CurrentCurrent, or electric current, is the rate of flow of electric charge through a medium. This charge is typically carried by moving electrons in a conductor such as a wire. It can also be carried by ions in an electrolyte, or by both ions and electrons in plasma.

Simple terms: Current is what flows on the wire like water flowing through a hose.

Current flows from negative to positive on the surface of a conductor. It is measured in amperes or amps.

Circuits

Video: http://www.youtube.com/watch?v=9iQhL3VTDb8

There are several kinds of circuits; however, we will only discuss the simple circuit.

Simple Circuit

A simple circuit is fund everywhere in the world, in homes, offices and other places.

The simple circuit requires three things for it to work:

1. A source of electrical potential difference or voltage2. A conductive path3. An electrical resistor

Despite the materials being simple, it is quite useful and common. They exist in flashlights, ceiling lights, doorbells, and many kitchen appliances. Below is an animation of what goes on in a simple circuit where the electrical energy is shown as gravitational potential energy (GPE).

In the animation you can see:

The battery or source is represented by an escalator which raises charges to a higher level of energy

As the charges move the resistor, represented by the paddle wheel, they do work on the resistor and as a result, they lose their energy

By the time each charge makes it back to the battery, it has lost all energy given by the battery

As the charges move through the wire, they lose no energy The potential drop (negative potential difference) across the

resistor is the same as the potential rise (positive potential difference) across the battery. This demonstrates that a charge can only do as much as was done on it by the battery.

The charges are positive so this is a representation of Conventional Current (the apparent flow of positive charges)

The charges are only flowing in one direction so this would be considered direct current

VoltageVideo: http://www.youtube.com/watch?v=Lklai_KXGxg

Also called:

Potential difference Potential drop Electrical potential difference Electrical potential energy

It is the difference in electrical potential between two points, or in the difference in electrical potential energy per unit charge between two points.

As the monkey does work on the positive charge, he increases the energy of that charge. The closer he brings it, the more electrical potential it has. When he releases the charge, work gets done on the charge which changes its energy from electrical potential energy to kinetic energy.

Every time he brings the charge back, he does work on the charge. If he brought the charge closer to the other object, it would have more electrical potential energy. If he brought 2 or 3 charges instead of one, then he would have had to do more work so he would have created more electrical potential energy. Electrical potential energy could be measured in joules just like any form of energy.

Since the electrical potential energy can change depending on the amount of charge you are moving, it is helpful to describe the electrical potential energy per unit of charge. This is known as electrical potential.

Electrical potential – at a point within a definite space is equal to the electrical energy (measured in joules) at that location divided by the charge there (measured in coulombs)

ResistanceResistance is how much an object resists the flow of electrons. It is the inverse of conductance. It measures how difficult it is for electrons to flow through a material. It is measures in ohms.

Think of current like water flowing through a hose, if there is nothing in the hose, then the water will flow freely. However, if there is a something blocking the hose such as leaves, small animals, pebbles, etc. then it will be harder for the water to flow through.

Conductors, Insulators and Resistors

1. Conductor – a material which contains movable electrical charges. In metallic conductors such as copper and aluminum, the movable charged particles are electrons

2. Insulator – a material that is a poor conductor of electricity.- An insulator, such as ceramic or rubber, has high

resistance and poor conductance. A metal has low resistance and high conductance

3. Resistors – objects that are designed to have a specific resistance so that they can dissipate electrical energy or modify how a circuit behaves

Objects such as wires that are designed to have low resistance so that they transfer currents with the least loss of electrical energy are called conductors. Objects that are designed to have a specific resistance so that they can dissipate electrical energy or otherwise modify how a circuit behaves are called resistors. Conductors are made of highly conductive materials such as metals, in particular copper and aluminum. Resistors, on the other hand, are made of a wide variety of materials depending on factors such as the desired resistance, amount of energy that it needs to dissipate, precision and cost.

Factors Affecting Resistance

1. The nature of the material – some materials are better conductors than others, causing less resistance. The more tightly an atom holds on to its outermost electrons the harder it will be to make a current flow. The electronic configuration of an atom determines how willing the atom will be to allow an electron to leave and wander through the lattice. If a shell is almost full, the atom is reluctant to let its electrons wander and the material it is in is in an insulator. If the outermost shell (or sub-shell with transition metals) is less than half full then the atom is willing to let those electrons wander and the material is a conductor

2. The thickness of the conductor/wire – the larger the wire, the less resistance; the bigger the cross sectional area of the wire the greater the number of electrons that experience the ‘electric slope’ from the potential difference. As the length of the wire does not change each cm still gets the same number of volts across it – the potential gradient does not change and so the average drift velocity of individual electrons does not change. Although they do not move any faster there are more of them moving, so the total charge movement in a given time is greater and current flow increases. This does not give rise to a straight line graph as cross sectional area is inversely proportional to resistance not directly proportional to it.

3. The length of the conductor/wire – the longer the wire, the more resistance; if you take a wire of different lengths and give each a particular potential difference across its ends. The longer the wire, the less volts each cm of it will get. This means that the ‘electric slope’ that makes the electrons move gets less steep as

the wire gets longer, and the average drift velocity of electrons decreases. The correct term for this ‘electric slope’ is the potential gradient. A smaller potential gradient (fewer volts per meter) means current decreases with increased length and resistance increases.

4. The temperature – heat affects resistance, the higher the temperature the higher the resistance. When you cool a certain material to a the point where it reaches its critical temperature, its resistance would equal zero. But if you increase its temperature, you’d give more energy to its atoms, this increasing the number of collisions between them resulting in increasing the resistance

Ohm’s lawOhm’s Law describes mathematically the relationship between current, voltage (potential difference) and resistance. The more potential difference you have, the greater your current is going to be. The more resistance a circuit has, the lower the current is going to be. The following equation describes Ohm’s Law. It holds true for any circuit as long as temperature does not change.

V=IR I=VR

∨R=VI

I is the current through the conductor in units of amperes;

V is the potential difference measured across the conductor in units of volts; and

R is the resistance of the conductor in units of ohms.

The potential difference (voltage) is proportional to the current through it. The current is indirectly proportional to resistance.

VRI