## Mains Electricity: safety

Posted in AQA GCSE P2, P2: Mains Electricity by Mr A on 1 Mar 2010

• Electricity is dangerous!
• Structure of cables
• Fuses
• Earth wire
• Three-pin plugs

Electricity is dangerous!

An electric shock can affect your muscles and nerves; it can paralyse you or stop your heart beating. You can get an electric shock from anything plugged in to the mains.

Structure of cables

Wires are coated in plastic for safety. The metal wires allow a current to flow as they conduct electricity. The plastic coating is an insulator which prevents people from being electrocuted.

Fuses

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Earth wire

The most dangerous thing that can go wrong with an appliance is that the live wire becomes loose inside and touches the casing. If the casing is metal it will become live. If you touch the casing, you will get an electric shock.

The earth wire is connected to the inside of the casing. Then, if the live wire touches the casing, charge will flow through the earth wire (rather than you), and the fuse will blow.

Three-pin plugs

## Dependence of nuclear radius on nucleon number

Posted in A2 Unit 5: Radioactivity, AQA A2 Unit 5 by Mr A on 22 Feb 2010

What is the relationship between the radius of a nucleus, R, and the number of nucleons in the nucleus , A (AKA the mass number, N)?

Use the following data to investigate this. Assuming it is a power relationship, recall that we can find the log of both sides in order to discover what this power is.

 Nucleon number, A Nuclear radius, R (fm) 7 2.30 14 2.89 31 3.77 88 5.34 120 5.92 157 6.47 197 6.98 239 7.45

## Diffraction gratings and patterns

Posted in AQA AS Unit 2, AS Unit 2: Waves by Mr A on 21 Feb 2010

Diffraction grating equation

Important: The following derivation assumes that all rays incident on each part of the screen are parallel. This is a fair assumption, provided the distance from the slits to the screen is much larger than the slit separation.

Thus, for the central fringe, the rays travel exactly the same distance as one another.

For the first order fringe, each successive ray travels an extra path difference of $d \sin{\theta}$. Incidentally, this extra path difference must also be equal to $\lambda$, for constructive interference to occur.

If we proceed to the second order fringe, each successive ray must travel an extra $n \lambda$. It, therefore, follows that

$\boxed{n \lambda = d \sin{\theta}}$

Worked example (class demo)

If a red laser is shone through a diffraction grating with ? lines per mm at a screen ? m away, and the first order fringe makes an anlge of ?, what is the wavelength of the light?

## Interference and Diffraction

Posted in AQA AS Unit 2, AS Unit 2: Waves by Mr A on 20 Feb 2010

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## Focussing with a Convex Lens

Posted in AQA AS Unit 2, AS Unit 2: Waves by Mr A on 14 Feb 2010
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## Investigating Resistance Wire

Posted in AQA GCSE P2, P2: Current Electricity by Mr A on 20 Jan 2010
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## Snell’s Law, Refraction and Total Internal Reflection

Posted in AQA AS Unit 2, AS Unit 2: Waves by Mr A on 13 Jan 2010

When light meets the boundary between two media it is either refracted or reflected. What happens depends on the refractive indices of the media and the angle of incidence.

$n_{1} \sin \theta_{1} = n_{2} \sin \theta_{2}$

• $n_{1}$ = refractive index of incident medium
• $n_{2}$ = refractive index of refractive medium
• $\theta_{1}$ = angle of incidence
• $\theta_{2}$ = angle of refraction

Special Cases

1) $n_{1} < n_{2}$

$\Rightarrow \sin \theta_{1} > \sin \theta_{2}$

$\Rightarrow \theta_{1} > \theta_{2}$

So, even at maximum incident angle,

$\theta_{1} = 90 \quad \Rightarrow \quad \theta_{2} < 90$

This means that the ray is always refracted.

2) $n_{1} > n_{2}$

$\Rightarrow \sin \theta_{2} > \sin \theta_{1}$

$\Rightarrow \theta_{2} > \theta_{1}$

Therefore, when the incident angle, $\theta_{1}$, is above some critical angle $\theta_{c}$,

$\theta_{1} > \theta_{c} \quad \Rightarrow \quad \theta_{2} > 90$

So, if the angle of incidence is large enough (larger than the critical angle), the ray will not be refracted, but instead reflect off the boundary. This is known as total internal reflection (TIR).

 Snell’s Law Applet

 Note on Refraction

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