7 - magnets Flashcards
Induced Magnetism
An induced magnet is a material that becomes a magnet when it is placed in a magnetic field. Induced magnetism always causes a force of attraction. When removed from the magnetic field an induced magnet loses most/all of its magnetism quickly. The force between a magnet and a magnetic material is always one of attraction..
magnet poles
The poles of a magnet are the places where the magnetic forces are strongest. When two magnets are brought close together they exert a force on each other. Two like poles repel each other. Two unlike poles attract each other. Attraction and repulsion between two magnetic poles are examples of non-contact force.
Magnetic Fields
The region around a magnet where a force acts on another magnet or on a magnetic material (iron, steel, cobalt and nickel) is called the magnetic field.
The strength of the magnetic field depends on the distance from the magnet. The field is strongest at the poles of the and the spacing between field lines indicates the strength of the field - The closer together the lines the stronger the magnetic field.
The direction of the magnetic field at any point is given by the direction of the force that would act on another north pole placed at that point.
The direction of a magnetic field line is from the north (seeking) pole of a magnet to the south(seeking) pole of the magnet.
Permanent Magnetism
A permanent magnet produces its own magnetic field
magnetic compass
A magnetic compass contains a small bar magnet. The Earth has a magnetic field. The compass needle points in the direction of the Earth’s magnetic field.
Electromagnets
When a current flows through a conducting wire a magnetic field is produced around the wire. The strength of the magnetic field depends on the current through the wire and the distance from the wire.
Shaping a wire to form a solenoid increases the strength of the magnetic field created by a current through the wire. The magnetic field inside a solenoid is strong and uniform.
The magnetic field around a solenoid has a similar shape to that of a bar magnet Adding an iron ore increases the strength of the magnetic field of a solenoid. An electromagnet is a solenoid with an iron core.
right hand grip rule for a straight wire carrying a current
right hand grip rule for a solenoid carrying a current
The Motor Effect:
When a conductor carrying a current is placed in a magnetic field (at right angles to the field) the magnet producing the field and the conductor exert a force on each other. This is called the motor effect
What effets the size of the force in the motor effect
Size of current
Strength of the magnetic field
The orientation of the wire (strongest when wire at right angles to field)
Fleming’s Left Hand Rule:
The direction of force, current and magnetic field can be linked by using the left hand in the following way:
motor effect formula
For a conductor at right angles to a magnetic field and carrying a current:
F = BIL
F: force - Newtons (N)
B: Magnetic flux density (no. of field lines in a region - shows the strength of a magnetic field) - Tesla (T)
I: current - Amps (A)
L: length - Metres (m)
Electric Motors
- The current flows through the coils producing a magnetic field which interacts with the magnetic field of the permanent magnet.
- As the current on each side of the flows in opposite directions each side experiences the force in opposite directions which causes moments in the same direction
- (each half-revolution) the two halves of the (rotating) commutator swap from one (carbon) brush to the other
- (each half-revolution) the commutator reverses the current (in the coil)
- keeping the forces in the same direction (keeping the coil rotating)
Loud Speakers and Head Phones
current creates a magnetic field around the coil
which interacts with the permanent magnet field
producing a resultant force causing the coil/cone to move
when the direction of the current reverses, the direction of the resultant force reverses causing the coil to vibrate
This results in the cone vibrating causing air molecules to move
the movement of the air molecules produces the pressure variations in the air needed for a sound wave
the air molecules bunch together forming compressions and spread apart forming rarefactions
generator effect
If an electrical conductor moves relative to a magnetic field it cuts through magnetic field lines inducing a potential difference across the ends of the conductor
If the conductor is part of a complete circuit, a current is induced in the conductor.
An induced current generates a magnetic field that oppposes the original change, either the movement of the conductor or the change in magnetic field.
Factors that affect the size of the induced potential difference/induced current:
strength of magnetic field (magnetic flux density)
speed of relative motion
number of turns on coil
Factors that affect the direction of the induced potential difference/induced current:
reverse magnetic field/reverse polarity of magnets
reverse direction of magnet
Microphone
sound waves cause the diaphragm to vibrate
the diaphragm causes the coil to vibrate
the coil cuts magnetic field lines
a potential difference is induced across the ends of the coil
Alternator - AC Generator
- the coil cuts magnetic field lines
- a potential difference is induced across the coil
- there is a complete circuit, so a current is induced in the coil
- every half turn the potential difference reverses direction
- so (every half turn) the current changes direction
Dynamo - DC Generator
the coil moves through the magnetic field or the coil cuts magnetic field lines
a potential difference is induced across the coil
there is a complete circuit, so a current is induced in the coil
because each half-revolution, the two ends of the coil swap from one brush to the other
so the direction of the induced current / potential difference does not reverse every half rotation
Generator p.d. graph - alternator
blue line is a sketch of how they would change if they were both rotated at twice the previous speed.
Generator p.d. graph - dynamo
blue line is a sketch of how they would change if they were both rotated at twice the previous speed.
Transformers
A basic transformer consists of a primary coil and a secondary coil wound on an iron core.
- (the alternating current causes) a changing magnetic field around the primary (coil)
- This creates a changing magnetic field in the core
- This induces an alternating potential difference across the secondary coil (causing an alternating current)
Step-up and step-down transformers
The ratio of the potential differences across the primary and secondary coils of a transformer Vp and Vs depends on the ratio of the number of turns on each coil, np and ns.
A step-down transformer has more turns on the primary coil than turns on the secondary coil. It decreases the p.d. and increases the current
A step-up transformer has fewer turns on the primary coil than turns on the secondary coil. It increases the p.d. and descreasesthe current
Power Transmission:
If transformers were 100% efficient the electrical power output would equal the electrical power input. The equation that shows this in terms of current and potential difference is:
This shows that if the p.d. is increased the current must decrease.
By decreasing the current in the transmission lines less energy is dissipated to surroundings as thermal energy so a higher percentage of energy is usefully transferred
