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Topic 7 Magnetism and Electromagnetism By Bethan Poole (7.2 The motor…
Topic 7 Magnetism and Electromagnetism
By Bethan Poole
7.1 Permanent and induced magnetism, magnetic forces and fields
7.1.1 Poles of a magnet
Two types of magnetic pole- North seeking pole and a south seeking pole
Poles are the places where the magnetic forces are the strongest
Unlike poles attract
(North attracts south)
Like poles repel
(north will repel north)
Permanent magnets
- produce their own magnetic fields
Induced magnets
- become magnetised when placed in a magnetic field, when removed they lose magnetism quickly
7.1.2 Magnetic fields
Magnetic field
- Region around a magnet where a force acts on another magnet/ magnetic material
Strength of field depends on distance from the magnet (stronger at the poles)
Arrows on field lines always run from north to south- how direction of the force that would act on a north pole placed at that point
Force between permanent magnet and a magnetic material or induced magnet is always on of attraction
Flux Density
- density of field lines and indicates strength of the field at that point - closer the lines, higher the flux density
Higher flux density= stronger the field and the greater the force felt by another magnet
Plotting Fields
Magnetic compass contains a small bar magnet
Compass needle aligns with the earths magnetic field and always points to the magnetic north
Provides evidence that the Earth's core is magnetic
Magnetic compass can be used to plot the magnetic field around a bar magnet...
1)
Place the bar magnet on a piece of paper
2)
Place the compass at one end of the magnet
3)
On the paper, mark where the point of the compass needle is
4)
Move the compass so the tail of the needle is at the point that has just been marked
5)
On the paper, mark a new point where the needle is
6)
Repeat and connect the marks until the whole field is plotted
7.2 The motor effect
7.2.1 Electromagnetism
When a current flows in a conducting wire a magnetic field is produced around the wire
Direction of field lines depends on the current- can be found with the right hand grip method
Grip wire in right hand (thumb in direction of current)
Fingers curled around wire will point in the direction that field lines should be drawn
Strength of field depends on size of current and distance of the wire
Effect can be seen by placing magnetic compass at different points along the wire and turning the power supply on and off
Solenoid
- formed when a wire is looped into a cylindrical coil- this increases the strength of the magnetic field creating a strong uniform field inside it
Iron core can be added to increase strength further (makes an
electromagnet
)
Solenoid increases magnetic field strength because
2)
Looped shape means magnetic field lines around the wire are all in the same direction
1)
It concentrates a longer piece of wire into a smaller area
Magnetic field around a solenoid has a similar shape to a bar magnet
North pole can be found with right hand grip method
1)
Hold solenoid in right hand with fingers following direction of current
2)
Thumb points to north pole of the solenoid
Electromagnetic devices
Many devices use electromagnets
Example
Electric bell
1)
When switch pushed electromagnet magnetised
2)
Electromagnet attracts armature
3)
Hammer strikes the gong breaking the circuit
4)
Armature springs back completing the circuit again and remagnetising the gong
5)
Cycle continues for however long the bell is pressed
7.2.2 Fleming's left-hand rule
Motor Effect
- When a current carrying conductor is placed in a magnetic field it experiences a force
Caused by the field created by the current interacting with the magnetic field
Force can be increased by increasing
Size of current
Length of conductor in the magnetic field
Flux density
Reversing direction of current or magnetic field with reverse force direction
Direction of force can be found with Fleming's left-hand rule
1)
Hold left hand so thumb, first finger and second finger are at right angles to one another
2)
Make 1st finger point in direction of magnetic field
3)
Rotate wrist so 2nd finger points in direction of current
4)
Thumb points in direction of force
Left hand rule can be used to find direction of either the field, current or force so long as two others are known
7.2.3 Electric motors
Current carrying the coil in a magnetic field will rotate due to the current going up one side of the coil is in the opposite direction to the current coming back down the other side so one side moves up and the other moves down
They are designed so that the coil rotates continuously
The brush contacts at the commutator ensure the current direction in the coil is always the same direction
This makes sure the motor is continuously turning and doesn't stop in an upright position
Fleming's left hand rule
can be used to work out which direction the motor will rotate
Increasing current or magnetic field will make the motor rotate fasted
Reversing current or magnetic field will make motor spin in the opposite direction
7.2.4 Loudspeakers
Use variations in alternating current to produce sound waves
1)
As current travels through coil a force is experienced due to magnetic field- makes speaker cone move
2)
As current in alternating, direction of force alternates so speaker cone oscillates
3)
Increasing electrical power produces a larger force so cone will oscillate with a higher amplitude= larger sound waves produced
4)
Freq. of sound produced matches freq. of alternating current
5)
Size of speaker affects the speed it can oscillate and the size of the oscillations
6)
Large cone better for low pitch sounds and small cone better for high pitch
7.3 Induced potential, transformers and the National Grid
7.3.1 Induced potential
Generator Effect
- When conductors and magnetic fields interact a potential difference can be induced
PD may be induced by:
Electrical conductor moving relative to a magnetic field
Change in magnetic field around a conductor
Increasing speed of movement or size of magnet will increases size of induced PD
Reversing direction of movement or magnetic field will reverse direction of induced PD
7.3.2 Uses of the generator effect
Alternators
Generator effect used to generate alternating current
Rotating magnet is used with a fixed coil wire
As the magnet rotates direction of the field through which the coil passes alternates
This induces an alternating potential and current
Speed of rotation is freq of alternating current
Dynamos
Generator effect used to generate a direct current
Coil rotates and the magnet is fixed
As coil rotates it generates a PD in one direction
Use of split-ring commutator means once coil passed upright, connections are reversed
Means direction of current output is always same direction
Both
Induced PD is greatest when magnetic field and coil are parallel so magnetic field is being cut by rotating the coil at a faster rate
During rotation- at one point coil and field are at right-angles so field lines not 'cut'- her induced potential is zero (can be seen on graphs
Graphs PD against time determine rotation speed- time between two 0 points is half a rotation
7.3.3 Microphones
Moving coil microphone uses the generator effect to convert sound waves-> electric signals
1)
Sound waves hit the microphone
2)
Change in air pressure related to the sound wave cause microphone diaphragm to oscillate
3)
Microphone diaphragm and coil vibrate at same freq. as incoming sound wave- bigger amplitude of sound= bigger amplitude of vibration
4)
This induces a PD and current in coil with same freq. as incoming sound wave and an amplitude dependent on the amplitude of the incoming wave
7.3.4 Transformers
Consists of a primary and secondary coil wrapped around a soft iron core
(iron as it's easily magnetised)
1)
Alternating current flows through primary coil (effectively a solenoid)
2)
Alternating current induces a magnetic field, which induces an alternating PD in secondary coil
3)
If Secondary coil part of a complete circuit, alternating current flows in secondary coil
If transformers were 100% efficient power output would = power input
Power= voltage x current
- if PD increases current decreases and vice versa