1.1 Charge and its movement

I declare: solidlikelyshaky

1. Two charges, one atomexplanation

   Explain electric charge: it comes in two signs, like charges repel, unlike charges attract, and ordinary matter is neutral.

   The rubric draft

   Proven if:

   • the learner names the two kinds of charge, positive and negative;
   • they say that two like charges repel and two unlike charges attract, and show it on an example (two balloons rubbed on the same sweater that push apart, a rubbed balloon that clings to the sweater that rubbed it);
   • they explain that ordinary matter is neutral because each atom carries, in its nucleus, as many positive protons as it has negative electrons around it;
   • they explain that an object becomes charged when electrons pass from one body to another, for example by rubbing: the one that gains them becomes negative, the one that loses them becomes positive.
2. Conductor and insulatorexplanation

   Explain why a metal has free electrons and why an insulator does not.

   Assumes: Two charges, one atom

   The rubric draft

   Proven if:

   • the learner explains that, in a metal, the outermost electrons of each atom are so loosely bound that they leave their atom and wander through the whole metal, between fixed positive ions: these are the free electrons;
   • they explain that, in an insulator (glass, rubber, plastic, dry air), each electron stays bound to its atom, so that almost none is free to move;
   • they point out that free electrons are not extra electrons: the metal stays neutral overall;
   • they conclude that electric current is a drift of these free electrons as a whole, and that this is why a metal conducts and an insulator does not.
3. A closed loopexplanation

   Explain why a current flows only in a closed loop, from the source back to the source, and what happens when the loop opens.

   Assumes: Conductor and insulator

   The rubric draft

   Proven if:

   • the learner says that current flows only in a continuous conducting path that leaves one terminal of the source and comes back to its other terminal: a closed loop;
   • they explain that the source does not make the electrons but pushes those already in the conductors, so that what leaves one terminal must be able to come back in through the other;
   • they explain that a single opening, anywhere in the loop, stops the current everywhere in the loop, not only at the break;
   • they take the example of the switch, an intended opening: closed, it closes the loop and the lamp lights; open, it opens the loop and the lamp goes out, even though it stays connected to the source by the other wire.
4. The two directions of currentexplanation

   Explain conventional current and electron flow, and why the two conventions coexist without disturbing the calculations.

   Assumes: Two charges, one atom, Conductor and insulator and A closed loop

   The rubric draft

   Proven if:

   • outside the source, conventional current goes from + to −; electrons go from − to +;
   • the origin: the convention was fixed before the electron was known;
   • why it does no harm: a negative charge moving one way is equivalent to an equal positive charge moving the other way; the calculations give the same result;
   • which one is used where: schematics and component arrows (the diode) follow conventional current.
5. One carrier per mediumexplanation

   Name what carries the current in a metal, electrons, and in an electrolyte (the cell, the pack, salt water), ions, and say which way each one goes.

   Assumes: Conductor and insulator and An ion is charged (assumed on entry)

   The rubric draft

   Proven if:

   • the learner says that, in a metal such as copper wire, the current is carried by the free electrons;
   • they say that, in an electrolyte such as salt water or the inside of a pack cell, the current is carried by positive ions and negative ions, not by electrons;
   • they give the direction of each: in the wire, outside the source, electrons go from the negative terminal toward the positive terminal; in salt water between two electrodes connected to a source, positive ions go toward the negative electrode and negative ions toward the positive electrode, so in opposite directions;
   • they do not carry this rule over to the inside of a cell that is discharging, where the reaction pushes the ions, and where positive ions go toward the positive terminal.
6. A current of ions inside the cellexplanation

   Explain what happens inside a cell: the reaction that separates the charges, the ions crossing the electrolyte, the electrons going round through the circuit — a battery is a current of ions inside.

   Assumes: One carrier per medium, Redox exchanges electrons (assumed on entry) and A closed loop

   The rubric draft

   Proven if:

   • the reaction is a redox reaction: electrons are given up at one electrode and taken up at the other;
   • electrons do not cross the electrolyte: they go round through the external circuit;
   • in the electrolyte, ions migrate from one electrode to the other and close the loop;
   • the conclusion stated: inside the cell, the current is carried by ions.
7. Where the movement is hindereddiagnosis

   Diagnosis: a contact that heats up, or a cell that no longer delivers in the cold — say which movement of charges is hindered, and where.

   Assumes: A current of ions inside the cell and Conductor and insulator

   The rubric draft

   Proven if:

   • for the case set, the learner names the fault: in the contact, electrons get through badly; in the cold cell, ions migrate badly;
   • for the contact, they locate the cause: an oxidized, loose or too small surface makes a resistance where there should be almost none, and that resistance heats up when the current goes through it;
   • for the cell, they locate the cause inside the cell, in the cold electrolyte where the ions move slowly, not in the wires;
   • they describe the step that confirms: for the contact, measure the voltage drop across it under load and compare it with that of a sound contact; for the cell, warm it up and measure its voltage under load again.