drop or loss at the light bulb (load). Look back at sources and loads.

A voltmeter is wired in parallel to the circuit. You do not need to break the circuit. All you have to do is touch the red (+) probe of the voltmeter to the positive terminal of the battery and the black (-) probe of the voltmeter to the negative terminal of the battery. If done properly the reading should be positive. If you get a negative value you either connected the probes backwards on the voltmeter or to the battery.

A parallel circuit has multiple paths. You are actually diverting a very tiny current through the voltmeter in order to get a reading. You will observe series and parallel circuits for light bulbs later.

The symbol for voltage is a capital V and the unit is a volt, V.

Voltage or Potential Difference

Touch the red (+) probe of the voltmeter to the positive terminal of the battery.

Orienting a voltmeter is a little more challenging than an ammeter.

Touch the black (-) probe of the voltmeter to the negative terminal of the battery.

If done properly the reading should be positive. If you get a negative value you either connected the probes backwards on the voltmeter or to the battery.

If you move the probes together around the circuit to the light bulb you should have the probes connected in the following orientation and the reading should be negative.

The value should be close to the reading at the battery, except it is negative.

WHY?

A

e-

Remember that electrons flow around the circuit from the negative terminal of the battery through the loads along the way and back to the positive terminal of the battery. The size of the current never changes.

One way to illustrate this is to say that the electrons pick up energy at the battery (positive voltage or voltage gain) and gives up its energy at the light bulb (negative voltage or voltage loss).

You may find that the voltage loss at the light bulb is a little bit less than the voltage gain at the battery. This is due to losses in the ammeter, switch, wires; poor connections; or a combination of both.

Keep in mind that energy is conserved. So as one goes around the circuit in a complete loop, the voltage gains must be equal to the voltage losses.

Two Bulbs in Series

You will probably find that the sum of the voltage losses at the bulbs, V1 + V2, is close in value to the voltage gain at the battery, VS. Remember there can be other losses.

You will notice that the bulbs are very dim or maybe out, but there is still current registered on the ammeter.

If you are lucky the bulbs are identical and they are both the same brightness. Chances are they are not and one is dim and the other is dimmer or out.

Note: In any complete loop around the circuit from the negative terminal of the battery, through the loads, to the positive terminal of the battery, and back to the negative terminal, that the sum of the voltage gains equal the sum of the voltage losses.

Two Bulbs in Series

What happens when you unscrew Bulb #1?

Bulb # 2 goes out.The ammeter indicates that the current, I ,is zero. The circuit is broken!

What happens when you unscrew Bulb #2?

Some of the older Christmas lights were wired in series and when one burned out they all went out. This drove many a person crazy trying to find out which one blew out.

Bulb # 1 goes out.The ammeter indicates that the current, I ,is zero. The circuit is broken!

Two Bulbs in Series

4. "Christmas Lights" are often wired in series just like the circuit you just tested.a) What happens when one of the lights burns out? Why?

As we just saw, as one bulb goes out (the filament breaks, which is just like unscrewing a bulb) the circuit is broken and there is no pathway for the current and all the bulbs go out.

The challenge then is to figure out which one went out and replace it.

Two Bulbs in Series4. "Christmas Lights" are often wired in series just like the circuit you just tested.b) We can make a string of lights in series just like the "Christmas Lights." If you were given light bulbs that are designed to operate with a voltage of 14 volts, how many would you have to wire in series before it is connected to a 120-volt outlet?

Remember that the sum of the voltage gains equals the sum of the voltage losses. If the bulb is designed for 14 volts then 14x = 120 or x = 8.57. I will need nine bulbs. I had a bunch of 14 volt bulbs and wired them and it does work.

CAUTION! I would not recommend trying this unless you really know what you are doing. And DEFINITELY, DO NOT LET A STUDENT DO THIS!

A better suggestion: If you have an old 80 or 100 bulb string. Cut off a socket and strip the wires and then wire it to a 1.5 volt AA, C or D battery and watch it light up.

Two Bulbs in Series

4. "Christmas Lights" are often wired in series just like the circuit you just tested.c) If you have a one hundred bulb string set of lights that are wired in series, what is the voltage drop across each bulb?

ΣVGAIN = ΣVLoss

120 V = V1 + V2 + … + V100

120 V = 100VV = 1.20 volts

ΣVGAIN = ΣVLoss

The voltage drop across each bulb is 1.2 volts

Two Bulbs in SeriesSo, what can we do to make both bulbs bright again?

