Electric circuits (and making speakers)

Types of Fields M M1 Gravitational Field M2 Gravitational Fields Mass ‘M’ has gravitational field ‘G’ ‘Gravitational’ means that the field is due to gravity All ‘masses’ have gravitational fields, people have gravitational fields, atoms have gravitational fields, but the ‘magnitude’ or ‘how big ‘ the gravitational field is depends on how big the mass is A good example of a large mass is the earth, it has a gravitational field that pulls the moon towards it A gravitational field is illustrated by ‘field lines’, direction of pull is denoted by arrows A gravitational field exists only because mass ‘M’ exists – this is an important point For all field lines, the closer they are together, the greater the force of the field d1 shows a smaller distance between the field lines than d2, so the force of the field is larger closer to mass ‘M’ This is shown is the second diagram, the force exists all around mass ‘M’, the darker green is a stronger force field and the lighter green is the weaker force field

Types of Fields + - M Gravitational Field Electric Field Electric Fields In an atom, we have three things: protons (positively charged), neutrons (neutrally charged) and electrons (negatively charged) An electric field only exists around things that are ‘charged’ So in an atom, the things that have a ‘charge’ are protons and electrons The electric field only exists because there is a charge present (whether it’s positive or negative) Protons have a repelling electric field while electrons have an attractive electric field, just like the gravitational field Protons Repelling - Electrons Attracting

Types of Fields + M Gravitational Field Electric Field Magnetic Field Magnetic Fields Here we have a diagram of a wire that is full atoms that have both positive and negative charges Like every charge has an electric field, every moving charge has a magnetic field Magnetic fields only exist because there is a moving charge, specifically, electrons moving through the wire The magnetic field produced ‘wraps’ around the wire, and we’ll learn how this happens in the next few slides Magnetic Field - - Electron Flow - - - - - - - - - Current Flow

N S Permanent Magnets Permanent Magnets All permanent magnets have a north and south pole, and the magnetic fields are illustrated, one again, using field lines The magnetic field goes from the north to the south pole

Permanent Magnets Permanent Magnets - We can visualize the magnetic field when we place iron filings around a magnetic, we can see the fields going from the north to the south pole

Magnetic Fields in an Electric Coil Electromagnet If we create a coil out of many wire loops stacked on top of each other, and send current through the coil, you get a lot of magnetic field around the wire The magnetic field of all the loops add up together, and you get a magnetic field that looks very much like a field around a bar magnet We get a north pole and a south pole around the coil, creating an electromagnet: a magnet created using an electric current The more loops we have the stronger the magnetic field. S

How Speakers Work S N Permanent Magnet The job of a speaker is to convert electrical signal from the sound source into an audible sound: we start out with a permanent magnet with a north and south pole. Permanent Magnet N

 How Speakers Work   N - S + Diaphragm Electromagnetic Coil We then create an electromagnetic coil around the permanent magnet, which has its own magnetic field around it with a north and south pole when current flows through the wire. The permanent magnet is fixed, but the electromagnet can move. This coil behaves like a normal permanent magnet but with one handy property: when the flow of current reverses direction, the poles on the magnet flip – where the north pole becomes south and vice versa. As pulses of electricity pass through the coil, the direction of the magnetic fields rapidly change and the poles of the electromagnet switch. This means that electromagnet is attracted and repelled from the permanent magnet and vibrates back and forth. The electromagnet is attached to a diaphragm or cone that amplifies these vibrations, pumping sound waves into the surrounding air and towards your ears. Permanent Magnet - S + Alternating Current

 How Speakers Work   - + Diaphragm Electromagnetic Coil Permanent Alternating Current

Materials Cup Phone Electrical Tape Copper Wire Magnet Audio Jack slide to unlock Aladdin Sane David Bowie Copper Wire Provide pupils with a copy of the handout: ‘Making speakers.pdf’. Magnet Audio Jack

Steps to Assemble Speaker  Stick magnet to the inside-bottom of the cup  Wrap copper wire coil and stick to outside-bottom of cup  Join copper wire to audio jack Stick magnet to the inside-bottom of the cup. Wrap copper wire coil and stick to outside-bottom of cup. Join copper wire to audio jack.

Steps to Assemble Speaker  Stick magnet to the bottom of the cup  Wrap copper wire coil and stick to outside-bottom of cup  Join copper wire to audio jack 1. Stick magnet to the bottom of the cup. 2. Wrap copper wire coil and stick to outside-bottom of cup. 3. Join copper wire to audio jack.

Steps to Assemble Speaker  Stick magnet to the bottom of the cup  Wrap copper wire coil around paper tube & hot glue around magnet  Join copper wire to audio jack Stick magnet to the bottom of the cup. Wrap copper wire coil around paper tube & hot glue around magnet. Join copper wire to audio jack. TIP: tape in an ‘X’ over coil TIP: wrap about 100-200 coils

Steps to Assemble Speaker  Stick magnet to the inside-bottom of the cup  Wrap copper wire coil and stick to outside-bottom of cup  Sand ends of copper wire & join to audio jack TIP: tape over ends Stick magnet to the inside-bottom of the cup. Wrap copper wire coil and stick to outside-bottom of cup. Sand ends of copper wire & join to audio jack.

RAF examples: electromagnets Using electromagnets isn’t something new. The RAF were using them a long time ago. Look at this example of a magnet fitted to an aircraft. It was used the detect mines during the Second World War.