phy chem bi

double / triple award igcse science

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6.20 know and use the relationship: input power = output power

input power = output power

since power = current x voltage

Vp x Ip = Vs x Is

p = primary, s = secondary 

when a voltage of 12V is applied across the primary coil of a step-down transformer, a current of 0.4A flows through the primary coil. calculate the current flowing through the secondary coil if the voltage induced across it is 2V. assume that the transformer is 100% efficient

12V x 0.4 = 2 x Is

I = (12 x 0.4) / 2

I = 2.4A 

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6.19 know and use the relationship between input (primary) and output (secondary) voltages and the turns ratio for a transformer

input (primary) voltage / output (secondary) voltage = primary turns / secondary turns 

Vp / Vs = Np / Ns

for example

a transformer has 100 turns on its primary coil and 500 turns on its secondary coil. if an alternating voltage of 2V is supplied across the primary, what is the voltage across the secondary coil?

100 / 500 = 2V / S

(500 x 2) ÷ 100 = S

S = 10V

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6.18 explain the use of step-up and step-down transformers in the large scale generation and transmission of electrical energy

transformers that increase voltage are called step-up transformers. transformers that decrease voltage are called step-down transformers 

transformers are really close to being 100% energy efficient

in step-up transformers, as voltage increases but power stays constant, the current decreases. vice-versa for step-down transformers.

as current passes through a wire, energy is lost as heat. this is why after generation, currents are passed through step-up transformers. during transmission of energy across long distances, a minimum amount of energy is lost. 

when it enters towns and cities, the supply is passed through a step-down transformer so it can be used in homes. 

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6.17 describe the structure of a transformer, and understand that a transformer changes the size of an alternating voltage by having different numbers of turns on the input and output sides

when there is an alternating current, the shape of the magnetic field around it increases and decreases with current. 

when an ac current-carrying coil is placed near another wire coil, the changing magnetic field from the first coil is also applied to the second coil

as the magnetic field is constantly going in and out of the coil, the direction of current is constantly changing (alternating output) 

the size of the induced voltage depends on the number of coils. if there are more coils, more voltage is induced and vice versa 

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6.16 describe the generation of electricity by the rotation of a magnet within a coil of wire and of a coil of wire within a magnetic field and describe the factors which affect the size of the induced voltage

motion causes the coil inside the magnetic field to turn

as it rotates, the wires cut through the magnetic field lines, and a current is induced in the wires

as a result, the current flows to the slip ring, (one side of the wire is connected to one slip ring)

when the force acting on each side of the wire is changed (every half rotation), the direction of the current changes

this makes a a.c. current, and this generator is called an alternator.

factors that affect how much voltage is induced - speed that the magnet / wire is moved - strength of the magnet - the number of turns in the coil (surface area that moves through the magnetic field) 

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6.15 understand that voltage is induced in a conductor or a coil when it moves through a magnetic field or when a magnetic field changes through and describe the factors which affect the size of the induced voltage

if a wire is moved across a magnetic field at right angles, a voltage is induced in the wire. if the wire is part of a complete circuit, a current flows. this can also occur if a magnet is moved through a coil of wire 

when the magnet goes into the coil, the current is positive, and when it moves out again, the current is negative. if the polarity of the magnets is reversed, direction of current is reversed as well 

if the magnet is stationary, (whether in or out of the coil, no current is produced)

this is called electromagnetic induction 

factors that affect amount of induced current: - speed that the magnet / wire is moved - strength of the magnet - the number of turns in the coil (surface area that moves through the magnetic field) 

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6.14 describe how the force on a current-carrying conductor in a magnetic field increases with the strength of the field and with the current

force on a current-carrying conductor increases if:

- magnetic field increase - current increases

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6.13 use the left hand rule to predict the direction of the resulting force when a wire carries a current perpendicular to a magnetic field

use the fleming left hand rule to predict motion, current or field direction when the other two are given

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6.11 understand that there is a force on a charged particle when it moves in a magnetic field as long as its motion is not parallel to the field

if any charged particle (eg. a current running through a wire), it will experience a force from the magnetic field.

the introduction of a new magnetic field causes the magnetic field lines to overlap, and the stronger field lines push the particle to an area of weaker field lines (see more information here)

if the wire is running parallel to the field, the force will be felt along the wire, not perpendicular to it, so it does not move

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6.12 understand that a force is exerted on a current-carrying wire in a magnetic field, and how this effect is applied in simple d.c. electric motors and loudspeakers

if a current through a piece of wire is held at right angles to a magnetic field, the wire will move (force is created)

when a current flowing through a wire is put through a magnet, the magnetic field lines overlap. 

in some places, the direction of the fields are the same, and the magnetic field is stronger. in other places, the direction of the fields are in opposite directions, so the magnetic field is weaker. 

the wire is pushed from the strong part of the field to the weak part (the magnetic field lines don’t like being squished together) 

electric motors

the magnetic on the two sides of the coil are at different directions. one side feels the force pushing it downwards, whilst the other is being pushed upwards, so the coil rotates. 

the direction current on each side of the coil is switched when the coil is vertical by the commutator, so the forces acting on each side is switched and the rotation direction is maintained 

to increase rate of motor turns: - increase the number of turns or loops of wire (to make a coil) - increase the strength of the magnetic field  - increase current flowing through the loop of wire 

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6.10 sketch and recognise magnetic field patterns for a straight wire, a flat circular coil and a solenoid when each is carrying a current

the direction of the magnetic field / current can be determined by the right hand grip rule

the magnetic field of a solenoid looks a bit like a magnetic field (red arrows show direction of current and blue show magnetic field lines) 

magnetic field of a flat coil

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6.9 describe the construction of electromagnets

wrapping the wire around an iron core also makes the whole contraption an electromagnet

ways to strengthen the magnetic field: - adding more turns to the coil strengthens the field - increasing the current flowing through the 

the magnetic field disappears when the circuit is switched off 

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6.8 understand that an electric current in a conductor produces a magnetic field round it

when a current flows through a wire, a magnetic field is created around it - the field is quite weak - the field is circular in shape 

this is called electromagnetism

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6.7 describe how to use two permanent magnets to produce a uniform magnetic field pattern

when magnetic field lines are the same distance away from each other, it forms a uniform magnetic field pattern 

when two opposite poles are put next to each other, the magnetic field lines will all be going from the north to south pole, perpendicular to the magnets

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6.6 describe experiments to investigate the magnetic field pattern for a permanent bar magnet and that between two bar magnets

magnetic field lines can be seen by sprinkling iron filings above and surrounding the magnet 

this can be repeated with two bar magnets, with like / opposite poles near to each other 

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6.5 understand that magnetism is induced in some materials when they are placed in a magnetic field

magnetism can be induced in some non-magnetic materials when they are placed in a magnetic field (materials containing iron, nickel or cobalt) 

this is because the magnetic field encourages the electron line up and form poles inside the material 

in magnetically soft materials, the magnetism will disappear after the magnet is moved away (iron is a magnetically soft material) 

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6.4 understand the term ‘magnetic field line’

magnetic field lines visualise the main features of a magnetic field

they: - show the shape of the magnetic field - show the direction of the magnetic field (field lines travel from north to south) - show the strength of the magnetic field - where the field lines are closest is where the magnetic field is strongest