6.1 Units
Ampere (A)
Volt (V)
Watt (W)
6.2 Magnets repel and attract
Same poles will repel, different poles attract. Magnets also attract other magnetic substances like:
- Iron (steel)
- Nickel
- Cobalt
- Neodymium
These substances are magnetic.
6.3 properties of magnetically hard and soft materials
Magnetically hard materials permanently retain their magnetic properties and are hard to demagnetize.
Magnetically soft materials will only temporarily retain their magnetic properties.
6.4 magnetic field lines
These are the lines that show the magnetic field of a magnet. The more compactly packed they are, the stronger the magnetic field in that area.
They may not be very accurate in representing magnetic field b/c:
- Does not show 3 dimensions
- Shows spaces between the fields lines, actually there are no spaces
6.5 magnetism induced in materials when placed in a magnetic field
Some materials which are not magnets can be magnetized when placed a magnetic field.
This is because the field encourages the magnetic domains to align up and form poles, temporarily magnetizing the substance.
6.6 experiment to investigate magnetic field patterns for a bar magnet
Using iron fillings
- Elevate a piece of paper by 1 cm over a bar magnet
- Slowly sprinkle iron fillings on the paper until they align to show the magnetic field lines
Using small compases
- Place a bar magnet on flat surface
- Align small cpmpasses in a way that north of compass points at the south of the compass placed after it. Make magnetic field line with the compass needles joint together like this.
Arrows are drawn from north to south.
NS
6.7 using two permanent magnets to produce a uniform field pattern
Join the north and south of two permanent magnets together. They will act as one one big magnet and the field lines will go around it in a loop.
6.8 electric current produces a magnetic field
A conductor with electric current flowing through creates a magnetic field.
6.9 construction of electromagnets
They have Insulated copper with electric current flowing through wrapped on a soft iron core in loops.
6.10 magnetic field patterns for straight wires, a flat circular coil and a solenoid when each is carrying current
Solenoid is very similar to a bar magnet.
6.12 Force exerted on a current carrying wire
If a current carrying wire is placed in a magnetic field, a force will be exerted on it, it will go up or down.
This principle is used in d.c electric motors as applying current courses the motors coil to repel or attract the magnetic field generated by the magnets in the motors.
This is also used in loudspeakers as current is applied to a coil which generates a magnetic field repelling or attracting the permanent magnet in the loudspeaker causing it to vibrate rapidly and producing sound by pushing air particles.
Thumb = Force
Magnet = Pointer
Current = Middle
6.14 force on a current carrying wire increases with current and magnet strength
The force increases on the current carrying wire when
- The magnet strength is increased
- The current is increased on the wire – which increases the strength of its field
- When the number of coils on the wire is increased – also increases the strength of its field
6.15 voltage induced in a conductor and factors that affect the size of voltage
A voltage is induced on the end of a current-carrying wire (conductor) when it moves through a magnetic field. This is called electromagnetic induction or the generator effect.
If the wire is a part of a complete circuit, a current is also induced.
The voltage gets bigger and visa versa when
- The speed of movement increases – for the magnet or the conductor
- The strength of the magnetic field increases – of the magnet
- Number of turns on the coil increases
- The area of the coil and the magnet is greater
6.16 generation of electricity using electromagnetic induction
A magnet can be rotated inside a coil of wire or a coil of wire can be rated inside a magnetic field to produce electric current.
The factors that affect the size of the induced voltage are:
- The speed of movement increases – for the magnet or the conductor
- The strength of the magnetic field increases – of the magnet
- Number of turns on the coil increases
- The area of the coil and the magnet is greater
6.17 transformers structure and working
It can change the voltage and current of electricity
Have a look at this image: http://www.s-cool.co.uk/gifs/g-phy-elmag-dia16.gif
Have a look at this image: http://www.s-cool.co.uk/gifs/g-phy-elmag-dia16.gif
More coils on the output sides increase the amount of voltage flowing and decreases the amount of current. The power is same on both the sides assuming efficiency is 100%. This is a step up transformer.
Less coils on the output side decreases the voltage but increases the current. This is a step down transformer.
The a.c current in the primary coil is always changing, this creates a dynamic magnetic field which causes electromagnetic induction in the secondary coil inducing a voltage in it.
6.18 use of step up and step down transformers in large scale generation of electricity
The national grid has to carry a of power across the long distances from power stations to cities. For the long distance, step up transformers are used to increase the voltage but reduce the current so the long wires with a lot of resistance don’t heat up.
A step down transformer is used to reduce the voltage before the electricity can be used by the appliances.
6.20 transformer power transfer equation
I1 x V1 = I2 x V2
Only true for 100% efficiency transformers. If the efficiency for the transformer is lets say 90% the second part of the equation will equal to 90% of the first part of the equation.