Electromagnetism
- A magnetic field is a region where magnetic materials experience a force.
- Magnetic field lines are used to show the size and direction of magnetic fields, which always point from North to South.
- Placing the N and S poles of two permanent bar magnets near each other creates a uniform field between the 2 magnets.
Magnetic Field Patterns
- Compasses and iron filings align themselves with magnetic fields. You can use multiple compasses to see magnetic field lines coming from a bar magnet or between two bar magnets.
- If you only have one compass, you can use it to move around. Before moving the compass, trace the direction of each position on paper.
- Avoid placing the compasses too close to each other, as they also produce magnetic fields. Ensure that you are measuring the field of the magnet, not the compasses nearby.
- You can also use iron filings to see magnetic field patterns.
→ Put a magnet under a piece of paper, scatter the iron filings on top, and tap the paper until the iron filings form a clear pattern.
Magnetic Induction
- Similar poles repel, while opposite poles attract (magnets).
- Both poles attract magnetic materials (that aren't magnets).
- Magnetic materials act as magnets when they come into contact with a magnetic field.
→ The original magnet has induced this magnetism.
- The closer the magnet and the magnetic material get, the stronger the induced magnetism will be.
Magnetic Materials
- A magnetic material is 'soft' if it loses its induced magnetism quickly and 'hard' if it keeps it permanently.
→ Iron = soft
→ Steel = hard
- Transformers use iron, which must magnetise and demagnetise 50 times per second.
- You can increase the strength of the magnetic field around a solenoid by adding a magnetically soft iron core through the middle of the coil.
Magnetic Fields
A current-carrying wire creates a magnetic field:
- An electric current in a conductor produces a magnetic field around it.
- The larger the electric current, the stronger the magnetic field.
- The direction of the magnetic field depends on the direction of the current.
- There's a magnetic field around a straight, current-carrying wire.
- The field is made up of concentric circles with the wire in the middle.
- The magnetic field in the centre of a flat circular coil of wire is similar to that of a bar magnet.
- These are concentric ellipses of magnetic field lines around the coil.
- The magnetic field inside a current-carrying solenoid (coil of wire) is strong and uniform.
- The field outside the coil is similar to that outside a bar magnet.
- This means the ends of the solenoid act like the N and S pole of a bar magnet.
- A solenoid is called an electromagnet.
The Motor Effect
- The motor effect happens when you put a current-carrying wire in a magnetic field.
- Place a wire carrying current between two magnetic poles. The magnetic fields affect one another. The result is a force on the wire. This can lead to the wire moving, a phenomenon known as the "motor effect."
→ This is because charged particles (e.g., electrons in a current) moving through a magnetic field will experience a force, as long as they're not moving parallel to the field lines.
- To experience the full force, the wire has to be at 90 degrees to the magnetic field.
- The force always acts in the same direction, relative to the magnetic field, the magnets, and the direction of the current in the wire.
- The magnitude of the force increases with the strength of the magnetic field.
- The force also increases with the amount of current passing through the conductor.
- Reversing the current/magnetic field also reverses the direction of the force.
- A useful way of showing the direction of a force is to apply a current to a set of rails inside a horseshoe magnet.
→ A bar is placed on the rails, completing the circuit. This action creates a force that propels the bar along the rails.
Fleming's Left Hand Rule
- Thumb = motion, 1st finger = field, 2nd finger = current
D.C. Electric Motor
Four factors that speed it up:
1) More current
2) More turns on the coil
3) Stronger magnetic field
4) The coil contains a soft iron core
- Because the coil is on a spindle and the forces act one up and one down, it rotates.
- The split-ring commutator alternates the electrical contacts every half turn to maintain the motor's rotation in the same direction.
- The direction of the motor can be reversed by:
→ Swapping the polarity of the DC supply.
→ Swapping the magnetic poles over.
- Fleming's LHR can be used to determine the best method for completing LHR for each side.
→ Draw the direction of current on both arms of the coil and complete LHR for each side.
Loudspeakers
- Loudspeakers work because of the motor effect.
- An amplifier feeds AC electrical signals to a coil of wire in the speaker.
- A permanent magnet surrounds the coil.
- Therefore, the AC signals exert a force on the coil, causing it to oscillate back and forth.
→ These movements make the cone vibrate, creating sound.
Electromagnetic Induction
- Electromagnetic induction is the creation of a voltage (and possibly current) in a wire that is experiencing a change in magnetic field.
The Dynamo Effect
- The dynamo effect uses electromagnetic induction to generate electricity using energy from KE stores.
- There are two situations where you can receive an EM induction:
1) An electrical conductor (often a coil of wire) moves through a magnetic field.
2) The magnetic field passing through an electrical conductor undergoes changes, becoming bigger, smaller, or reversing.
- You can test this by connecting an ammeter to a conductor, moving the conductor through a magnetic field, or moving a magnet through the conductor.
→ The ammeter shows the magnitude and direction of the induced current.
- If the movement direction reverses and the ammeter's direction reveals the magnitude, the induced voltage/current also reverses.
- To get a bigger voltage, you can:
→ Increase the strength of the magnet.
→ Increase the speed of movement.
→ Increase the number of turns on the coil.
- The generator has a magnetic field and movement; this induces a current/voltage.
A.C. Generators
- Generators rotate a coil in a magnetic field or a magnet within a coil.
- Their constructions are like a motor.
- The coil induces a current that changes direction every half turn as it spins.
- Instead of a split-ring commutator, AC generators have slip rings and brushes, so the contacts don't swap every half-turn.
→ This implies that they generate an AC voltage, which can be observed on a CRO display. Faster revolutions also produce more peaks and a higher overall voltage.
- Power stations produce electricity using AC generators; they obtain the energy required to spin the coil/magnet field in various ways.
Transformers
- Only work with AC.
- Transformers alter the voltage of an alternating current.
- They all have 2 coils: primary and secondary, joined with an iron core.
- An alternating voltage across the primary coil quickly magnetises and demagnetises the magnetically soft iron core, causing an alternating voltage in the secondary coil.
- The ratio between the primary and secondary voltages is the same as the ratio between the number of turns on the primary and secondary coils.
- Step-up transformers increase voltage.
→ More turns on the secondary coil than the primary coil.
- Step-down transformers decrease voltage.
→ More turns on the primary coil than the secondary coil.
- Transformers are nearly 100% efficient, so power in = power out.
- Step-up and step-down transformers are used when transmitting electricity across the country.
- The voltage produced by power stations is too low to be transmitted efficiently. P = IV ∴ the lower the voltage, the higher the current required to deliver the same power, and current causes wires to heat up.
- A step-up transformer boosts the voltage right before it's transmitted.
- Step-down transformers are used at the end of a journey to reduce the voltage so it's more useful and safer to use.