Tuesday, Oct 26, 14:00-15:30 PM CEST
The term electricity is used generically to refer to different related physical concepts:
- Electric charge: a fundamental property of matter measured in coulombs.
- Electric current: the rate of flow of electric charge, measured in amperes.
- Electric potential: the potential difference in electrical energy between two points e.g. between the positive and negative terminals of a battery. It is measured in volts.
- Electromagnetism: the relationship between electricity and magnetism, which enables electrical energy to be generated from mechanical energy (as in a generator) and vice versa (as in a motor).
Electricity can be appreciated in different ways such as lightning, sparks and static charge.
In daily life, most people use electricity to refer to what we use to power our daily lives and fulfill our energy requirements.
It is the common name we give to the energy flow we get from the grid or a battery. The technical name is: ELECTRIC CURRENT
Electrical systems have many things in common with water systems, so understanding how water acts in a water system helps in understanding how electricity acts in a electrical system.
To understand a water system, there are a few things that you should know. Things such as:
- How much water there is
- How much force is pushing the water through the pipe
- How much water is flowing through the pipe over a certain time
It is important to measure these things in a water system, just as it is to measure similar things in an electrical system.
ELECTRIC CURRENT refers to the amount of charges flowing through a specific medium at a particular rate.
The symbol is I and is measured in Amperes (A) by the International Unit System.
When water moves through a pipe, it is said to flow. The volume of water that flows through a pipe in one unit of time is called the flow rate. When electricity moves through a wire, it is sometimes said to flow like water but it is usually said to have a current rather than a flow rate.
To understand the meaning of amperage, it is important to remember the concept of coulomb of charge. In terms of electrons, 1 coulomb of charge is roughly equal to 6.24 x 1018. Therefore, a current of 1 ampere is 1 coulomb of charge going past a given point per second.
It is common to see in batteries the expression ampere-hour (Ah), which is a unit of electric charge. It is equal to the charge transferred by a steady current of 1A for 1h.
Let's think in practical terms
- The current that flows through a solar-powered light may be less than 1A, while that needed to run a large solar-powered video may be 30 A
- 0.005 A is the maximum current a human body can receive without even noticing</span>
VOLTAGE is a quantitative expression of the potential difference in charge between two points in an electrical field.
Its symbol is V and is measured in Volts by the International Unit System.
Water pressure is a measure of the force that pushes water through a pipe.
Looking at the picture, we can see a water tank at a certain height above the ground. At the bottom of this tank there is a hose. The pressure at the end of the hose is the water pressure, which can be compared to the voltage in an electrical system.
Similarly, electrical pressure pushes electricity through a wire. Volts (V) measure how much electrical potential energy there is between two points. It is important to notice that voltage is always a difference.
Electricity will always flow from a high voltage to a low voltage.
Let's think in practical terms
- A low is around of 1.5V and is common in appliances such as radios.
- A medium electrical pressure is between 120V to 240V and it can be found in power outlets in modern homes.
- High voltages of more than 1,000V are needed to move electricity along long distances or to provide very high power.
- Most home photovoltaic (PV) systems operate at 12V.
- AC from the grid operates at 240 V.
RESISTANCE is the degree of difficulty encountered by the current in a wire by the electron flow.
Its symbol is R and its unit is the ohm (Ω).An Ohm is defined as the amount of resistance that exists between two points that have a potential difference of 1 Volt while 1 Ampere of Current is flowing.
Electricity flows through wires like water flows through pipes. Pipes allow water to be carried from one place to another just as wires allow electricity to be carried from place to place.
Consider two pipes of the same diameter but different lengths. If we have the same pressure or force applied to a very long pipe and to a very short pipe, we will have a much lower flow of water in the long pipe than the short one.
This is because the longer the pipe, the more difficult it is to push water through it. This force that opposes the flow of water is called flow resistance or just resistance. The resistance to water flow in a pipe increases proportionally with the length of the pipe, so a pipe twice as long resists flow twice as much. We say that it has a resistance of twice as much.
