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Mains Electricity: safety
- 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
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
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Dependence of nuclear radius on nucleon number
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 |
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Diffraction gratings and patterns
- Derive diffraction grating equation
- Worked example
- Diffraction patterns
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 
If we proceed to the second order fringe, each successive ray must travel an extra 
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?
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Diffraction patterns |
Fleming: Left or Right Hand?
Fleming, like most people, had two hands. Unlike most, he had a rule for each. But when should you use the left, and when the right?
| LEFT HAND | RIGHT HAND |
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| A motion occurs due to the current. To work out what direction this motion is in, use the Left Hand Rule. | A current is induced in the wire due to the applied motion. To find out in what direction this current flows, use the Right Hand Rule. |
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