ELECTRICITY
Is an invisible force. The force is the attraction or opposition between charges. I am not going to get into the electron and proton in the atom but you should have some understanding of how electrons flow through a conductor. The main principal behind current flow or the flow of electrons through a wire is the potential difference between charges. As we are aware Magnets have north and south poles and if we bring the same poles together there is a force that is trying to repel them but on the same note if we use opposite sides of a magnet the will attract. On a battery we have a positive and a negative terminal ( opposites) . If we were to attach a wire to one end of the battery and bring it close to the other end of the battery, as soon as we were close enough where the potential difference ( voltage ) was able to overcome the resistance of the air, electrons would flow or attract and we would have a spark. Electrons with a path to travel will be attracted to the opposite terminal and repelled from the alike terminal.

Electricity is energy that can be transported.
An electric circuit consists of an energy source, such as a battery or power supply, and interconnected electrical components implementing a useful function. The connections are formed by wires, also known as conductors, which are made of materials such as copper or some other metal that can conduct electricity.
Electrical charge transported across a conductor is called electric current. Charge is carried by electrons, which are negatively charged, or by positively charged ions in the conductor.

Current
Current is the intensity of the flow of charge in electric circuit. Between two points in a circuit, electrons flow from the more negatively charged point toward the one that is more positively charged. Positive charges, sometimes called holes, move in the opposite direction. When we measure how much current is flowing through a wire, it is based on the number of electrons flowing past that point in one second. There is a unit of measure called the Coulomb that enables us measure the amount of charge an object has (e.g. an electron). Since there are billions upon
billions of electrons flowing through the wires, we instead measure the charge with the Coulomb, which is 6,240,000,000,000,000,000 (6.24 billion-billion) electrons.
When one Coulomb of electrons passes through a wire in one second, that is one Ampere of current. Ampere is the basic unit of electric current. It is sometimes referred to as amps. When writing down a value of current, it is usually abbreviated with an "A" (e.g. 1 A = 1 Ampere).
We use an ammeter to measure electric current.
By convention, current flows in the direction of holes, which is opposite to the direction of electron flow.
Voltage
Named after Alessandro Volta. Fundamentally, the volt is a measure of the work needed to move an electrical charge. We can also use the terms pressure or force to explain voltage. The symbol for the potential difference is V. Sometimes the symbol E is used for emf. (Electro motive force).
If you wanted to measure how much voltage a circuit or battery had, you would use a voltmeter.
Voltage (electromotive force) is associated with any two points in the circuit, and represents the difference in electrical potential between those points. Stated differently, voltage is the electric force that causes electrons to flow in a circuit. Voltage is defined as a relative quantity. The basic unit of electromotive force is the Volt.
In a typical digital circuit, the lowest possible voltage is called ground and is arbitrarily assigned 0 volts. In most existing digital circuits, the highest possible voltage value is defined not to exceed 5.5 volts (industry is starting to move toward a 3.3-volt power supply).
In digital systems, we assign logic 1 to "high" voltages and 0 to "low" voltages, but these assignments are somewhat arbitrary. For TTL technology of the kinds described in this book, a voltage in the range of 0 to 0.4 volts is interpreted as logic 0, while 2.5 to 5.5 volts is interpreted as a logic 1. Voltages outside these ranges are not guaranteed to be interpreted as either a 0 or a 1.
The fundamental concepts of electricity can often be described by analogy with water. The greater the electrical potential, the larger the voltage, and the greater the force on the flow of the charge-carrying electrons. Think of a waterfall. A large voltage corresponds to a waterfall of great height. As a water molecule flows "downhill," a good deal of pressure is exerted on it by gravity and the force of water behind it. By analogy, water molecules correspond to electrons and electrical current corresponds to the speed of the water flow.
Suppose that the voltage difference is 0, so that both points in the circuit are at the same potential. In this case, the water is a stagnant pool, with no water flow, and there is no current. Given a waterfall of only modest height, the water trickles slowly downhill. This is analogous to a small current. But if the waterfall is of a great height, the flow of water will be forceful and the current is large.

Voltage drop
With current I through a resistance, by ohm's law the voltage across R is equal to I x R .
Now that we know the current is flowing through the circuit We can calculate the voltage drop across each resistor buy applying ohm's law.
V1 = I x R1 , 1amp x 4 ohms = 4V Voltage drop of 4 volts from A to B
V2 = I x R2 , 1amp x 6 ohms = 6 V Voltage drop of 6 volts from B to C
Total voltage drop 10 V The IR voltage across each resistance is called voltage drop the total of all combined voltage drops will always match the exact supply voltage.
Power
Power is the measure of the amount of energy produced or used by a circuit per unit of time. When current flows through a resistance, heat is produced. This heat is the evidence that power is being used. This is how a fuse opens, as the heat resulting from the excessive current, melts the metal link and opens the fuse. Power is expressed in watts or VA.
Formula P= V x I

Yes that's right that 40va transformer is 40 watts VA = Volts x Amps.
Every circuit uses a certain amount of power. Power describes how fast electrical energy is used. A good example is the light bulbs used in each circuit of your home. When you turn on a light bulb, light (and heat) are produced. This is because of the current flowing through a resistor built into the bulb. The resistance turns the electrical power into primarily heat, and secondarily light (assuming an incandescent bulb).
Each light bulb is rated at a certain power rating. This is how much power the bulb will use in a normal 110 Volt house circuit. Three of the most popular power values for inside light bulbs are 60, 75, and 100 Watts (Power is measured in Watts). Which of these light bulbs uses the most power? The 100 Watt bulb uses the most power.

Conductors and Insulators
There are some materials that electricity flows through easily. These materials are called conductors. Most conductors are metals.Three good electrical conductors are gold, silver, and aluminum.

Whereas conductors transport electricity, other materials, called insulators, are impervious to electricity. Insulators are materials that do not let electricity flow through them. Four good insulators are glass, air, plastic, and porcelain.
An important class of materials is the semiconductors, materials that can change from being conductors at one moment to being insulators at the next. This makes it possible to form electrically controlled switches, which are at the heart of all digital logic circuits.