It is also harder to push water through a thinner pipe than a thick one. The resistance increases with less amount of space for the water to flow. Furthermore, the resistance depends on the material the pipes are made of. Imagine a pipe made of steel and a pipe made from wood. The smooth surface of the steel pipe will give less resistance to the flow of water as opposed to the wood pipe. The type of material we use will tend to stop the flow of water through friction: a smooth pipe is better than a rugged one because it has less friction.
Electricity flowing through a wire acts in the same way as water flowing through a pipe. If the wire length is doubled, the resistance of the wire is also doubled, and it is twice as hard to force electricity through the wire. If the wire size (cross-sectional area) is cut in half, the resistance is doubled, and it is twice as hard to push electricity through the wire.
The wire material will also influence the amount of resistance. Silver, gold and copper are the best metallic conductors and they disperse less electricity than other conductors, such as iron, tin or steel. This dispersed electricity is energy that is eventually converted into heat.
In mathematical terms, the resistance of a wire is: R = ρL/A,where ρ is a material property called resistivity, L is the length of the wire and A is the cross-sectional area.
In electrical circuits, the temperature also plays an important role, if the temperature increases so does the resistance of the material.
Ohm’s Law states that the voltage (V) is equal to the product of the resistance (R) and the current:
V = I x R
Looking at this differently, if you find that you have to double the length of a pipe and cannot change the pressure that forces the water through the pipe, then the only way you can keep the same flow rate as before is to cut the resistance to flow in half.
To do this, you can lay two identical pipes and join them together. This gives double the space for water to flow and cuts the resistance in half. Another way is to take out the old pipe and put in a single new pipe with double the cross-sectional area of the old one.
Electricity acts in the same way. If wire length (resistance) is doubled and voltage (electrical pressure) kept the same, the amperes flowing (electrical flow rate) are cut in half. If voltage is kept the same and wire length doubled, you can only have the same current by cutting the wire resistance in half. This can be done by doubling up the wire with another of the same size, or by replacing the old wire with a wire that has twice the cross-sectional area.
The interaction between electrical pressure in volts, electrical flow rate in amperes and flow resistance in ohms is:V = A x Ω.
herefore knowing any two of the three units, amperes, volts or ohms, the third can always be calculated.
A piping system for rainwater may be simply a short pipe with a tap at the end or it may have many branches going to various places. The pipe and its connections are called a water circuit. For water to flow all the way from the tank to the appliance there must be a continuous pipe connecting them. If the pipe is disconnected or broken, the water will not flow to the appliance and it will not work.
In a similar way to water circuits, electrical circuits can be very simple, such as a battery joined to a light. A circuit can also be complex with several batteries and many appliances all joined together. An electrical circuit may also include other electrical elements such as batteries, resistors, motors and appliances.
There is one big difference between the way a water circuit and an electrical circuit work. A water circuit usually ends with the appliance and the water flowing away into a drain. In an electrical circuit, the electricity cannot flow away outside an appliance, so there must be a wire to carry electricity away from the appliance as well as a wire to carry electricity to the appliance. This return wire goes back to the power source, where the returned electricity is sent up to full voltage and sent back to the appliance. Is as if, we were pumping the water back again to the top of a building.
In an electrical circuit, the electricity must have a continuous path not only to the appliance but also from the appliance back to the source. If the path is broken at any point, the flow of electricity stops. If a continuous path does not exist, we say that the circuit is open. If a continuous path is present, then the circuit is closed. Electricity will flow through a closed circuit but will not flow through an open circuit.
Besides open circuit, we can have a short circuit.A short circuit occurs when two points that are not connected and normally have different voltages are suddenly joined together eliminating such difference.
→ In an open circuit the current is zero, and the resistance is infinite.
→ In a short circuit the voltage is zero, and the resistance is zero as well.