magnetism/electromagnetism Flashcards
What phenomenon does electromagnetism describe?
Electromagnetism describes the phenomenon whereby electric currents produce their own magnetic fields.
How does an electric current in a wire affect its surrounding magnetic field?
An electric current flowing through a wire produces a magnetic field around the wire.
What is the shape of the magnetic field lines around a straight wire carrying current?
- The magnetic field lines are concentric circles around the wire.
- They are closest together near the wire where the magnetic field is strongest.
What does the direction of the magnetic field depend on?
The direction of the magnetic field depends on the direction of the electric current.
How can you determine the direction of the magnetic field around a wire using your hand?
You can use the right hand rule: point your thumb in the direction of the current and your fingers will curl in the direction of the magnetic field.
What happens to the magnetic field direction when the current direction in a wire is reversed?
The direction of the magnetic field also reverses when the current direction is reversed.
What occurs when two sides of a circular coil interact with each other?
- The concentric circular magnetic fields get stretched out and form ellipses.
- A single magnetic field runs straight through the center of the coil
What is a solenoid?
A solenoid is a long piece of wire wound into a coil with many turns.
How does the magnetic field within a solenoid compare to that of a bar magnet?
The magnetic field within a solenoid is strong and uniform, similar to that of a bar magnet.
What is an electromagnet?
An electromagnet is a magnet created by using electricity to produce a magnetic field.
What happens to an electromagnet when the current is turned off?
The magnetic field disappears when the current is turned off.
How can the direction of the magnetic field in an electromagnet be reversed?
The direction of the magnetic field can be reversed by reversing the direction of the current flow.
What are the four ways to increase the strength of an electromagnet?
- Increase the current flowing through the solenoid.
- Increase the number of turns in the coil while keeping the length the same.
- Decrease the length of the coil while keeping the number of turns the same.
- Add an iron core inside the solenoid.
Why does adding an iron core to a solenoid increase its magnetic field strength?
Iron is a soft magnetic material that becomes an induced magnet when the solenoid is switched on, increasing the magnetic field strength.
What happens to the induced magnetism of the iron core when the current is turned off?
The induced magnetism of the iron core disappears when the current is turned off.
What material was discovered to create the first magnet?
Lodestone or magnetite
What property of lodestone was observed when it was freely suspended?
It settles facing the north-south direction.
What did Hans Christian Oersted discover in 1819?
Electric current can influence compass needles.
What did Michael Faraday discover in 1831?
The motion of a magnet towards or away from a circuit can produce a current.
What are the two main properties of a magnet?
Magnetic pole (polarity) and directional property.
What happens when a bar magnet is dipped into iron filings?
The iron filings cling mainly around the ends of the bar magnet.
What does the observation of iron filings around a bar magnet indicate?
Magnetic attraction is strongest at the ends of the bar magnet.
How can you demonstrate the polarity of a magnet?
By dipping a bar magnet in iron filings and observing where most filings cling.
How can a magnet be used as a compass?
It shows which side is the north pole and which side is the south pole.
Give examples of magnetic materials.
Iron, nickel, and cobalt.
Give examples of non-magnetic materials.
Plastic, wood, and rubber.
What are ferromagnetic materials?
Materials that are strongly attracted by a magnet.
how to use the right-hand thumb rule?
Using your right hand, point your thumb in the direction of current and curl you fingers. The direction of your fingers is the direction of the magnetic field
how to use the left-hand rule?
1) Using you left hand point your first finger in the direction of the Field
2) point your seCond finger in the direction of the Current
3) your thuMb will then point in the direction of the force (the Motion)
how can you reverse the direction of a motor?
- swap the polarity of the dc supply (reversing the current)
- swapping the magnetic poles over (reversing the field)
what is the motor effect?
1) an alternating current - ac - is sent through a coil of wire attached to the base of a paper cone
2) the coil surrounds one pole of a permanent magnet, and is surrounded by the other pole so the current causes a force on the cones - which means the cone moves
3) when the current reverses, the force acts on the cone in the opposite direction, causing the cone to move in the opposite direction
4) So variations in the current make the cone vibrate, which makes the air around the cone vibrate. This creates the variation in pressure that causes a sound wave
5) the frequency of the sound wave is the same as the frequency of the alternating current, so if you control the frequency of the ac, you can alter the sound wave that is produced.
what is the generator effect?
The induction of a potential difference - and current if its a complete circuit - in a wire that is moving relative to a magnetic field or experiencing a change in magnetic field.
what is generated because of the generator effect?
Electricity
how do Alternators generate an alternating current?
1) generators rotate a coil in a magnetic field
2) their construction is pretty much like a motor
3) as the coil spins, a current is induced in the coil. This current changes direction every half turn
4) ac generators have slop rings and brushes so the contacts do not swap every half turn
5) this means they produce an alternating potential difference
how do Dynamos generate Direct Current?
1) Dynamos work in the same way as alternators, apart from one key difference.
2) Dynamos use split-ring commutators instead of slip rings
3) this means the connection swaps every half turn to keep the current flowing in the same direction
what do you use an oscilloscope for?
To see the generated potential difference.
AC - line that goes up and down, crossing the horizontal axis
DC - the line is not straight, but it stays above the axis (always positive)
How do microphones generate current from sound waves?
1) sound waves hit a flexible diaphragm that is attached to a coil of wire, wrapped around a magnet
2) this causes the coil to move in the magnetic field, generating the current
3) the movement of the coil and generated current depends on the properties of the sound wave (like volume)
4) this is how microphones can convert the pressure variations of a sound wave into variations in current in an electric circuit
what do transformers do?
change the potential difference - but only for an alternating current.
