Magnetism - Physics

• Basic Magnet Principles


  Magnets repel and attract other magnets and attract magnetic substances.
• Magnetism


  The force exerted by magnets when they attract or repel each other
• Hard Magnetic Materials


  Substances that can be permanently magnetised
• Soft Magnetic Materials


  Substances that can only be temporarily magnetised
• Magnetic Field


  The area around the magnet where another magnet would feel a force
• Magnetic Field Line


  A line that shows the shape and direction of a magnetic field
• 3 Magnetic Materials


  Iron, Nickel and Cobalt
• What is the direction of a magnetic field?


  From north to south
• How can you tell which object is magnetic/the magnet: Experiment


  1. Place Bar A next to Bar B/C/D. Note if they attract/repel.

  2. Place Bar B next to Bar A/C/D. Note if they attract/repel.

  3. Place Bar C next to Bar A/B/D. Note if they attract/repel.

  4. Place Bar D next to Bar A/B/C. Note if they attract/repel.

  5. If 2 materials repel, they're the magnets.

  6. If nothing happens, one of the bars is non-magnetic.

  7. Use the discovered magnet against the others to find the magnetic bar.
• Investigating the Magnetic Field Pattern of a Magnet using a Compass: Experiment 1


  1. Place the compass around the magnet and draw the direction of the arrow

  2. Join up the arrows in the same direction.

  3. You now have the direction of the magnetic field.
• Investigating the Magnetic Field Pattern of a Magnet using Iron Filings: Experiment 2


  1. Spread out the iron filings into a plastic tray.

  2. Place the bar magnet in the centre of the tray.

  3. Observe the concentration of filings near the poles.
• Compass vs Iron Filings


  Iron filings don't show the field's direction; a compass does. A compass doesn't show the shape and magnitude of the field; iron filings do.
• How do you use 2 permanent magnets to produce a uniform magnetic field pattern?


  Two opposite poles form a uniform magnetic field. Place the opposite poles of two magnets close to each other to produce a uniform magnetic field.
• How is magnetism induced?


  1. Placing a soft magnetic material in the field of a magnet

  2. Rubbing a magnetic material with a magnet

  3. Running a current through a wire; a field is created around the wire
• What happens when an electric current passes through a wire?


  It will produce a magnetic field around the wire
• How do you create an electromagnet: Experiment


  1. Wrap wire 10 times around an iorn core.

  2. Connect 1 crocodile clip to each end of the wire.

  3. Connect the clips to a power supply.

  4. Turn on the power supply

  5. Change the current/no. coils as seen fit
• What is the shape of a magnetic field around a straight wire?


  A circle

  
• What is the shape of a magnetic field around a flat, circular coil?


  Two circles around each end of the wire with a straight line through the middle.

  
• What is the shape of a magnetic field around a solenoid?


  Like a normal bar magnet; two ovals flowing from north to south and more coming out of the poles.

  
• Charged Particle in a Magnetic Field


  There is a force on a charged particle when it moves in a magnetic field as long as its motion is not parallel to the field
• How do you increase the strength of an electromagnet?


  * Increase the current

  * Increase the number of coils

  * Add an iron core/nail
• What is the difference between a solenoid and an electromagnet?


  A solenoid is a coil and an electromagnet is a coil around an iron core.
• Advantages of Electrogmagnets


  * Can control the strength

  * Can turn it on/off
• How does an electromagnet bell work?


  1. The push switch is closed so current flows through the electromagnet.

  2. The electromagnet attracts the iron hammer.

  3. The hammer moves and strikes the gong.

  4. The contacts separate and the circuit is broken.

  5. The electromagnet is switched off and the hammer springs back.
• Motor Effect [3 mark answer]


  1. Current flows through the wire, inducing a magnetic field around it.

  2. The wire is placed in the field of a permanent magnet.

  3. The induced magnetic field interacts with the permanent magnetic field, creating a force.
• How do you increase the force in the Motor Effect?


  Increase the current flowing through the wire.
• How do you change the direction of the force in the Motor Effect?


  Reverse the direction of the current flowing through wire
• How does a Loudspeaker work?


  1. Current flows through the coil.

  2. A magnetic field is created around the coil.

  3. The permanent magnetic field interacts with the induced magnetic field.

  4. The Motor Effect occurs.

  5. As the current is alternating, it changes direction, moving back and forth.

  6. This happens very quickly, creating vibrations.
• How does a motor work?


  1. The power supply applies a potential difference around the coil.

  2. A current flows through the coil.

  3. A magnetic field is created around the coil.

  4. The induced magnetic field interacts with the magnetic field of the permanent magnet.

  5. This creates a force that makes the coil spin.
• SRC


  Split-Ring Commutator
• Fleming's Left Hand Rule


  First finger: field

  Knuckle: on the North

  Second finger: current

  Thumb: direction
• When is voltage induced in a conductor/coil?


  When it moves through a magnetic field or when when a magnetic field changes through it.
• How do you increase induced voltage?


  * Move the magnets faster.

  * Increase the number of turns between the primary and secondary coils.
• Induction [3 mark answer]


  1. The coil/wire is placed relative to the permanent magnet.

  2. The coil is moved up and down, cutting through the field lines.

  3. The movement trasnfers kinetic energy to the wire, creating a potential difference.

  4. The magnetic field interacts with the electrons, causing the flow of charge.
• A.C Generator

  A device that produces a potential difference. It uses a coil of wire rotating in a magnetic field.
• How does a Microphone work?


  1. The pressure varations in sound waves cause the diaphragm to vibrate.

  2. These vibrations cause vibrations in the coil.

  3. The coil moves relative to the permanent magnets so potential difference is induced in the coil.

  4. The coil is part of a complete circuit so the induced potential difference causes current to flow around the circuit.

  5. The changing size and direction of induced current matches the coil's vibrations.

  6. The electrical signals generated match the pressure vibrations in the sound waves.
• How do you change the direction of the voltage produced?


  Reverse the current by: changing the polarity or moving the magnet out of the coil.
• Transformer


  An object that changes the size of an alternating voltage by having different numbers of turns on the input and output sides.
• How does a step-up transformer work? [5 mark answer]


  1. A changing a.c current flows through the primary coil.

  2. This produces a changing magnetic field around the coil.

  3. The changing magnetic field is carried across by the iron core.

  4. A changing magnetic field is induced around the secondary coil.

  5. This induces a changing (a.c) voltage at the output p.d.
• Why are transformers used in the national grid?


  * If too much current flows thorugh a wire, it heats up.

  * This causes energy to be lost to the thermal store in the surroundings.

  * As this is not very efficient, we increase the voltage to combat this issue.

  * This is because P = IV so if voltage increases, current decreases, ensuing less heat is lost thermally.