Electric Charge & The Atom

An object is charged if it has gained or lost electrons.

http://commons.wikimedia.org/wiki/File:Stylised_Lithium_Atom.png

Like Charges Repel & Opposites Attract

http://www.datasync.com/~rsf1/eas.htm

http://blog.taser.com/how-does-a-taser-work-electricity-101/

The removed images show that two positive charges attract, two negative charges attract, while a positive and a negative charge will attract each other.

There is no need to load the image or read the pages linked below.

Electric Fields

An electric field is a region of space in which electrical forces act.

Electric charges (eg ____________) create electric fields around them.

http://www.madsci.org/posts/archives/2004-09/1095325697.Es.r.html

The removed image showed a woman standing on top of a mountain with her hair standing up. This was because there was a strong electric field between the ground and the clouds.

Moments after her and her brother left the spot it was struck by lightning.

So NEVER stay around if your hair stands up go somewhere safe!

Lightning

Lightning occurs during a thunderstorm when cloud movement causes a negative charge to build up in the cloud, which 'induces' a positive charge beneath it on the ground.

If these charges become large enough, the electrons will 'jump' to the Earth, causing lightning.

Thunder is the sound of lightning, and occurs at the same time, but is heard separately since sound travels so much slower than light.

Most lightning occurs within or between clouds.

http://upload.wikimedia.org/wikipedia/commons/thumb/b/b1/Lightning_hits_tree.jpg/220px-Lightning_hits_tree.jpg

Is this a Lightning Rod?

Lightning Rod?

A lightning rod provides an 'easy' way for the electrons to pass through a building or other structure, instead of it passing through the material it's made of, which often results in fires.

Many famous structures, including Osaka castle (twice in the 1600s) have been destroyed by lightning. The Eiffel tower lost its top in 1902.

One man has survived seven lightning strikes.

Never shelter under a tree during a thunderstorm.

Lightning Videos

Watch the slow motion lightning strike here:

http://en.wikipedia.org/wiki/Lightning

Creating a Current

Instead of passing through the atmosphere, electricity can run through a conductor.

Electrons are very small, so physicists measure them in Coulombs.

http://commons.wikimedia.org/wiki/File:Stranded_lamp_wire.jpg

How do we Measure Charge

Charge is measured in Coulombs.

One Coulomb (C) = 6 * 1018 electrons (but not by definition).

1. How many electrons in half a Coulomb?
2. What is the elementary charge (on one electron)?

The unit of the Coulomb was used before the elementary charge was known.

Image: Charles-Augustin de Coulomb
http://upload.wikimedia.org/wikipedia/commons/thumb/5/59/Charles_de_coulomb.jpg/250px-Charles_de_coulomb.jpg

Electric Current

Current is a measure of how much charge are flowing past a given point in a conductor per second.

Its units are Amperes.

One Amp = 1 C/s.
1. If five Coulombs flow past a point in a wire in ten seconds?
a) what is the current in the wire?
b) how many electrons flow past in two seconds?

Image: Charles-Augustin de Coulomb
http://upload.wikimedia.org/wikipedia/commons/thumb/5/59/Charles_de_coulomb.jpg/250px-Charles_de_coulomb.jpg

1) 12 * 1018 electrons pass a point in one second. What is the current in Amps?2) 3 * 1018 electrons pass a point in one second. What is the current in Amps?3) How many Coulombs pass a given point in three seconds if the current is five Amps? Harder: how many electrons is this?
4) Draw a diagram to explain an analogy between electric current and either
a) a bakery delivering bread to a grocery store
b) cars on a freeway
c) water flowing down a river5) An average lightning bolt carries 30 000 Amps and transfers 15 Coulombs.
a) How many electrons is this?
b) How long does the lightning bolt last?
6) A new Macbook Air contains a 7150mAh battery. If it takes five hours to charge the battery, calculate the average charging current.
Optional extension: Do some research and explain why we calculated the 'average' current. Sketch a charging curve for a (lithium ion) battery.

