4. (b) Magnetism Flashcards
State the factors that can affect the magnitude of induced an e.m.f
Speed in which the magnet moves
Strength of the magnet
The number of coils of the solenoid
State Lenzes Law
The direction of an induced current opposes the change causing it
Describe the simple construction of an A.C generator
Consists of:
+ rectangular coil which is forced to spin in a magnetic field
+ the coil is connected to an ammeter via metal brushes that press on two metal slip rings
Function of brushes and slip ring in an a.c generator
Provide a continuous connection between the coil and the ammeter
Graph of voltage output against time for a simple a.c generator
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Describe an experiment that shows that a changing magnetic field can induce an e.m.f in a circuit
Connect a solenoid with an ammeter
Move the magnet in and out of the solenoid
Observe the ammeter reading - a current flowing in both direction can be observe -> there is e.m.f induced
Move the magnet in 1 direction -> ammeter shows positive value
Move the magnet in reverse direction -> negative value
Magnet remains still: no voltage induced
To have an induced e.m.f, a magnet must cut through the magnetic field line
State the properties of magnets
A magnet has a magnetic field around it
The field is strongest at its poles
Field movement : N -> S
Can be created with an induced electric current
Magnetic field can be detected when they produce force
How to induce magnetism in a bar of ferrous material
Place a magnet near it
When the magnet is removed, the magnetism induced is lost
What are ferrous materials?
Metals which contains iron
Any ferrous materials can be magnetized or demagnetized
What are non-ferrous materials?
Metals which do not contain any iron
Only a few can be magnetized or demagnetized
Eg. Cobalt, Nickel
How to magnetise a metal?
Moving one end of a bar magnet along the material repeatedly in the same direction.
The head of the metal will become the same pole of the end of the magnet as it tries to repel the magnet as it approaches.
The tip of the metal will become the opposite pole of the end of the magnet as it tries to attract the magnet as it moves away
Or
Place the metal in a long coil of wire and pass a large DC current through the coil
How to demagnetise a metal?
Smash it with a hammer, dropping, etc
Heating to a high temperature
Place the metal in a coil of wire with AC current passing through then gradually removing it from the coil
Distinguish between the magnetic properties of iron and steel
Steel is a hard magnetic material, it is hard to magnetise but keeps it magnetism
Iron is a soft magnetic material, it is easily magnetised but easily loses its magnetism
Describe the elctromagnets
Also called temporary magnet
Created by an iron core with insulated wire coiled around
Made from soft ferromagnetic materials
When an electric current pass through the wire, the core becomes magnetised
When the current is switched off, the core loses most of its magnetism
Eg of uses: lift car in a rubbish dumb, circuit breaker
Describe the permanent magnets
Made from hard ferromagnetic materials eg. Steel
Made in different shapes including bars, flat rectangle, discs, cylinders,…
Eg of uses. Fridge magnet, horseshoe magnet, loudspeaker magnet
Advantages of temporary magnet over permanent ones
Can be switch on and off
Strength of magnetic field can be change, by changing the current
Can easily be made into a variety of shapes and are less expensive to make
Describe how an a.c generator works
Coil is rotated -> cuts magnetic field line -> emf generated -> current flow
Each side of the coil travels upward then downward then upward, etc
-> the current flows backward and forward -> alternating current
How can the emf induced by an ac generator can be increased
Increase the area of the coil
Using stronger magnet
Rotate the coil faster
Describe a transformer
Used for voltage transformation
Consists of two coil of insulated wire, both wound around the same iron core
ac current applied to the primary coil -> produces a changing magnetic field in the core
The line of the alternating magnetic field pass through the second coil and induce an ac current in the second coil
Why can’t a dc current be used for a transformer
The voltage applied to the primary coil must be ac because constant current cannot cause electromagnet induction
Two equations for transformer
Vp / Vs = Np / Ns - p= primary; s= secondary; N= number of turns
Vp / Ip = Vs / Is
Describe the use of the transformer in high voltage transmission of electricity
When a current flows through a wire, some energy is lost as heat
The higher the current, the more heat is lost
-> need to transmit electricity at a low current to reduce theses losses -> requires high voltage ( P= IV)
Step up transformer is used at power station -> produce very high voltage
But too high voltage are dangerous for the use in home -> step down transformer is used to reduce the voltage to a lower level before it is used
Advantages of high voltage transmission
Less power lost
Thinner, light and cheaper cables can be used since current is reduced
Save money
Why energy losses in cables are lower when the voltage is high
Due to the flow of electrons in the wire, there is heat loss while transferring electrical energy
The larger the current, the more energy is lost as heat to the surrounding
Base on the equation P = IV, when a generator supplies an amount of power, high voltage -> smaller current -> less energy is lost -> more efficient at supplying energy
