Electricity

Electrical energy is transferred into other types of energy by electrical appliances. Some examples are:

-Light bulbs to produce light and, unfortunately, heat,
-Speakers to produce sound,
-Drills to produce kinetic energy, and waste heat and sound.

All appliances will waste some energy but in their design, they will waste as little as possible.

Power is an important electrical measure and is measured in Watts. 1 watt of power is the equivalent to 1 joule of energy being transferred every second. so 1W=1J/s.

As watts are rather small, we commonly use kilowatts (kW) which are 1000 times bigger than watts. Power, measured in watts is found by this formula:

Power in watts = Energy in joules / Time taken in seconds

We can revisit our equation for efficiency here and substitute the energy for power and still get the same answer as long as both energy terms are substituted for power as shown:


Efficiency = (Useful power out / Total power in) x 100

When we pay for our electricity bills, we meet another new unit of measure. If you look at a bill, or even at your electricity meter, you will see the unit kWh or kilowatt hour. This means using one thousand watts for an hour. For example, if no electricity is being used in your house and you put on a 1kW heater for 1 hour, your bill will be 1kWh. Likewise, using a 100W bulb for 10 hours will be 1kWh. If you haven't already worked this out, this is how we come to these numbers:

Energy transferred (kWh) = Power of the appliance (kW) x Time appliance is on (Hours)

Heat loss home

The last part of your bill will all be about money and power companies charge per unit, or more specifically, per kWh. They then convert your usage into a cost using this formula:

Cost of energy used = number of kWh units used x cost per kWh

For example, if you used 185 kWh and the power company charge 14.5p per kWh...

Cost = 185 x 14.5
So cost = £26.825

The final consideration in this topic, is the cost effectiveness of appliances. We must consider many things such as:
-Purchase price
-Installation price
-Electricity cost
-Maintenance cost
-Environmental impact cost
etc...

As electricity and gas get more and more expensive, it is more important to us to get energy efficient appliances and to reduce the amount of energy that we waste in our homes. As well as energy efficient bulbs and turning the TV off standby, we can reduce the amount of heat that escapes from our homes so that we save gas and electricity, not only to save money, but also to reduce our carbon footprint.
 
We prevent heat loss with glass fibre loft insulation and cavity wall insulation. They both trap air to prevent convection and the glass is a poor conductor of heat. Double glazing and draught excluders also contribute, however, there is a very important financial consideration to make.

This is called payback time. Going back to the considerations list above when purchasing an appliance, you must be aware of how long it will take to recoup the money invested in an energy saving device such as loft insulation. If an item is extremely expensive and saves very little energy, it may simply not be worth buying.



Coal fired power station
Wind power farm

We rely on tremendous amounts of electricity. You are either reading this on a screen powered by electricity, powered by a battery charged by electricity or on paper, printed using electricity. In most power stations, fuels are burned to generate steam under pressure, the steam then turns a turbine which is connected to an electrical generator.

This power station can easily be turned up, slowed down or even turned off. With demand fluctuating, this is a very important consideration. During the advert breaks on the most popular TV shown, millions of people put the kettle on and in the middle of the night, nobody is using the electricity so it can be turned off.

Most of these power stations rely on fossil fuels like coal, gas and oil. Some use a small amount of biofuel which is the remains of recently dead plant matter rather than ancient ones as with coal, gas and oil. Biofuels can be oils from plants such as cooking oils. In a nuclear power station, either Uranium or Plutonium is used and as the nuclei decay (called nuclear fission), they release energy but much more concentrated than a fossil fuel per kg. Although nuclear does not emit any greenhouse gasses, the waste has to be stored for thousands of years and accidents like Chernobyl and Fukushima are devastating to people, animals and plants.

Renewable sources of energy are very popular but as well as their green advantages, they have many disadvantages. For example, despite not producing greenhouse gasses nor relying on dwindling fossil fuels reserves, they are not reliable, e.g. no solar power at night or wind power on a calm day. That won't be much use to the millions of kettles going on at half time on FA cup final day.

Hoover Dam hydroelectric power generation
Solar panels on a roof

Wind turbines are just like the other power stations except that steam doesn't turn the generator, the wind does. These are getting better and better, however, they can do nothing if it isn't windy. In 2012 we had storms in Scotland that made them spin too fast and damaged them.

Wave power forces floats to bend at joints which generated electricity and turbines under the water in tidal estuaries are forced to turn when the tides come in and out.

 A much more tried and tested method is hydroelectric power generation. Here at the Hoover Dam (left), water is stored behind the dam wall and allowed to flow out only through turbines. The fast flowing water spins the turbines and generated electricity. This power station can be speeded up and even turned off.




Solar cells transfer the electromagnetic radiation from the Sun into electrical energy. They are most successful on watches and calculators but by joining enough together we make a solar panel.

Alternatively, water can flow through a solar heating panel and the water is heated directly.


Solar Power TowerNational grid

On the left is a Solar Power Tower. Around the base of this water tower, are hundreds of mobile mirrors all pointing the Sun's rays at the water tower to heat it up to produce steam.


The final alternative source we'll look at is Geothermal. Basically, to heat the water that fills the radiators in our homes or even heating the water that becomes steam that will drive a turbine, water is pumped underground into hot volcanic rocks where it absorbs energy from the rocks and becomes very hot water or steam.

As a part of your understanding of energy and alternatives, you must generate a list of positives and negatives about all of the energy types discussed. Reread through the above and make a simple table putting the pros and the cons next to each source of energy.

