Charge and Current The comic may go over some students’ heads… I taught this topic immediately after static electricity, so it made sense in that context. It refers to the fact that, due to historical accident in defining ‘positive’ and ‘negative’, conventional current is said to flow from positive to negative, even though in a wire the electrons flow from negative to positive.

What is Charge? Nobody really knows! Best to admit this from the outset…

But we do know that… It comes in two types, which we call positive and negative Similar charges repel; opposite charges attract Charge is ‘carried’ by electrons (negative) and protons (positive) Although a proton is much bigger than an electron, they both carry the same amount of charge (but opposite)

Atoms To remind students what the atom is composed of, and where the electrons fit in. A few different ways of depicting the atom.

Atoms (negative) (positive) Students may wish to draw a diagram of the atom in their books, to remind themselves of the names, charges and locations of the subatomic particles.

You can’t have these on their own – they are all carried by other things: Charge Beauty Confidence Momentum possessed by electrons and protons possessed by people, paintings, ornaments, flowers, etc. possessed by people possessed by objects ‘Charge’ is a rather abstract idea. An electron is not charge; it carries charge. You cannot have pure charge on its own any more than you can have pure mass or pure beauty – all are characteristics of objects.

If you are into a bit of cross-curricular experience (I am), you might like to tell the story of Helen of Troy. The following four slides are all depictions of her. In ancient Greek myth, Helen of Troy, daughter of the god Zeus, was said to be the most beautiful woman in the world. She had many suitors, and married King Menelaus of Sparta. She was later abducted by (or some versions of the story say she ran away with) Paris, prince of Troy. A fleet of a thousand ships was launched to attack Troy and get Helen back – and so the Trojan War began. Scientist and writer Isaac Asimov came up with the tongue-in-cheek unit of beauty, the Helen: the beauty required to launch a thousand ships. Mere mortal women only have a fraction of a Helen’s-worth of beauty each.

Helen of Troy played by Diane Kruger in the 2004 film ‘Troy’.

Evelyn DeMorgan: Helen of Troy, 1895

Dante Gabriel Rossetti: Helen of Troy, 1863.

Anthony Frederick Sandys: Helen of Troy, 1867 Anthony Frederick Sandys: Helen of Troy, 1867. (Presumably she looks miserable because of all the trouble she has caused.)

Beauty Charge Carried by electrons and protons Carried by people Measured in coulombs Takes a lot of electrons (about 6,200,000,000,000, 000,000!) to carry one Coulomb of charge between them Carried by people Measured in Helens Takes a lot of people (about 1000 ordinary women!) to carry one Helen of beauty between them

It takes many women to make a Helen… x10 makes up 1 Helen (1 H) 1 thousandth of a Helen (1 mH)

It takes many electrons to make a coulomb… 0.00000000000000000016 of a Coulomb x a gazillion (or so) makes up one Coulomb

i.e. One coulomb of electric charge is one GREAT, BIG, HONKING quantity of charge, while one electron carries a pathetically minute quantity of charge.

Typical Amounts of Charge When you rub a balloon against your hair, you build up around a trillionth of a coulomb of unbalanced charge on the balloon (and on your hair). The balloon has gained around one extra electron for every ten trillion it had already. The Van de Graaff generator contains more unbalanced charge: around 0.00005 C. About 20 C of charge are transferred in a lightning strike. I did this lesson immediately after a topic on static electricity, so the idea of unbalanced charge and the Van de Graaff was still fresh in the students’ minds.

Neutral Matter A cubic centimetre of copper contains a total of about 750,000 C of charge altogether (half negative – in electrons; half positive – in protons). About 13,000 C of this is carried by free electrons. The point being that in ordinary, neutral matter, there are huge amounts of charge, but the positive and negative are exactly balanced. They are like two enormously strong but perfectly matched wrestlers locked in combat: you don’t notice anything because they do not appear to be doing anything. But even a tiny amount of unbalanced charge has very noticeable effects.

Charge Questions We don’t really know what charge actually is – but what do we know about it? What are the tiny particles that carry charge? Which is positive and which is negative? Where are they each found? What is the unit of charge (give the full word and also the symbol)? Copy and use your own more interesting adjectives to fill in the gaps: The amount of charge in a coulomb is ______ [very large]. The amount of charge on an electron is ______ [very small]. Add your own illustrations to this piece of work (optional).

Current Current is the rate of flow of charge (i.e. how much charge is flowing past per second) Animation: flowing electrons Animation: electrons in a circuit So current is measured in Coulombs per second (C/s) BUT the C/s has its own particular name: the ampère, amp, or A. For the first animation, you could use the one on the webpage: http://www.rkm.com.au/ANIMATIONS/animation-electrical-circuit.html The second animation needs to be linked to the file ‘Electrons in Circuit Animation.gif’. (If played in QuickTime, the electrons are animated and go round the circuit.) It illustrates the point that the battery or power supply does not supply electrons; the electrons are already there in the wires and components. The power supply simply provides the energy to move the electrons around the circuit. They act rather like a bicycle chain: a force on one part of the chain causes the entire chain to move at once. (A common misconception is that the power supply contains a store of electrons, and these come rushing out of the supply when the circuit is switched on.)

