What the curriculum thinks you need to know:
PC_BK_37 Basic concepts of electricity and magnetism
PC_BK_38 Electrical voltage, AC and DC current, resistance, impedance
PC_BK_39 Electrical circuits: series and parallel
PC_BK_40 Symbols of basic components of electrical circuits
This post is the sister post of ‘Electrical Components’, Read this one First!
What you need to know (The theory):
The absolute basics of electronics are often forgotten from learning for FRCA, and as a result, people lose easy marks in the MCQ. Most of the questions asked are based around definitions and as such are easy marks if you know it!
We all know from school that electrons carry electrical charge. Electrons are tiny negatively charged particles which are a constituent of atoms. Metals and conductors have a ‘sea’ of electrons around them as they all have ‘free electrons’. Its all GCSE chemistry again!
The unit of charge is the coulomb (see Capacitors for more). One electron carries 1.602 x 10ˆ-19 coulomb, so a very large number of electrons are needed to make up a coulomb (6,240,000,000,000,000,000 if you’re interested!).
A Battery or power supply gives the driving force to move these electrons. Remember electrons move from the negative terminal to the positive terminal. Its like Magnets, the negatively charged electrons are ‘attracted’ to the positive electrode.
Also remember that the movement of electrons and electrical charge through a conductor will produce a magnetic field around the conductor. This is how electro-magnets work (you remember those from school right?!). In reverse, moving a conductor through an magnetic field will induce an electric current. This is how a dynamo on a bike works (if you’re old enough to have had one on your bike as a kid, if not, google it. They were cool, honest).
Electrical current is the flow of electrical charge.
If we measure the flow, we always do it across a specific point and measure the amount that flows across this point per unit time. Hence the flow of electrical charge is measured in coulombs per second.
1 Ampere = 1 Coulomb / Second
Electrical Voltage (aka potential difference) is the energy required to move one coulomb of charge between two points in a circuit.
Moving this current around a circuit requires energy. In a fluid circuit this would be the pressure differential which drives the fluid around. The voltage is the same as this. It is simply the amount of energy required to move a unit of charge around a circuit. Think of electrical circuits you own, the more complex circuits need higher voltages to operate, they need more ‘oomph’ to push the electrons around the circuit.
1 Volt = 1 Joule / Coulomb
Resistance and Conductance (Including Impedence)
Materials can be classified as Conductors, Semiconductors or insulators.
Conductance is the ability of the material to allow the flow of electrical charge.
- Conductors have a high conductance, as you’d expect from the name!.
- Semi conductors have a low conductance, but the conductance can be modified by changing the conditions the semi conductor is in. This usually means changing the amount of electrons available in the material (excess or starving). This change then allows the flow of electrical charge. This is how transistors and similar work.
- Insulators do not allow the flow of electrons and hence charge, and have a non existant conductance.
Conductance is measured in the Siemen (no sniggers please).
Resistance is the ability of a material to resist the flow of electrical charge.
The measurement of resistance is in the Ohm.
1 Ohm = Resistance across two points when one volt applied across the points gives a current flow of 1 ampere
Sounds like the conductance and resistance are kind of the same thing but opposites? (please physics geeks, don’t kill me for saying that). Well, the ohm is the reciprocal of the siemen. So they kind of… are.
Impedance can be thought of simply as the resistance in an AC circuit. describing it accurately is outside the scope of this post! Again, see the post on capacitors/inductors for this.
So how does all this fit together? Remember Ohm’s Law from school? Drawing all those triangles and things? That’s how. See it was useful, Honest.
From this we can quite easily work out the remaining value in a circuit if we know the other two values.
Alright, so why is my ‘leccy bill in kWh and all this nonsense? (‘Leccy is south wales speak for electricity if you were wondering!)
Power is measured in Watts…
Power (Watts) = Energy (Joules) / Time (Seconds)
Watt = Joules / Second
Great, but how does that work with all this Ohm’s Law stuff? Well If we look at the equation for Volts, we see that 1 Volt = 1 joule / Coulomb. If we then look at the equation for Amps, 1 Amp = 1 Coulomb / Second. so….
Power = Voltage x Current
Power = Volts x Amps
Power = Joules / Coulomb x Coulomb / Second
****Those coulombs then cancel out****
Power = Joules / Second
So back to your electricity bill, 1kWh is simply one kilowatt running for one hour. So 1000 watts for one hour or 1000 joules/second for one hour or (yawn!) 1000j/s x 3600sec or 3,600,000 joules. Now you see why its not in joules!
The college has had some MCQs regarding energy usage in circuits. these usually involve an appliance which consumes a certain number of watts and then they ask how many joules this is. Remember an hour is 3600 seconds (60×60).
Series/Parallel in circuits
Be careful with these, the college likes British symbols (‘cous they’re the best, obviously).
What you need to know (How it works in practice):
These definitions are pretty useless in daily life (hence why you forgot them from school). Learn them for the exam and then replace that knowledge with useful stuff after the exam.
Random Exam factoids (i.e. the things the college like asking):
- Conversion of units, Common examples are watts/kWh to joules etc.
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