Magnetism & Electromagnetism

  • Created by: AMcL1
  • Created on: 28-05-19 13:12

Permanent and Induced Magnetism

  • Magnetic Material - A material that uses non-contact force to attract other magnetic materials or to be attracted by a magnet.
  • North Seeking Pole - The end of a magnet pointing north. For example, a compass needle is a bar magnet and points north.
  • South Seeking Pole - The end of a magnet pointing south. With magnets, like poles repel and opposite poles attract.
  • Magnetic Field - The region of force around a magnet. If there's a strong field, there is a large force, and if there is a weak field, there is a smaller force. The field is strongest at the poles.
  • Permanent Magnet - A magnet that produces its own magnetic field. One of these will repel or attract other magnets or magnetic materials.
  • Induced Magnet - A temporary magnet that only becomes a magnet when placed in a magnetic field.
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Magnetic Field Around A Wire

  • An electric current flowing in a wire produces a magnetic field around it. If the current is small, the magnetic field is very weak, but, if the current is large enough, iron filings can be used to show the circular magnetic field. Also, further away from the wire, the magnetic field is weaker, and if the current is reversed, so is the direction of the magnetic field. 
  • Solenoid - A long coil of wire where the magnetic field from each loop adds to the next.
  • Electromagnet - In an electromagnet, more turns of wire increase the magnetising effect when the current flows, so, if the current is turned off, the magnetism is lost. 
  • Relay - This is a device that uses a small current to control a larger current in another circuit. This is how it is made and how it works: A solenoid is wound around an iron core. A small current magnetises the solenoid. This attracts to electrical contacts, making a complete circuit. The current then flows from the battery to start the motor.
  • In order to increase the strength of a magnetic field, you can: use a larger current, use more coils of wire, put the coils of wire closer together or use an iron core in the middle. 
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The Motor Effect - Part 1

  • Electric Motor - In an electric motor, a coil of wire rotates about an axle. The current flows through the wire causing a downward movement on one side and an upward movement on the other side.
  • Generators - A generator is a coil of wire rotating inside a magnetic field. The end of the coil is connected to slip rings. Generators produce an alternating current (AC).
  • Split-ring Communicator - This is a split ring touching two carbon brush contacts.
  • Loud Speakers - These convert variations in electrical current into sound waves. In them, a varying current flows through a coil that is in a magnetic field. As this current varies, a force on the wire moves backwards and forwards. The coil is connected to a diaphragm. The diaphragm movements then produce sound waves.
  • Microphones - These convert pressure variations in sound waves into variations in current in electrical circuits.
  • Fleming's Left Hand Rule - This is used to predict the direction a straight conductor moves in a magnetic field. The thumb represents the direction of the force/F, the first finger represents the direction of the magnetic field/B (always travels from north to south) and the second finger represents the direction of the current/I (whether it is going away from or towards you). 
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The Motor Effect - Part 2

  • The motor effect is where magnetic fields from the permanent magnet and the current in the foil interact. If the current is reversed, the foil moves upwards. If the aluminium foil is placed between two poles of a strong magnet, it will move downwards when current flows through it. The size of the force acting on the foil depends on magnetic flux density between poles, size of current and the length of foil between poles.
  • Magnetic Flux - The lines drawn to show a magnetic field. If there are lots of lines, it is a stronger magnet.
  • Magnetic Flux Density - The number of lines of magnetic flux in a given area. This measures the strength of the magnetic force.
  • Force = Magnetic Flux Density x Current x Length (F = BIL)
  • If the current and magnetic field are parallel to each other, there is no force on the wire. If they are perpendicular to each other, there is a force acting on the wire.
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Induced Potential, Transformers & National Grid

  • The National Grid - The network that distributes the eletricity produced in power stations around the UK.
  • Induced Potential - This is when a conducting wire moves through a magnetic field, causing a potential difference to be produced.
  • The Generator Effect  This is where electricty is generated by inducing a current or a potential difference. It has many uses, such as in microphones.
  • Transformer - A transformer is two coils of wire onto an iron core. In a transformer, an alternating current (AC) is supplied to the primary coil, causing the magnetic field to change. Then, the iron core becomes magnetised, and so it carries the changing magnetic field to the secondary coil. This induces a potential difference.
  • Step-up Transformers - These increase the voltage but decrease the current, which increases efficiency by reducing the amount of heat lost from the wires. 
  • Step-down Transformers - These decrease the voltage (to 230V) but increase the current, which makes the value of the voltage safer for houses and factories to use. 
  • Voltage of the Coil x No. of Primary Coils = Voltage of the Coil x No. of Secondary Coils
  • Power = Current x Potential Difference (P=IV)
  • Power Supplied to Primary Coil = Power Supplied to Secondary Coil (Ip x Vp = Is x Vs)
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Units

  • Force - Newtons (N)
  • Magnetic Flux Density - Tesla (T)
  • Current - Amperes (A)
  • Length - Metres (M)
  • Power - Watts (W)
  • Potential Difference/Voltage - Volts (V)
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