If we now put two batteries in series, connecting the negative terminal of the second with the positive terminal of the first, what happens?

AGAIN Note: In any complete loop around the circuit from the negative terminal of the battery, through the loads, to the positive terminal of the battery, and back to the negative terminal, that the sum of the voltage gains equal the sum of the voltage losses. There is still only one path for current to travel through the circuit.

The voltage gain is doubled, the current increases, and the bulbs return to their former brightness when one bulb was wired to one battery.

Two Bulbs in ParallelWhat is different when you wire two bulbs in parallel?

You might want to try this with one battery first.

Both bulbs are bright.V1 = V2 = VS

Trace the path taken by the current.

The current leaves the battery and it all goes through the ammeter illustrated.

e-

The current comes to a junction and splits, some going through Bulb #1 and the other going through Bulb #2.

The current rejoins at the next junction goes through the switch and returns to the battery. Note that there are two pathways between the junctions, Parallel circuit.

Two Bulbs in ParallelWhat happens when you unscrew bulb #1?

Bulb #1 goes out.Bulb #2 stays lit about the same.The current drops (roughly in half).VS = V2 and stay about the same as before bulb #1 was unscrewed.

What happens when you screw Bulb #1 back in and unscrew Bulb #2?

Bulb #2 goes out.Bulb #1 stays lit about the same.The current drops (roughly in half).VS = V1 and stay about the same as before bulb #1 was unscrewed..

How is a home wired? Using what you observed in series and parallel circuits, do you think that your house in wired in series or parallel? Explain your answer. Let’s look at the options.

If your house were wired in series, what would happen if one light bulb blew out or was turned off?

All the other lights and appliances would go out! That’s no good!

If your house were wired in parallel, what would happen if one light bulb blew out or was turned off?

All the other lights and appliances would stay on! That’s good!

How is a home wired?To be thorough, a circuit breaker or fuse is wired in series with all the outlets and lights that are wired in parallel to each other on a given circuit.

There are many circuits in a home that are wired in parallel with all the other circuits.

Each circuit has a circuit or fuse wired in series with all the outlets and lights on that circuit.

If one fuse blows or circuit breaker is tripped only the appliances on that circuit go out.

ResistanceWhen you place a meter long piece of copper wire in the circuit, there should be no significant change in the current.

The voltage drop across the copper wire, VCW, will be very small. At most VCW = 0.02 V

Copper wire is a good conductor so it is said to have little resistance.

The voltage drop across the light bulb, VB, will be close to VS.

ResistanceWhen you replace the copper wire with a meter long piece of nichrome wire, what is different?

It works best if you have 28 or 30 gage nichrome wire. You may be able to get a piece from your physics department in the high school.

The current in the ammeter goes down quite a bit.VNW becomes sizable usually more than VB.The bulb is barely lit or may not glow at all.VS = VNW + VB.

ResistanceNicrome wire is said to have a lot of resistance. What is resistance?

Resistance is the property of the wire that inhibits the flow of charge. It acts like friction.

Resistance (R) is the ratio of the voltage drop across the device to the current passing through the device.

R = V/I and its unit is called an ohm abbreviated Ω.Named after Georg Simon Ohm.

ResistanceAs you shorten the nichrome wire you will notice that:1) The current increases.2) VNW decreases.3) VB increases.4) The bulb finally starts to glow or get

brighter.

What qualities of a given wire affects its resistance?

Length, the longer the wire the greater the resistance.

Thickness or diameter, the thicker the wire the resistance is less.

ResistanceNichrome wire will glow and get hot with a great enough voltage and current. Nichrome wire is used to give off heat in a toaster, hair dryer or space heater.

This is fun to do. If you go to Radio Shack and get a cheap rheostat and replace the nichrome wire. As you turn the knob the same thing will happen when you changed the length of the nichrome wire.

A rheostat acts as a dimmer switch or a volume control by varying the resistance.

ResistanceThe symbol for a rheostat is:

A

The schematic is:

The arrow on the resistor indicates that you can change the position of the wire (arrow) and increase or decrease the resistance. This is a variable resistor.

Resistance

Ohm’s Law: V = IR

R = V/I resistance in ohms, Ω

For a resistor the resistance is constant.

