P15 Flashcards
Which metals are magnetic?
Colbalt, iron, nickel
How can you stop the effect of a magnet ?
You can stop the effect of a magnet by shielding it with a magnetic material.
What happens when a current is passed through a coil of wire ?
When a current is passed through a coil of wire, the coil has a magnetic field around it. This is called an electromagnet.
What do all magnets have?
A north and south pole
What is a magnetic field ?
A region where other magnets or magnetic materials experience a force ( this is a non-contact force )
What do all magnetic materials produce?
All magnetic materials produce a magnetic field
The lines in a magnetic field diagram travel from which end to which end?
The lines travel from north to south
Where is the magnetic field strongest at ?
It is strongest at the poles
What does a current carrying wire produce ?
It produces a magnetic field
What are the 3 things that can effect the power of an electromagnet
-the core materials
-number of coils
-size of the current
What is a solenoid?
It is a long coil of insulated wire
How does an electric bell work ?
- When the current flows through the circuit, the electromagnet makes a magnetic field.
- The electromagnet attracts the springy metal arm.
- The arm hits the gong, which makes a sound.
- The circuit is broken now the arm is out of position.
- The electromagnet is turned off and the springy metal arm moves back.
- The circuit is complete again.
- The cycle repeats as long as the switch is closed.
What is the motor effect ?
When a current-carrying wire (or any other conductor) is put between magnetic poles, the magnetio field around the wire interacts with the magnetic field it has been placed in. This causes the magnet and the conductor to exert a force on each other. This is called the motor effect and can cause the wire to move.
How can you increase the motor effect?
The size of the force can be increased by:
-increasing the current
-using a stronger magnet
How can the wire experience the full force of the motor ?
To experience the full force, the wire has to be at 90 degrees to the magnetic field. If the wire runs parallel to the magnetic field, it won’t experience any force at all. At angles in between, it will feel some force
Where does the force always act at?
The force always acts at right angles to the magnetic field of the magnets and the direction of the current in the wire
The force acting on a conductor in a magnetic field depends on which three things ?
- the magnetic flux density - how many field (flux) lines there are in a region. This shows the strength of the magnetic field
- the size of the current through the conductor
- the length of the conductor that’s in the magnetic field
Whats the equation of the force that acting when the current is at 90 degrees to the magnetic field it is in ?
Force (N) = Magnetic flux density (T) x Current (A) x Length (m)
How can you find the direction of the force ?
You can find the direction of the force with Flemings left hand rule
Using your left hand point your index finger in the direction of the Field
Point your middle finger in the direction of the current
Your thumb will then point in the direction of the force (motion)
How does a current-carrying coil of wire rotate in a magnetic field ?
the coil is on a spindle and the forces act one up and one down, it rotates.
The split-ring commutator swaps the contacts every half turn to keep the motor rotating in the same direction.
The direction of the motor can be reversed either by swapping the polarity of the de supply (reversing the current) or swapping the magnetic poles over (reversing the field).
What does it mean by induced potential difference ? What is the generator effect
If an electrical conductor moves relative to a magnetic field or if there is a change in the magnetic field around a conductor, a potential difference is induced across the ends of the conductor.
If the conductor is part of a complete circuit, a current is induced in the conductor - this is called the generator effect.
What is the definition of the generator effect ?
The induction of a potential difference (and current if there’s a complete circuit) in a wire which is moving relative to a magnetic field, or experiencing a change in magnetic field
How can you do the generator effect ?
-You can do this by moving a magnet in a coil of wire OR moving a conductor (wire) in a magnetic field (“cutting” magnetic field lines).
-Shifting the magnet from side to side creates a little “blip” of current if the conductor is part of a complete circuit.
-If you move the magnet (or conductor) in the opposite direction, then the potential difference/current will be reversed. Likewise if the polarity of the magnet is reversed, then the potential difference/current will be reversed too.
-If you keep the magnet (or the coil) moving backwards and forwards, you produce a potential difference that keeps swapping direction - an alternating current.
Where is the north and south for a solenoid ?
The south pole is the end where current is clockwise
The north pole is the end where current is anti-clockwise
What do transformers change the size of ?
Transformers change the size of the potential difference of an alternating current
What do all transformers have ?
They have two coils of wire, the primary and secondary, joined with an iron core
What happens when a alternating pd is applied across a coil ?
When an alternating pd is applied across the primary coil, the iron core magnetises and demagnetises quickly. This changing magnetic field induces an alternating pd in the secondary coil
The ratio between the primary and secondary potential difference is the same as the ratio…
Between the number of turns on the primary and secondary coils
What do step up transformers do? What do they have more of on the secondary coil
Makes the pd across the secondary coil greater than the pd across the primary coil
There are more turns in the secondary coil than in the primary coil
What do step down transformers do ?
Makes the pd across the secondary coil less than the pd across the primary coil
There are fewer turns in the secondary coil than in the primary coil
What does it mean when it says that the core is laminated (layered) ?
It means that it cuts out induced currents in the iron and this increases the efficiency of the transformer
What is the transformer equation ?
Vp/Vs = np/ns
Np meaning the number of turns on the primary coil
Ns meaning the number of turns on the secondary coil
How efficient are transformers ? And why ?
Transformers are almost 100% efficient
Becuasw When a device is connected to the secondary coil, almost all of the electrical power supplied to the transformer is delivered to the device.
How does a loudspeaker work ?
• The moving coil loudspeaker creates sound waves when an alternating p.d is applied to its coil
• The coil is in an magnetic field, the current in the coil cause a force on the coil due to the motor effect
• The alternating current causes the force to constantly change direction, making the coil and the diaphragm to vibrate
• The vibration causes sound waves to be produced of the same frequency as the alternating p.d source
How do microphones work ?
1) Microphones are basically loudspeakers in reverse.
2) Sound waves hit a flexible diaphram that is attached to a coil of wire, wrapped around a magnet.
3) This causes the coil of wire to move in the magnetic field, which generates a current.
4) The movement of the coil (and so the generated current) depends on the properties of the sound wave (louder sounds make the diaphragm move further).
5) This is how microphones can convert the pressure variations of a sound wave into variations in current in an electric circuit.
