Electric bill

[Jabbajaws]

Whilst at college, my tutor told us that in our households, where we use a wide number of appliances, anything that contained a coil, gives electricity back to the supply.

Please can anyone elaborate as to whether or not this is true and why it happens if anyone is able to explain....

 

[Adam W]

A coil of wire produces a moving magnetic field, and a moving magnetic field induces a voltage which works against the current supplying it, known as 'back emf'.
Rather than 'giving electricity back to the supply' it is really working against it which would cause the appliance to use more power - a phenomenon known as LENZ's law (had to check the spelling because I dropped a clangar when trying to explain kirchhoff's law), otherwise known as the 'law of self-destruction'.
This effectively adds an impedance to the circuit, known as 'Inductive Reactance' (XL).

This effect is countered by installing a capacitor in parallel with the terminals of, say, a motor or fluorescent light fitting; an effect known as 'Capacitive Reactance' (XC). In factories where there are a lot of motors the capacitors are banked together near the point of supply, and cut in as and when they are needed.

If a circuit has more of either Inductive or Capacitive reactance it is said to have a low 'Power Factor'.

Think of 4 people trying to push a broken down car - the most efficient way is for all 4 to push in the same direction, but if one decided to try pushing against the other 3, the car would still move but they would have to work harder.

If you're at college your tutors will go over it in the 2nd year. Don't worry if like me you haven't got a scooby what they're on about to start with - you don't really need it until the 3rd year exam.

 

[Jason Smith]

quick tip on learning methods- if you havent a clue what someone is on about, write down what they say and put it in the too difficult section of your notes. When you refer to it in the future, you may find yourself saying to yourself " cant believe I ever wrote that down ".

 

[Adam W]

I think my "difficult section" was the whole of my 2nd year notes; I didn't 'get' most of it - the whole XL and XC business in science, and cable calcs in Ins Tech all went straight over my head, but then in the 3rd year everything just seemed to fall into place.
All my tutors kept saying the 3rd year would be the hardest, but i found it OK because I had prior knowledge from the first 2 years.
I had a separate folder for each year arranged in 4 sections - college stuff (tutorials etc), practical, science, Ins Tech, all in chronological order. New stuff was always added to the back of each section so if i needed to find something I just had to try to remember how long ago we'd learnt it.

My top tips would be:
If you don't understand something - ask. You might feel like an idiot if you're the only one that doesn't get it but you'll feel more of an idiot if you fail and have to retake.
Turn up to every lesson. Sounds obvious but in my year the guys who got bad results or failed were the ones who didn't bother turning up to half the lessons. I went to every lesson and got distinctions in every module