P4.2 - Uses of Magnetism

• Fields can combine because of a wire due to a permanent magnet. This produces a force on the wire.
• Two fields in the same direction add up, in opposite directions they cancel out.
• If you place a wire with current in between a magnetic field, the force on a wire produces a catapult effect. There are stretched line above the wire and few unstretched wires underneath it.

  
• The current, magnetic force, and force are all at right angles to each other. You can remember this with Fleming's Left Hand Rule.

  
• Force on a conductor (at right angles to the magnetic field) carrying a current (N) = magnetic flux density (T) * current (A) * length (m)
• One amp is the size of current that produces a force of 0.2 micronewtons on two wires 1 metre apart.
• To make a simple motor, put a loop of wire inside a magnetic field. Once connected to a battery, a current flows and one side of the wire goes up and the other goes down. This motor would not spin very well as the coils would rotate the other way once it reaches its vertical positon.
• Split-ring commutators are used to keep current moving from the right to left of a wire at all times while also allowing it to spin.
• The split ring commutators allow current to flow the same way from a battery, but it halves once it reaches the coil so it stays moving from right to left.
• The speed of a motor can be changed by:

  • The magnitude of current flowing in the wire.
  • The strength of the magnetic field.
  • The number of coils in the wire.
  • The length of the coil.
• An induced potential difference happens across the ends of a wire if the wire is in a changing magnetic field, so that it cuts the field line.
• An induced potential difference can happen if a conductor is placed inside a magnetic field.
• If the wire does not cut the lines of the magnets, no induced potential difference occurs.
• The potential difference that you induce depends on:

  • the length of the wire.
  • The rate field lines are cut.
• To increase the rating on a voltmeter:

  • moving the wire faster
  • using a stronger magnetic field
  • using more wires / coils.
• Moving the wire faster and using a stronger magnetic field means more field lines are being cut per second.
• More loops means there is an induced potential difference in each loop, so potential difference increases.
• If the force acting on a wire is in the same direction as the force applied, then the wire would "fly off".
• The magnetic field is produced in the opposite direction to the field that produces a potential difference.
• When you drop a magnetic inside a metal coil, it takes longer to fall because of the changing magnetic field caused by the current in the metal tube. This produces a potential difference and current flows because it is a complete circuit.
• Electromagnetic induction is the production of an electromotive force across an electrical conductor in a changing magnetic field.
• The output of the coil is a potential difference that changes in direction, it is called an alternating potential difference.
• Alternating potential difference can happen if a magnet is shaken inside a tube with wire around it. If it is connected to a bulb, it will light up.
• In an alternator, which is an alternating current generator, it is the coil that spins between the poles of a magnet. This is the equivalent of moving a magnet in and out of a coil.
• The brushes are not attached to the slip rings, they brush against the rings so that the voltmeter is always connected to the ends of a coil, but the coil does not get tangled.
• A graph of potential difference against time shows that the potential difference changes in direction from positive to negative.

  
• In a commercial generator in a power station, there is a rotating electromagnet inside a coil of wire.
• A dynamo is a direct current generator. The potential difference produced by a dynamo that drives a current does not change direction. You can use the same arrangement as a motor to produce a direct potential difference. The potential difference can change in magnitude, but it does not change direction because the coil is connected to a split ring commutator.

  
• In both alternators and dynamos, the output can be increased by:

  • using a stronger magnetic field.
  • using more turns on a coil.
  • spinning the coil faster.
• A transformer can be used to increase or decrease potential difference.
• There are three ways to change the field lines that cut a coil:

  • moving a magnet or the coil.
  • using another coil and turning it on and off. (changes the number of field lines being cut)
  • using another coil that has alternating current running through it. (current flows in one direction, then the other. The magnetic field lines around the coil changes).
• A transformer is a loop of iron with two coils around it. The magnetic field is "trapped" inside the iron core.

  • an alternating potential difference across the primary coil produces an alternating potential difference in it.
  • an alternating current in the primary coil produces a magnetic field in the core that is always changing.
  • The changing magnetic field from the primary coil induces a changing potential difference in the secondary coil

  
• P.d across primary / p.d across secondary = number of turns in primary / number of turns in secondary
• A step up transformer increases potential difference as the secondary coil has more turns than the primary one.
• A step down transformer has more turns in its primary coil than secondary coil.
• In a generator, movement produces a potential difference in a wire (and a current flows if there is a circuit).
• In a motor, the current that flows because of potential difference across the ends of a wire, produces movement.
• A dynamic microphone acts like a generator. A sound wave is a pressure wave. As sound hits the diaphragm, areas of high pressure (compressions) push the diaphragm in. And areas of low pressure (rarefaction) pulls it out. As the diaphragm moves, the coil moves too. This causes a potential difference to be induced into the wires. This is the "electrical signal" a microphone produces.

  
• In a carbon microphone, there are carbon granules behind the diaphragm. As sound waves hit the diaphragm, it changes the resistance of carbon. Current passing through the carbon changes as resistance does.
• A loudspeaker acts like a motor. If you connect the ends of a wire to a changing potential difference of a suitable frequency, you will hear a sound.