Except for super conductors, all electrical objects have resistance.

An incandescent light bulb is not a resistor. As the filament heats up and glows the resistance increases.

Resistance

What properties of a wire affects its resistance?

Length. As the length increases the resistance increases. There is a direct proportion between resistance and length. R ~ L

Cross-section area (diameter). As the cross-section area increases the resistance decreases. This is and inverse proportion.R ~ 1/A Then R ~ 1/d2

Resistance

Joule’s Law P ~ I2

P = I2R Power is measured in watts, W

But V = IR Ohm’s Lawso P = I2R = I(IR) = IV

Also I = V/R so P = I2R = (V/R)2R = (V2/R2)R = V2/R

P = I2R = IV = V2/R

Series Circuits

VS = V1 + V2 = …

IS = I1 = I2 = …

Then: ISRS = I1R1 + I2R2

Source

1 2

But: IS = I1 = I2

So: RS = R1 + R2 = …

Parallel Circuits

IS = I1 + I2 = …

VS = V1 = V2 = …

Then: VP/RP = V1/R1 + V2/R2

But: VS = V1 = V2

So: 1/RP = 1/R1 + 1/R2 = …

1

2

Short CircuitWhen you do this section, only close the switch long enough to take the required readings. You are KILLING the battery!

What happens when you place the wire between the terminals of Bulb #1?

the current, I, increases.Bulb #1 goes out.Bulb #2 gets brighter.

VS stays the same or increases slightly.The wire gave a low resistance path around Bulb #1.

Short CircuitWhen you do this section, only close the switch long enough to take the required readings. You are KILLING the battery!

What happens when you place the wire between the terminals shown?

The current, I, increases a lot.Bulbs #1 & #2 go out.

VS goes to zero or close to it.The wire gave a low resistance path from one terminal of the battery to the other, mostly bypassing the bulbs.

Short CircuitWhen you do this section, only close the switch long enough to take the required readings. You are KILLING the battery!

What happens when you place the wire between the terminals shown?

the current, I, increases a lot.Bulbs #1 & #2 go out.

VS goes to zero or close to it.The wire gave a low resistance path from one terminal of the battery to the other, mostly bypassing the bulbs.

All the bulbs are the same and all the batteries are the same at 1.5 volts. None of the bulbs blow out.

In which case(s) will the bulb(s) be brightest?

A

1 2 3

B

4 5 6

All the bulbs are the same and the batteries are both 1.5 volts. The only difference between the two circuits is the wire that goes around bulb 5.

What are the voltages across each of the bulbs?0.5 V 0.5 V 0.5 V 0.75 V 0.75 V0 V

Which bulb(s) have the greatest current passing through it?

Greatest Greatest

Which bulb(s) have the least current passing through it?

Least

Magnetism

Shock Value

A magnet sets up are area around it that is called a magnetic field. Each magnet has two types of poles, North (N) and South (S).

Law of Magnetic Poles.

Opposite poles attract each other.

Like poles repel each other.

N S N S

N S S N

S N N S

It was discovered that when a magnetized needle was free to rotate that is always oriented itself in a North/South direction. The end of the needle that pointed in a northerly direction is the “North Seeking Pole” or the North Pole of the needle. From this idea magnetized needles were made free to rotate in a device called a compass. In our drawings the tip of the arrow is the North end of the compass needle.

North seeking poleNorth end of Needle

South end of Needle

Remember that a compass is a small magnet that is free to rotate. The end that points toward the Arctic pole of the Earth is the N pole of the compass.

Compasses that are placed near strong magnets can easily become reverse polarized. Any time you pick up a compass, make sure it is properly magnetized.

On many compasses the end that is supposed to point North has an “N” on it, is painted red or blue, or has an arrow point. Hold the compass and make sure that the proper end points in a northerly direction. If it does not, then it is reverse magnetized.

You can use a compass that is reversed magnetized as long as you realize that the painted end is now the S end of the compass needle and the N end is the other end. It still is a little magnet.

Good

Good

Reversed

This leads to an interesting question. Magnetically which pole is in the northern hemisphere of the Earth?

Geographic North Pole of the Earth

Geographic South Pole of the Earth

S

N

The Earth is like a big bar magnet with the magnetic South Pole in the Northern Hemisphere and the magnetic North Pole in the Southern Hemisphere.

