Magnetism and Electromagnetism Flashcards
What are the 4 magnetic metals?
Iron, steel, cobalt, nickel.
What are the two types of magnets?
Permanent and induced (can be induced by stroking an iron nail along a magnet).
How do you plot the magnetic field pattern of a magnet using a compass?
- Place the plotting compass near the magnet on a piece of paper
mark the direction the compass needle points. - Move the plotting compass to many different positions in the magnetic field, marking the needle direction each time.
- Join the points to show the field lines, marking on arrows based on the needle pointing to the south pole.
Can you draw the magnetic field pattern of a bar magnet?
Check it.
How is the behaviour of a magnetic compass prove the Earth’s core must be magnetic?
The compass’ north pole always points to Earth’s north pole, and the same with the south poles. This shows the compass’ magnet is affected by the earth, and therefore must be experiencing a force within a magnetic field, therefore the Earth has a magnetic field and this must be caused by the earth’s core. As opposite poles attract, if the compass’ north pole points to the Earth’s north pole, that must actually be its magnetic south pole.
When a current flows it causes a magnetic field, the strength of this field is the greatest when…
Close to the wire and if the current is increased.
How do you increase the magnetic field strength around a solenoid?
Size of current, no. of turns, iron core.
There is no motor effect force if… the force is greatest when?
The current and magnetic field are parallel. They are at right angles.
How do motors work? Draw a diagram.
- Current - magnetic fields interacting - force. In the left this is downwards, in the right it is up so the coil rotates anti-clockwise.
- When the coil is vertical it is parallel to the magnetic field and therefore should not move, but momentum and the split-ring commutator cause the motor to keep rotating.
- The conducting brushes reconnect, current flows in the opposite direction so each side of the coil is near the opposite pole and will keep rotating.
How do loudspeakers work? Draw a diagram.
- Alternating current in a coil generates a magnetic field.
- The electromagnetic field and permanent magnet interact, so the motor effect takes place and pushes the cone outwards.
- The current is reversed, so the direction of the field reverses and the cone is pulled back in.
- Repeatedly doing this makes the cone vibrate in and out, this creates pressure variations in the air, these are sound waves.
How is the result of the generator effect different regarding whether the circuit is complete?
If the circuit isn’t, potential difference will be induced across the ends of the conductor, if it is then potential difference will be induced in the conductor.
What will increase the induced current?
The speed of the movement (rate of rotation in alternators/dynamos), the magnetic field strength or the number of turns on the coil is increased.
How do alternators and dynamos work and differ? Draw diagrams.
- A coil is rotated in a magnetic field, so a current is induced.
- In an alternator, slip rings connect to the coil have carbon brushes that make continuous contact between the external circuit and slip rings, so current flows in the external circuit. Therefore, every half rotation as movement changes, so will the direction of current.
In a dynamo, the slip rings are instead split-ring commutators, which changes the coil connections every half-turn, so the current to the external circuit flows in the same direction.
What would be the properties of the potential difference-time graphs plotted of alternators and dynamos?
- An alternator: would be a sine curve, at 0=0, 90=maximum, 180=0, 270=-maximum, 360=0.
- A dynamo: sine curve with only positive values/one direction, 0=0, 90=maximum, 180=0, 270=maximum, 360=0.
How does a microphone work? Draw a diagram.
- Pressure variations in the air cause the diaphragm to vibrate and therefore the coil to vibrate.
- This movement relative to the magnetic field of the magnet and the conducting coil ( in a circuit ) causes induced potential and current.
- The changing size and direction of the current matches the vibrations of the coil, and the electrical signals generated match the pressure variations of the sound waves.
