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Electricity

When I studied A Level Physics I found this one of my trickiest areas. I found that analogies helped me visualise what is happeneing in the circuits, then you've got to know the formulas and apply these in turn to solve any problems.

Intensity of Current

Why does electric current have the symbol 'I' and is it just the flow of electrons? Watch this short video to find out a bit more.

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Quantity of Charge

You can try this simple experiment at home, with an energy saving light bulb and a balloon. I also explain a bit more about the property of electric charge and why is has the symbol 'Q'. 

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Kirchhoff's First Law 

Kirchhoff's 1st Law states that at any point in an electrical circuit, the sum of currents into that point is equal to the sum of currents out, electrical charge is conserved.

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Mean Drift Velocity

How fast do electrons move in electrical circuits? Here I show you how to derive the I = nAev equation you can use to work out the mean drift velocity of electrons in a wire.

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Conductors, Insulators and Semi-Conductors 

How can you change the density of charge carriers in a semi-conductor? This video looks at the number density, n, in conductors, insulators and semi-conductors including thermistors and LDRs.

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Circuit Symbols

The essential symbols you need to know to understand the rest of electricity.

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Potential Difference vs. Electromotive Force 

This is where it gets tricky! You may previously have used the term 'voltage' but now you need to understand the difference between the e.m.f. of a supply and the p.d. across a component. Please note that this analogy is not perfect, it doesn't really explain how energy can be transferred almost instantaneously or how alternating current works but it's a start...

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  • Electromotive Force: The energy gained per unit charge by charges passing through a supply (from chemical to electrical).

  • Potential Difference: The energy lost per unit charge by charges passing through a component (from electrical to other forms).

Kirchhoff's Second Law

This will help you to understand Kirchhoff's 2nd Law when you consider the conservation of energy in a circuit.

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The Volt and W=VQ 

How can you define the volt?

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  • '1 volt is the potential difference between two points when 1 joule of work is done to move a charge of 1 coulomb.' 

or

  • '1V is the p.d across a component when 1 joule of energy is transferred per coulomb of charge passing through the component.'

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Energy gained by Particles through a Potential Difference

The energy gained by charged particle moving through a potential difference can be converted into kinetic energy. This is the basis of old style TVs, oscilloscopes and also particle detectors.

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Resistance and Ohm's Law

V is always equal to IR, but V is not always proportional to I. If that doesn't make sense then you need to see this video and the next one on IV characteristics.

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Resistance and Temperature

Why does the resistance of a conductor increase with temperature? This model of the vibrating ions is one way to visualise what is happening inside the lattice.

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Resistance and Cold Temperatures (with Liquid Nitrogen)

As you cool a substance its resistance decreases (if you go extremely low you get into the realms of superconductivity where R = 0). 

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Conductance (the Anti-Resistance)

Just like resistance is an indication of how much current is impeded, conductance tells us how well something conducts electricity.

OCR Spec B

IV Characteristics of Resistors, Lamps, Diodes and LEDs 

To investigate various components you can set up a simple circuit to vary the current and potential difference. This shows you the IV characteristics, with their distinctive curves for resistors, filament lamps, diodes and LEDs.

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The Light Dependent Resistor 

LDRs: what are they made from and how do they work?

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The NTC Thermistor 

This semiconductor changes its resistance with temperature which makes it an ideal component in circuits that monitor the temperature of their surroundings.

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Resistivity

Resistivity, with the Greek symbol rho (hence the thumbnail - row), is a material property that you need to understand.

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Strain Gauges

Strain gauges are small sensors that can be attached to real objects, perhaps the foundations of a building or an aircraft wing, to monitor and detect changes in length. A metallic strain gauge has a wire that zig zags up and down inside. As this stretches the changes in length of the wire, along with a decrease in its cross sectional area, increases the resistance of that component.

OCR B and CIE

Electrical Power

The rate at which energy is transferred in electrical components is the electrical power. Here I show you how to derive P=IV and the other equations that you need to know.

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The Kilowatt Hour

This is not power - it's another unit that we can use tio measure large quantities of energy.

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Resistors in Series

Resistors in series: so easy even a 7 year old can explain it (well mostly).

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Resistors in Parallel

Resistors in parallel cause the overall resistance to decrease. I also show how you can rearrange the equation when you have just two resistors in parallel to make it even quicker to work out their combined resistance.

How to make circuits that work every time

Finding it hard to make your circuits work? Use these simple tips to make sure they work every time.

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Internal Resistance and How to Measure it 

Battery and power sources are made of 'stuff' which have their own internal resistance, which we call 'r'. Here I show what it looks like if you look inside a battery (you probably shouldn't try this at home - there could be all sorts of dangerous chemicals). I also show how if you record values of V and I you can calculate E and r from a graph.

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Potential Divider Circuits

These are not too bad - just follow the basic rules of circuits and you can solve any problem. These potential divider circuits divide the potential and are often used with thermistors or LDRs as a sensing circuit.

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How to Solve Circuit Problems 

Just a brief look at some of the most important equations you should use, and why you should label circuit diagrams in different colours as you work through solutions.

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The Wheatstone Bridge

Not invented by Sir Wheatstone, this is an example of a 'bridging circuit' where a sensitive ammeter bridges two parallel parts of the circuit. It can be used to find the resistance of an unknown resistor as it acts like a potential divider. It does this using two known resistors and a variable resistor, which is adjusted until no current flows through the ammeter in the bridge. We need to understand Kirchhoff's Laws in order to fully understand the Wheatstone Bridge Circuit.

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