## ELECTRICITY from 2017

Here is the topic song

Updated November 2019

Electricity 2017 Final word version of the Electricity Unit.

The booklet is large as it contains lots of questions for you to practice, practicals for you to complete and notes.

They are large notes so that you ought to be able to work your way through whether you are in class or away at college etc.

The section numbers are linked to the compendium with all the things to cover in National 5 Physics.

I can now upload ppp to mrsphysics, so here is one of the first. It is to cover section 9 and 10 of the outcomes. Thanks to the kind person who produced the stuff on A.C and D.C. I’ve been using it for years. Let me know if I’ve nicked it from you and I’ll add my grateful thanks.

#### Components

resistor network Try this when you think you have got to grips with resistances in series and parallel.

AC_DC This is a powerpoint presentation that someone passed to be in the days of SG. It covers AC and DC traces

Voltage (2)

Voltage Analogy

#### Voltage Dividers

VOLTAGE DIVIDER FORMULAE The formula sheet for voltage dividers

VOLTAGE divider circuits (2)

VOLTAGE divider circuits2

POTENTIAL DIVIDERS2

POTENTIAL DIVIDERS

VOLTAGE divider Q  Practice those horrible voltage divider questions with this pdf version of the document below. The answers are given for you to check. VOLTAGE divider Q

#### Mains- Electricity in the home

Here are some additional notes that might help as you go through the materials. Check out the post on using your calculators to measure resistance (I’ll add the link here when I’ve found the post!)

Ring main   Based on the SG course notes and not really in the N5 course, but it might give a little background to why when calculating the fuse rating for an appliance you use 240V and not the 230 V as stated.

Traces

#### Older Notes

EE1 – Electricity LOCKERBIE The old electricity notes (based on a colleagues work- thank you and I’ll find out who you are), these will be superceded when the document above is completed.

Elect & elect D&G Prob Book no answers These are some great little questions by Mr Belford from Dumfries Academy, but some of the numbers are a little bit fictional!

Elect & elect D&G Prob Book no answers The above document as a pdf file.

…… to be continued!

## A.C/D.C

As Mr Clydesdale says “A very good band”

The date is really 2020, but I need this post under the main Electricity notes section.

Mr Sharkey demanded I take screenshots of the traces for his OneNote ClassNotebook as he was made to leave against his will! So he asked, and I did!

## Here are the A.C /D.C traces

#### from Nat 5C 2020

We’ve plugged in a 1.5 V cell to the picoscope, put a voltmeter in parallel and noted the reading on the voltmeter and the looked at the value on the picoscope.

The picoscope was picking up some of the electrical signals from the computers and power around the room.

Notice on these images the reading on the picoscope and the voltmeter are the same. The cell is a source of D.C, direct current. In direct current the current /charges only flows in one direction. The free electrons in the ciruit are always drifting around the circuit in one direction.

When the polarity is reversed (swapping the positive and negative connections to the cell) the trace moves below the zero line showing that the current is now in the opposite direction. The voltmeter reads -1.5 V (the negative indicating that the current is in the opposite direction).

When we connect up to the A.C supply of the usual school power supplies we can see that the trace indicates the current flows in both directions. We can tell this as the trace of the voltage goes above and below the 0 V line on the picoscope. The trace shows a wave indicating the voltage and hence current is changing direction and magnitude many times per second. In the case of the mains voltage the frequency of the supply is 50 Hz.

Notice that the reading on the voltmeter reads 6.69 V. The power supply is set to 6V, but the peak of the trace is greater than this, about 9.5 V. The peak voltage of an A.C. trace is always greater than the quoted voltage of the supply. This is because we want to be able to compare A.C and D.C traces and so the quoted value is 1.414 times smaller than the peak voltage, try this.

When the polarity is reversed it makes no difference to the trace.

Another power supply in the Department is the 5.0 V regulated power supply. We can see this is a D.C trace and that the value of the voltage and hence the current is steady.