The next section concerns itself with how the energy gets to you. Electricity to transferred to you home and school by the National Grid. This network of pylons and cables travel to every building. The cables are overhead when they travel distances but underground near to your homes. At low voltages and high currents, lots of energy is lost as heat through resistance in wires. To avoid this and to prevent you from receiving pitifully low power to your home, step-up transformers are used just outside the power stations to make the electricity very high voltage and very low current to reduce resistance. Closer to your home, a step-down transformer returns the voltage and current to a normal level.



In the sockets you have at home, the current is supplied by alternating current (AC) which means that the + and - swap regularly. This frequency of change is measured in Hertz and the frequency feeding your computer now is 50 Hz. The electrons moving in the conductors change direction 50 times in a second. It is so fast a cycle because if slower, light bulbs would visibly change in brightness and give a flickering effect that would have us all in a permanent headache.

Each plug has a live wire, a neutral wire and an earth. The live is where the pushing and pulling of electrons is driven, the neutral completes the circuit and the earth is connected to Earth so if a short circuit occurs, the extra current flows to Earth rather than to your heart!

Remember back to KS3 science, if you put bulbs in series, they don't all get the same power or energy, the first is brighter but in parallel, they all receive the same output. This is why sockets and lights in your home are powered in parallel to ensure that lamps aren't dim at one end of our home and they don't blow out at the other.

Cables are different thicknesses depending on how much load they have to carry, a kettle's cable is a lot thicker than your phone charger.
From time to time, devices malfunction and could be potentially dangerous. If they draw too much current when they brake, that energy could be transferred into heat and start a fire. Fuses have a special "fuse wire" inside that can only carry a certain amount of current above which, it melts and breaks the circuit. This will stop potential harm through electrocution or fire. Alternatively, there are circuit breakers which rely on an electromagnetic switch. These cut off the power faster so are safer. They can also be reset much faster. If there is no Earth, or if you want to be extra specially safe, use a RCCB or Residual Current Circuit Breaker. It turns off the power if the current flowing in the live wire is different from the current in the neutral wire meaning that some must be escaping.

There are 2 equations that you need to know for this section, the first is for electrical Power:

Power (in Watts) = Energy transferred (in Joules)
                                    Time (in Seconds)
Shortened to P=E/t
The second is for electrical energy:

Charge (in Coulombs) = Current (in Amperes) x Time (in Seconds)
Shortened to Q=IT


To begin this topic, we look at electricity with no current. If you rub two insulators together, electrons are ripped from the surface of one and collect on the surface of the other. This localised concentration of charge is called static electricity.
We see this if we rub a balloon on a jumper then it either attracts someone's hair or may stick to a wall. These occur because like charges repel and opposite charges attract.
Current is the flow of charge. Charge in electricity is provided by the electrons moving through a conductor. Current is then the flow of electrons. We can measure the current in relation to charge using this equation:

Current (amps) = Charge (coulombs) x Time (seconds)


When measuring the current through a circuit, the current must flow through it so the ammeter must be in series. If you are measuring the voltage (potential difference), you are measuring the change in energy across the component so it must be in parallel. The voltage can be calculated as the amount of energy or work done per coulomb of charge, using this formula:

 

Voltage (V) = Work done or Energy (J) x Charge (C)


These equations lead us to being able to calculate resistance. This is the opposite to the flow of current, this is a measure of how difficult it is for the electrons to get around the conductor. Graphs will show the relationship between voltage and resistance, they show a clear pattern that as the voltage goes up, so does the resistance. This links us to Ohm's law. It states that at a constant temperature, the relationship between voltage and current is a straight line (directly proportional) graph through the origin.

Resistance can be calculated using:

Voltage (V) = Current (A) x Resistance (Ω)

In reality, even a basic component like a filament bulb does not have a simple plot like this one. This is because as the current increases, so does the temperature which goes against Ohm's law, making the graph a curve that still passes through the origin. These graphs are both positive and negative as current can flow either way through a bulb, however, resistance is too high in one direction for a diode so the current remains zero in one direction.

Series circuits have all of the components connected one after another like the carriages on a train. The electrons have no choice of route through them so they all pass through, which is why the current is the same in each device. The voltage across each is different as some energy will be used in the first then some in the next and so on. This can be visualised if you connect 4 bulbs in series to a battery, the first is much brighter than the last. The voltages across each can be added to a "total voltage" and the resistance in each can also be added to give a total resistance.  In a parallel circuit each component is connected across the supply so if you have 4 bulbs attached and one breaks, the other 3 will still be lit. Here, the voltage across each item is exactly the same but now the electrons have a "choice" as to which way they can flow so current through each component is different. The biggest resistance component has the smallest current and the smallest resistance component has the largest current. Total current is all of the individual currents added up.

Key words and terms for this topic: electrical appliance, power, watt, kilowatt, kilowatt-hour, cost effectiveness, payback time, turbine, generator, fossil fuel, biofuel, nuclear fission, renewable, non-renewable, tide, tidal, wave, solar energy, solar cell, solar power cell, solar power tower, geothermal, national grid, step-up transformers, step-down transformers, base load, start-up time, direct current, alternating current, frequency, live - neutral and earth wire, oscilloscope, socket, cable, three-pin plug, fuse, circuit breaker, residual current circuit breaker (RCCB) insulating, electron, attract, repel, series, parallel, volt, resistance, Ohm's law, ohmic conductor, filament bulb, diode, light-dependent resistor, thermistor. .
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