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• 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

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.

• 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 . Incidentally, this extra path difference must also be equal to , for constructive interference to occur.

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

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?

Diffraction patterns