We can see when the polarity is changed (the connections to the power supply are swapped over) We can see the the trace of the voltage goes below the zero line, indicating the current is moving in the opposite direction. The voltmeter reading is the same as the value on the picoscope.

However, when we connect the picoscope to the usual Lockmaster power supply on the D.C. setting we get a rather unusual trace. The trace is D.C, remember direct current tells us that the current remains in one direction. However, the voltage and hence current isn’t constant. This is an unsmoothed D.C trace, and is common in cheaper power supplies. The trace never goes below the zero value on the screen.

Reversing the polarity shows us that the voltage is opposite, we get a negative value on the power supply but the trace never goes above the line. The current remains in one direction.

So in summary

In DIRECT CURRENT the current always moves in one direction.

In ALTERNATING CURRENT the current changes direction, usually many times per second. The current also usually changes magnitude (size).

With cells or regulated power supplies the D.C trace gives a constant value. In an unregulated trace the current also changes magnitude, but never direction.

## Voltage Many teachers think that voltage is too difficult a concept for S1 students to understand. By the time students get to AH we expect them to be fully knowledgeable about voltage, but we don’t clearly explain it to them as we go along. I am as guilty as the next person of doing this so… . My new mission is to teach voltage as best and as fully as I can to S1 and build on the concept each year so that by AH they will feel confident about this work.

Having met Gill Arbuthnott at the Edinburgh International Book Festival (see post in Blog) I was really impressed with the way she tries to explain difficult concepts early on. She has given me permission to reproduce her page 16 on The Volt here.

The Volt

This was named after Alessandro Volta It is a unit of measurement in electricity. It tells us how much energy an electric charge has. You sometimes hear people saying things like, “The number of volts running through the circuit is…”. This doesn’t actually make sense! It’s like saying, “The height running through the mountain is 1000 metres.” Heights don’t run, and neither do volts. There is no Usain Volt!

What is a volt?

So what is a volt? Imagine you are in a building with stairs and a lift. You carry a tennis ball up one floor in the lift, and let it roll back to ground level down the stairs. A battery is like the lift – it’s a way of giving energy to something. In the building this is the ball – in electrical terms it’s an electron.

The ball rolling down the stairs is losing energy. In our circuit the equivalent is the electrons losing their energy to power a bulb. The voltage is equivalent to the height you take the ball up in the lift – more height is equivalent to greater voltage. And the distance the ball goes up in the lift must be the same as the distance it comes down by the stairs.

There are plenty of pictures in the book, but I didn’t think it was as easy to reproduce them. The book is full of more really interesting stuff, and even material about coins that Mr Chemistry opposite Mrs Physics didn’t know about (but then he’s far too young!)

http://www.bloomsbury.com/uk/a-beginners-guide-to-electricity-and-magnetism-9781472915740/

Definition: Potential difference is the amount of work done to move an electric charge from one point to another.

or

Definition: The definition of voltage is the electromotive force or the electrical potential difference between two points in a circuit expressed in volts.

Voltage is a scalar quantity. The SI unit of voltage is the volt, such that 1 volt = 1 joule/coulomb.

The easiest way to understand voltage is to use a water analogy. Using a hose as an example, think of voltage as the amount of pressure forcing water through a garden hose. The higher the pressure in the pipe the more water is forced through the pipe each second. The greater the voltage, the greater the flow of electrical current (that is, the quantity of charge carriers that pass a fixed point per unit of time Q=It) through a conducting or semiconducting medium for a given resistance to the flow.

One volt will drive one coulomb (6.24 × 10 18 ) charge carriers, electrons, through a resistance of one ohm in one second.

Voltage can be direct or alternating. A direct voltage maintains the same polarity at all times. So charges always flow in one direction. In an alternating voltage, the polarity reverses direction periodically. The number of complete cycles per second is the frequency, which is measured in hertz (one cycle per second). An example of direct voltage is the potential difference between the terminals of a cell. Alternating voltage exists between the mains positive and negative.  