Moving Electrons When electrons carry charge along a wire, they move slower than the minute hand on a clock. Animation: free electrons flowing The animation needs to be linked either to ‘Free Electrons Flowing animation’ (I can’t find who to credit this to – I can’t remember where I got it from originally) or to one of the animations from the following websites: http://www.members.shaw.ca/len92/electrons_moving_wire.gif or http://micro.magnet.fsu.edu/electromag/java/filamentresistance/ or http://www.furryelephant.com/player.php?subject=physics&jumpTo=ee/3Ms1. One way or another, all of these show free electrons moving when a current flows. It could be confusing for some students – though for those familiar with Chemistry and electronic structure it may be quite enlightening. I’ve read that there is a way to directly measure the drift velocity of charges in a conductor, though I haven’t tried it myself and I can’t find a video which shows it. Connect metal electrodes to the ends of a large salt crystal (NaCl), then heat it to 700 degrees C and apply high voltage to the electrodes. At this temperature the salt becomes conductive, but as electrons flow through it they discolour the crystal, and a wave of darkness moves across the clear crystal. The velocity of this slow-moving wave can be measured. (And if you double the current, the speed of the wave doubles.) This demonstration appears in: Physics Demonstration Experiments (two volumes) H. F. Meiners, ed. Ronald Press Co 1970 .

Relationship between Charge, Current and Time Write a little three-word formula showing the relationship between these three quantities. Charge = current x time (in C) (in A) (in s) Q t I

Example questions on current Charge (C) Current (A) Time (s) 5 2 0.4 1 20 0.5 50 250 3 60 A circuit is switched on for 30s with a current of 3A. How much charge flowed? During electrolysis 6A was passed through some copper chloride and a charge of 1200C flowed. How long was the experiment on for? A bed lamp is switched on for 10 minutes. It works on a current of 0.5A. How much charge flowed? Questions posed in ‘Charge and Current.ppt’.

Example questions on current Charge (C) Current (A) Time (s) 10 5 2 0.4 1 20 0.5 40 50 0.2 250 180 3 60 A circuit is switched on for 30s with a current of 3A. How much charge flowed? During electrolysis 6A was passed through some copper chloride and a charge of 1200C flowed. How long was the experiment on for? A bed lamp is switched on for 10 minutes. It works on a current of 0.5A. How much charge flowed?

Example questions on current Charge (C) Current (A) Time (s) 10 5 2 0.4 1 20 0.5 40 50 0.2 250 180 3 60 A circuit is switched on for 30s with a current of 3A. How much charge flowed? Q = I x t = 3 x 30 = 90 C During electrolysis 6A was passed through some copper chloride and a charge of 1200C flowed. How long was the experiment on for? A bed lamp is switched on for 10 minutes. It works on a current of 0.5A. How much charge flowed?

Example questions on current Charge (C) Current (A) Time (s) 10 5 2 0.4 1 20 0.5 40 50 0.2 250 180 3 60 A circuit is switched on for 30s with a current of 3A. How much charge flowed? Q = I x t = 3 x 30 = 90 C During electrolysis 6A was passed through some copper chloride and a charge of 1200C flowed. How long was the experiment on for? t = Q / I = 1200 / 6 = 200 s A bed lamp is switched on for 10 minutes. It works on a current of 0.5A. How much charge flowed?

Example questions on current Charge (C) Current (A) Time (s) 10 5 2 0.4 1 20 0.5 40 50 0.2 250 180 3 60 A circuit is switched on for 30s with a current of 3A. How much charge flowed? Q = I x t = 3 x 30 = 90 C During electrolysis 6A was passed through some copper chloride and a charge of 1200C flowed. How long was the experiment on for? t = Q / I = 1200 / 6 = 200 s A bed lamp is switched on for 10 minutes. It works on a current of 0.5A. How much charge flowed? Q = I x t = 0.5 x 600 = 300 C

Current Practical Use the bulbs, leads and cells to build a simple series circuit. Draw the circuit diagram in your book. Measure the current. (Where should you put the ammeter?) Mark the current(s) on your diagram. Use the equipment to answer the following questions: If you have a fixed number of cells of a particular voltage each, do they always give exactly the same current or can you make the current more or less? Draw diagrams of the circuits you make. Mark the current(s) on your diagrams.

Reminder: how to draw circuit diagrams

Reminder: how to draw component symbols Cell

One day you might be designing your own guitar amps…