Chapter 9 (9.9 - 9.12) - Generators, Transformers, Power across d country Flashcards
What are alternators
Generators that give out Alternating Current (AC)
Components of an AC generator
2 slip rings
2 brushes
Rectangular coil
magnet
How does an AC generator work
The coil rotates. This cuts the magnetic fields so an emf is generated (faraday’s law)
Each side of the coil travels upwards, downwards, upwards, downwards, etc. Therefore the current flows backwards, forwards, etc.
When is the current from an ac gen maximum
When coil is horizontal and cutting field lines at fastest rate
How to increase EMF (and current) in AC generator
Increasing number of turns in coils
Increasing area of coil
Using stronger magnet
Rotating coil faster
What is mutual induction
When coils are magnetically linked so that the changing of current in one causes an induced emf in the other
How to increase the increased mutual induction
Core of the electromagnet goes right throuhg the second coil
Number of turns in the coil on the second coil is increasd
When is the emf induced in mutual induction
WHen the electromagnet’s current is switched on or off. This is equivalent to pushing a magnet in or pulling it out respectively. Induces a current for a fraction of a second
What does a transformer do
2 types
Increases or decreases AC current
Step up and Step down transformers
How does a transformer work
AC flows into the primary coil which is wrapped around an iron core. This leads to an alternating magnetic field being generated. The changing field induces an alternating voltage in the secondary coil
Formula for transformer
Output voltage/input voltage = turns on output coil/turns on input coil
What is a step up transformer
The secondary coil has more turns than the primary coil
Increases the voltage
Reduces the current (ohm’s law)
What is a step down transformer
The secondary coil has less turns than the primary coil
Reduces the voltage
Increases the current (ohm’s law)
Power formula in a transformer
Input voltage * Input current = output voltage * output current
What happens to power in a transformer
Remains constant since as the voltage increases, the current decreases in prop
vice versa
3 stages of power in a country
Generation
Transmission
Distribution
Explain entire process of power in a country
Power station
Step-up transformer
High voltage power lines
transformer substation
Power lines
transformer substation
Heavy industry
transformer substation
Light industry
transformer substation
Homes, Schools, Offices, farms, etc.
What is the grid
A nationwide supply network for power
Adv of the grid
Power stations can be sited away from heavily populated locations
No matter where the power station is located – a power station in an area where the demand is low can be used to supply areas where the demand is high.
How is power in a grid distributed
using a series of substations
What is a substation
Contain step-down transformers, which reduce the voltage in stages to the level needed by consumers.
What is the voltage for homes
This voltage is usually between 110 V and 230 V for home consumers, depending on the country.
Why is AC used compared to DC
Transformers can be used with AC which make it easier to distribute
Transformers don’t work with DC
Formula for power loss
power loss = current * voltage drop
voltage = current * resistance
hence power loss = current^2 * resistance
Why is a high voltage used
current is reduced and the heating effect is also reduced. This leads to less power loss because resistance is lesser.
Thinner, lighter and cheaper cables can be used as transmission cables. This saves a lot of money as well as energy.
Adv and disadv of overhead cables
Adv:
Cheaper
Easier to insulate (air is the insulator)
Money is saved since digging operation costs are avoided
Disadv:
Spoil the environment and its beauty (therefore underground cables are used then)
how is emf generated in transformers
alternating current causes magnetic field in iron core
magnetic field changes
field lines get cut
emf is hence induced
why do transformers not work with a dc voltage supply?
changing magnetic flux, induces emf in the secondary, no change of flux with a constant dc supply
in generators which hand is used
right hand
we can think of it as generator has 2 Rs so right hand