Look at your compass. Opposite poles attract. The end of the needle that points in a Northerly direction is the North pole of the compass needle.

Shape of magnetic field illustrated by iron filings. Iron filings will identify the poles, but not whether they are North or South.

Compasses can also indicate the shape of the magnetic field, identify the poles and whether they are North or South.

Iron filings trace out patterns of magnetic field lines about a bar magnet. In the field are some compasses. The compass needle in only one compass is shown. Draw the needles with proper orientation in the other compasses.

N S

Where are the poles of the magnet?

Field lines outside a magnet go from the North pole of the magnet to the South pole of the magnet.

Iron filings trace out patterns of magnetic field lines about a bar magnet. In the field are some compasses. The compass needle in only one compass is shown. Draw the needles with proper orientation in the other compasses.

N SA compass with point tangential (along) a field line.

Horseshoe Magnet: This is just a bar magnet bent into the shape of a horseshoe.

N

S

When compasses are placed around the magnet we can get the idea of the shape of the magnetic field and identify the poles.

Magnets are stacked as shown at the right. When a compass is placed above the top magnet it points down. All the magnets are identical disk magnets. Label the poles on each of the four magnets in the stack.

S

NN

SNS

S

N

It was also discovered that when current passes through a wire that a nearby compass was affected. You will be asked to map the field around a current carrying wire. It is important to show the direction of the current. We adopted that the flow of electrons in a wire is the current. We are going to use arrows to show the direction of the magnetic field and the direction of the current. To differentiate, we will put and “I” for current next to the arrow that shows the direction of the current.  

I

We are going to examine the field around a wire. To show which way a compass points when it is placed above the wire we show the compass blocking the wire (at the right below) and if the compass is placed under the wire, we can see the wire on top of the compass (at the left below).

I

Now, what happens if you are holding a wire vertically and looking down from the top? The end view of a wire is a circle.

How do you show the current coming towards you? Think of an arrow that is coming towards you, what do you see first?

How do you show current going away from you? What part of the arrow do you see when it goes away from you?

.

X

The point!

The tail feathers!

I

I

So what is the shape of the magnetic field around a current carrying wire?

I

I

Top WireLooking from

the top.

I

Bottom Wire

I X I .Left Wire

Right Wire

In order to help us remember which way the field goes we will develop a hand rule. There are many different hand rules and if you go to different text books, they may hold their hand differently. We also use the direction of the flow of electrons as the direction of the current. Purists use the direction of flow of positive charges, conventional flow (blame Franklin), so the hand rule will be a little different (opposite hand). Physics books adopt one convention or the other. Electronics books usually use the flow of electrons.

We will use the LEFT hand and grab the wire with the thumb in the direction of the current and the fingers wrap around the wire in the direction of the magnetic field.

I X

Left Hand Rule:•Grab the wire with your left hand.•Thumb in the direction of current (electron flow)•Finger wrap around the wire pointing in the direction of the field.

I I .

I

Practice: Horizontal Wire.

Compass under the wire.

Fingers under the wire point to top of the page.

Compass on top of the wire.

Fingers on top of the wire point to bottom of the page.

I X

I

Practice

I

ccw

. Current out of page towards you.

I

No Current, I = 0

When bend wire into a coil, all the field lines in the coil point in the same direction. Thus there is a stronger field inside the coil.

Looking from the top.

.x

Current comes up on the right, moves to the left and goes down on the left.

Magnetic Field Lines Note that all the field lines inside the coil ALL point towards the top.

I

.x

.x

.x

.x

.x

.x

.x

.x

.x

.x

.x

.x

What happens if you have many coils close together?

The field inside the magnet gets very strong and poles form.

N

S

This coil system is called a solenoid.

If you put a soft iron core in the solenoid you then have an electromagnet.

.x

.x

.x

.x

.x

.x

.x

.x

.x

.x

.x

.x

N

S

Left Hand Rule for Solenoids:•Fingers wrap around solenoid•Finger in direction of current•Thumb points to North Pole

N

.x

.x

.x

.x

.x

.x

.x

.x

.x

.x

.x

.x

N

S

Field lines outside the solenoid go from the North pole to the South pole.

Field lines inside the solenoid go from the South pole to the North pole.

This field looks like the field around a bar magnet. Field lines inside the bar magnet must also go from South to North, we just can’t put a compass in there.

THE END!