Questions+about+electricity.


 * //Questions from the first electricity session. Feel free to chip in on these answers!//**

//What's the link between p.d, charge and energy?// //What is voltage/difference between p.d and voltage?//

Potential difference and voltage are the same thing. It helps here to consider what we mean by voltage: Voltage means the potential of electric charge to do work - one volt is one joule per coulomb of electrical charge. A coulomb is a unit of electrical charge and from the point of view of circuits, you may find it helpful to consider it as a 'bucketful' of electrons. (I do). If one ampere of current is flowing that means that a coulomb of charge is passing through the circuit every second. If the potential difference (change in voltage) across that part of the circuit is one volt, then one joule per second (1 watt) of energy is transferred.

//Why is voltage split in series circuits, same in parallel?/Why does the current split in a parallel circuit? How does it know what to do?//

Thinking about voltage as potential difference helps here. It's not massively helpful to think of voltage being 'split' - instead consider the change in potential across different components. We considered the rope model as a way of looking at energy transfer in circuits. In a series circuit, the effect of resistances in series is to reduce the current flowing in the whole circuit. The reduced flow of current means that energy transferred less quickly in all components in that circuit. In a parallel circuit, though, there are multiple branches of the circuit, each with an independent path to and from the cell. No one path in the curuit affects the resistance of any other, so the flow of current through each branch is not reduced by other resistances. Each branch has a greater flow of current, therefore energy is transferred more quicky in the bulb; there is a greater energy change i.e. a greater change in voltage.

//Why does adding a bulb to a circuit make it dimmer? Why does increasing the voltage make it brighter?//

As in the example given above, adding a bulb to a series circuit will add resistance, meaning that for a goven potential difference, the flow of current is reduced, and therefor the rate of energy transfer in the bulb is reduced. Increasing the voltage causes a greater current to flow so energy is transferred more quickly in the bulb.

//How fast do electrons travel around a circuit (relative to current)/How fast does electricity flow?//

Much more slowly than you might think! Read this on drift velocities. It's from 'Hyperphysics'. The 'signal' produced, however, is very quick. Consider a piece of string. If I had a really long string and tied it around (let's pick on Dani, as she's a long way off and thus a good example) Dani's finger, as long as we kept the string taut (unlikely, I realise) I could pull the string quite slowly, say 1cm over a second. Dani's finger would get pulled (insert your own punchline) immediately, even though at 1cm/sec it would take a long time to get from Bristol to Cape Cornwall (just ask anyone who's made the journey on the August Bank Holiday)

//Why are electrons negative?//

As it's nearly the weekend, I'm just going to answer that question with this.

//Where does it come from?//

I was a little unsure of this one! Did we mean 'electricity'? The electricity that we use day in, day out is generated via electromagnetic induction. We'll cover that as a separate topic but in brief, we use movement provided by wind, wave or tide or, far more commonly, steam produced by nuclear or fossil fuels heating water, to turn generators. Generators provide a rapidly changing magentic field across a coil of wire and this in turn produces electricity. Imagine a giant bicycle dynamo whirring around in an aircraft hangar and you're not too far off the mark.

//How does a battery work?//

Well, one answer is that from the point of view of this topic at KS3 and, to an extent, KS4, we just don't care. The things that are pertinent to learning about how circuits work are that the battery produces a potential difference in the circuit and that it is //not the source of electrons//. What actually happens in the battery isn't relevant to either point. However, it helps to understand roughly what's going on to avoid carrying misconceptions. To dispel the idea of a battery being a 'reservoir' of electricity you can get students to make cells from citrus fruit and rods of differing metals then ask them if they think that Jamie Oliver is creeping around Sainsburys charging up the citrus fruit every morning. To explain how a battery actually works there's a nice, comprehensive set of resources on HowStuffWorks and for the chemists there's a thorough discussion of the science here from the University of Washington (State, not DC).

//Why do electricity pylons hum?//

I had my suspicions about this one but I still had to look it up. The hum, usually in the 50-60 Hz range, is at the transmission frequency of the electricity. It's thought to be caused by an effect called 'magnetostriction'; essentially, the magnetisation and demagnetisation of steel cables caused by an alternating current causes minute changes in the shape of the metal, causing the wire to oscillate slightly. The naked scientists discussion on this suggests modulating the generation frequency of the National Grid to play the power lines like the world's largest musical instrument. this sounds like a brilliant idea to me although I suspect that it might cause havoc with all manner of electronic equipment. The crackling sound made by pylons is just your plain old corona discharge.

//How does current flow between live and neutral wires?//

We'll come on to mains electricity at a later date.

I'm not quite sure what this question means! Any elucidation is welcome. //Step up/step down transformer vs Ohm's Law in a wire and the effect of changing voltage on current?//