Quick Review

One Coulomb is 6*1018 electrons. I = Q/t a) How many Coulombs in 18 *1018 electrons?b) How many Coulombs in 3 *1018 electrons?

c) If Three Coulombs pass a point in 1.5 seconds, what is the current in the wire?d) Why do we use Coulombs instead of electrons? Give another example of a unit representing a number of objects.

Conductors and Insulators

Make the bulb light up.Materials: 1 wire, 1 bulb and 1 battery.

Make a circuit to test whether or not something conducts electricity.
A) Draw the circuit in your book (without using symbols). It should be possible for another student to reconstruct it the same way.
B) Test the following for conductivity:
glass, iron, plastic, aluminium, copper, wood, graphite, zinc.
Complete everything on the worksheet.

Voltage

Electrons carry electrical energy. Work is done to move electrons through a conductor, so the electrons lose potential energy.

Potential difference is the difference in energy between one point and another. Potential difference is measured in Volts.

1 Volt = 1 Joule per Coulomb (J/C or V)

E.M.F

e.m.f = electromotive force is the voltage generated by a power source (eg ______). It drives the charge around a circuit.

e.m.f and P.D. Measure the same quantity (voltage) so of course have the same units.

Voltmeters

Voltmeters measure the potential difference between two points in a circuit.

It is difficult for electricity to get through a voltmeter (we say its __________ is high next class) so that it has minimal disruption on the circuit.

This image is worth loading to show how voltmeters are connected.http://www.gcsescience.com/pe5.htm

Resistance

Energy is required to push electrons through most conductors.

Resistance measures how difficult it is for electrons to get through.

Resistance is measured in Ohms ().1. Calculate the resistance of a light bulb which uses three Amps from a 9V battery.2. Calculate how many Amps will flow through a heater with an effective resistance of 10 when connected to the Japanese 100V mains.

Image: Georg Simon Ohm (Wikipedia)
http://en.wikipedia.org/wiki/Georg_Simon_Ohm

More Resistance Problems

1. Tammy takes a flashlight to Phuket. It runs on two 1.5Volt AA batteries (total 3V) and draws a current of 0.2A. Calculate the resistance of the LED (light source).

2. A kettle connected to the Japanese mains voltage (100V) draws five Amps. Calculate its resistance.

3. How much current would the kettle use if it were connected to the mains power in Europe (240V). Why might this be a problem?

4. A small solar-powered motor with an effective resistance of 100 uses 0.5 Amps. What is the voltage of its power source?

Length and Thickness

The resistance of a resistor (or anything else) is greater if the resistor is thinner, since there is less area for the electrons to pass.

The resistance is greater if the resistor is longer, because the electrons need to travel further through the resistor.

Example

A length of metal has a resistance of 10. It is cut in half, widthwise, and the two ends are placed lengthwise. What is its new resistance?

Pylons

Why are pylons made of aluminium, when copper is a better conductor?

http://www.flickr.com/photos/italianstylelover/4720689749/sizes/z/in/photostream/

Photo of pylons under Mt Fuji unnecessary.

Series and Parallel Circuits.

Current that leaves the battery must all come back. It can not disappear. If 10 Amps leaves, 10 Amps must come back.

Voltage is 'used up'.If a Coulomb leaves a battery with 12V, it must 'use it all up' in the circuit.
ie. If the EMF of the power source is 12V, the sum of the PD of all components in one circuit must be 12V.

Series Circuit

In a series circuit, the electricity can only flow one way.

The electricity goes through each bulb.

The current is the same everywhere in the circuit.

The P.D. Of all the components equals the EMF of the power supply.

Parallel Circuit

In a parallel circuit, each bulb has its own path to the power supply.

The electricity goes through each bulb only once.

The current from the power source is shared between all the different branches.

The PD of each component is the same.

