PMT Notes

• Magnetic materials


  Materials that are attracted to magnets and can be magnetised (e.g. iron, steel, cobalt, nickel)
• Non-magnetic materials


  Materials that are not attracted to magnets and cannot be magnetised (e.g. glass, plastic)
• Induced magnetism


  1. Magnetic materials can be magnetised by stroking them with a magnet, hammering them in a magnetic field, or putting them inside a coil with a direct current through it

  2. They can be demagnetised by hammering them, heating them or putting them inside a coil with an alternating current through it
• Magnetically hard


  Magnetic materials that can be permanently magnetised (e.g. steel)
• Magnetically soft


  Magnetic materials that are only temporarily magnetised (e.g. soft iron)
• Permanent magnets


  Hard-magnetic material that has been permanently magnetised
• Electromagnets


  Consist of a coil of wire wrapped around a magnetically soft core and can be turned on and off
• Permanent magnets are more useful when they do not need to be turned off such as a fridge magnet, whereas electromagnets have the ability to be turned on and off so they can be used for situations such as moving scrap metal
• Magnetic field lines


  • Point from north to south
  • The direction of a magnetic field line shows the direction of the force on a north pole at that point
  • Field strength decreases with distance from the magnet
• Plotting compasses are small compasses which show the direction and shape of a magnetic field
• Electric charge


  Measured in coulombs, there are positive and negative charges, unlike charges attract and like charges repel
• Charging a body


  Involves the addition or removal of electrons
• Conductors


  Allow electrons to flow through them
• Insulators


  Impede the flow of electrons
• Charging by induction


  Electrons flow from the ground into the disc as they are attracted to the rod, and the electroscope now contains a net negative charge
• Electric field lines


  • Point away from positive charges and towards negative charges
  • The field lines around a charged conducting sphere are as if the charge was concentrated at the centre of the sphere
  • The field lines between two charged plates go in straight lines from the positive plate to the negative plate and are equally spaced apart
• Current


  Measured in amps, the rate of flow of charge at a point in the circuit
• In metals, current is due to a flow of electrons. Because electrons are negatively charged, conventional current (which is the rate of flow of positive charge) is in the opposite direction to the flow of electrons
• Electromotive force (e.m.f.)


  Measured in volts, the energy supplied by the source per unit charge in driving the charge round a complete circuit
• Potential difference


  Measured in volts, the work done per unit charge in moving between two points in a circuit
• Resistance


  The greater the resistance, the harder it is for current to flow through the component
• As the length of a resistor increases

  The resistance increases
• As the diameter of a resistor increases

  The resistance decreases
• Ohmic conductor


  The current is directly proportional to the voltage (i.e. it has constant resistance)
• Non-ohmic conductor


  The resistance changes as the voltage and current change (e.g. a filament lamp)
• As the current increases through a filament lamp

  The temperature increases, electrons and ions vibrate more and collide more, increasing resistance
• Power


  P=IV, P=I^2R, P=V^2/R
• Series circuits


  • Components are connected end to end in one loop
  • The same current flows through every component
  • The potential difference is shared across each component
  • The total resistance is the sum of the resistances of each component
• Parallel circuits


  • Components are connected to the power supply in separate branches
  • The current is shared between each branch
  • The potential difference is the same across every branch
  • The total resistance of two resistors in parallel is less than the resistance of either resistor by itself
• Potential divider circuit


  Divides the source voltage into smaller parts
• Thermistor


  A resistor whose resistance decreases as the temperature increases
• Light dependent resistor


  A resistor whose resistance decreases as light intensity increases
• Relay


  An electromagnetically operated switch
• Relays are used to switch on a circuit with a high current using a circuit with a small current
• Diode


  Only allows current to flow in one direction, can be used as a rectifier to convert AC into DC
• Analogue signals


  Vary continuously in amplitude, frequency or both
• Digital signals


  A series of pulses with two states, a high state and a low state
• Digital signals carry more information per second and maintain their quality better over longer distances compared to analogue signals
• All signals get weaker as they travel longer distances and need to be amplified so they can be returned to their original strength
• Relay


  • Switches on and attracts an iron arm
  • Iron arm rotates about a pivot and pushes the contacts in another circuit together