Current and Voltage

A1A3A2

V1

V1

12V

3

MeterReading

V1A1A2A3

Current and Voltage

A1A3A2

V1

V1

V2

12V

All bulbs are (equivalent) 4

MeterReading

V1V2A1A2A3A4

A4

Current and Voltage

A1A3A4A2

V1

V3

V1

V2

12V

All bulbs are (equivalent) 6

MeterReading

V1V2V3A1A2A3A4

Resistors in Series

Two resistors in series are just like one long resistor.

RT = R1 + R2 + R3...Calculate the current
in this circuit.

http://en.wikibooks.org/wiki/GCSE_Science/Parallel_and_series_circuits

1

2

Resistors in Parallel

With two resistors in parallel, there are two paths for the electricity to travel through, so it is easier, so the resistance is lower.

Calculate the resistance of a 10, 1 and 5 resistor, all in parallel.ANS: 0.77.Don't forget the final reciprocal at the end!

Note for IGCSE students:IGCSE only requires two in parallel.

http://www.learnabout-electronics.org/resistors_20.php

Extension Exercise

Five 10 resistors are connected in parallel to a 10V power source.

A) Calculate the current through each resistor.
B) Calculate the current in the whole circuit.
C) If all the resistors were to be replaced with one resistor (to save space) what would its resistance be?
D) Can you find a connection between the total resistance and the additional resistances? What if they were different resistances?

How is Power Different in Different Countries?

http://bionicbong.com/wp-content/uploads/2011/03/socket.jpg

http://img.archiexpo.com/images_ae/photo-m/power-socket-50417-2252991.jpg

1.

2.

3.

Photos of different power sockets in different countries unnecessary.

Power

Five Amps flow through a 12 Volt car bulb. How many Joules flow through the light bulb each second?

This is called the power. Power measures how much energy something uses or produces per second.

A Joule is a measure of energy.One Joule is about the amount of energy required to lift a calculator out of a bag onto a desk.

Image: James Joule (Wikipedia)

Energy and Power

Power (Watts) = Voltage (Volts) * Current (Amps)Mr Duffield bought a small heater in Japan. It was labelled 600W. Japan's mains voltage is 100V.
1) Calculate the current through the heater element in Japan.
2) Calculate the resistance of the heater element in Japan.

He then took it to Taiwan, where the voltage is 110V.
3) Assuming the resistance stays the same in Taiwan (?), calculate the new current through the heater.
4) Calculate the new power of the heater in Taiwan.
5) How many extra Joules of energy must be dissipated by the fan in the heater each second in Taiwan (instead of Japan)? Is this likely to be dangerous?
6) How many Joules of heat does the heater produce in Taiwan in ten seconds?
7) Repeat calculations for New Zealand (240V). What will happen if the heater is used there?

Iphones and Fridges

Does an iphone use more power than a fridge? Some studies have claimed this, but the results have been disputed. Research this and explain your conclusion.

http://theweek.com/article/index/248273/your-iphone-uses-more-energy-than-a-refrigerator

Kilowatt Hours

One Kilowatt hour (kWh) is a measure of energy.

It is the amount of energy something which has a power of 1kW uses in one hour.

It is a useful unit for the general public since electricity is sold in kWh. In Japan 1 kWh costs approximately 20 Yen.

How many Joules in 1 kWh?

If a vending machine uses 3 000 kWh per year, calculate its power in Watts (assuming it is on all the time).

ANS: ~ 340 Watts.

Why Different Voltages?

Electric current interferes with the body's nerve system, and as a resistor the body produces heat (thus shocks burn).

Assume a person has a resistance of 1000 Ohms (it varies a lot person to person, and depends on the skin in particular).1. Calculate how much current they will have pass through them in Japan (100V).

2. Calculate how much current the same person would have pass through them if they were electrocuted in Australia (240V). 3. On a cold winter's day a particular house uses 2000 Watts. Calculate the current through the wires coming into the house in Japan and Europe. Why might a higher current be more dangerous?
4. Draw a table to show the advantages and disadvantages of high and low voltage mains electricity.