Physics Flashcards
Conductors will only retain charge if
they are insulated from their surroundings
How can an objected be charged
- friction
- induction
If something is earthed
Can not become charged
Which way the electrons move?
Determined by whichever object has nuclei that attract the electrons less strongly (loses electrons)
Two factors which affect the electrostatic force
Larger the charges = larger the force
Larger distance = smaller the force
Sparking
Air between two objected becomes ionised by a large voltage and starts conducting Two charged objects that have air between them can discharge by a spark between them (when charge is large enough/distance is small enough)
How can risk of sparking be eliminated
By earthing; or if they are connected together by a wire then electrostatic charging cannot take place
Photocopying and printing
Charge being placed on the paper; exposed to toner powder which sticks to the paper at those locations as a result of electrostatic induction
Aircraft refuelling
Large volumes of fuel flow through the pipe - large amounts of friction - pipe is electrostatically charged - thus pipe is always earthed to prevent build up of charge
Wires crossing - NOT connected
Wires connected
Battery
Group of cells
Dc power supply symbol
Ac power supply symbol
The output from a power supply from mains electricity can be converted from ac to dc using
diodes as a ‘rectifier’.
A diode only allows current in one direction, in the direction of the arrow on the symbol.
Uk maims supply
50 Hz
(i.e. the current changes direction 100 times each second, producing 50 complete ‘to and fro’ cycles in one second).
Examples of good conductors:
- all metals, particularly copper, gold and silver
- carbon (in the form of graphite)
- ionic solutions.
Examples of good insulators (poor conductors):
- most non-metals, particularly plastics, rubber, dry wood, air, vacuum.
Water, unless extremely pure, is a conductor, so wet or damp materials are not good insulators.
all materials allow…
electric charge to move through them to some extent
Q
quantity’ of charge.
why metals are good conductors
ey contain free electrons that can move about through the metal and carry their (negative) charge with them
If a voltage is connected across a metal…
positive end of the metal attracts electrons and the negative end repels electrons.
In this way the electrons can move along the metal and cause a current (a flow of charge).
CURRENT DIRECTION
from the positive end of a conductor to the negative end
ELECTRON DIRECTION
Negative to positive
voltmeter is connected in
parallel
Why does a voltmeter need to have a HIGH resistance
otherwise it would tend to ‘short circuit’ the component across which it was connected
(because there would be a significant amount of current in the voltmeter instead of in the component).
Why does an ammeter need to have a low resistance
otherwise it would tend to reduce the amount of current that it was being used to measure.
Ohm’s law:
current is directly proportional to the voltage causing it at constant temp
resistance of the filament is not constant because
its temperature changes as the current in it changes
V-I graphs for fixed resistor + filament light bulb
thermistor
with a resistance that depends on its temperature.
Thermistor resistance
As its temperature increases, its resistance decreases.
Semi-conductor
LDR resistance
As the light intensity increases, the resistance of the LDR decreases.
Diode shows
Direction of current
Which one works
The temperature of the thermistor decreases.
What happens to the readings on meters 1 and 2?
1 = ammeter = decrease as more resistance from thermistors
2 = voltmeter = reduced current means reduced voltage in FIXED resistor as R is a constant in V=IR = causes voltage across thermistors to INCREASE, because total voltage supplied by battery has not changed
= voltmeter increases
Series
Current is same
Voltage adds up to total
Resistance adds up total
Parallel
Voltage is same
Current adds up in each branch
voltage
The difference in energy carried by each unit of charge either side of a circuit component (the energy lost or gained per unit charge)
How to find voltage across 2 resistors
Two resistors, of resistances 2Ω and 3Ω, are connected in series.
Combined resistance = 2 + 3 = 5Ω.
Note that 5Ω is greater than both 2Ω and 3Ω.
If these resistors are connected in series to a 10V supply, then the supply current = 10 / 5 = 2A.
The voltages across the two resistors are therefore 2 × 2 = 4V and 2 ×3 = 6V respectively.
Note that 4V and 6V add up to the supply voltage, 10V.
For two resistors of resistance R connected in series, the combined resistance is
R/2
Units of voltage
1V = 1JC-1
Two resistors, of resistances 3Ω and 6Ω, are connected in parallel. The parallel combination is connected in series with a third resistor, of resistance 4Ω, to a supply voltage of 18V. The 4Ω resistor dissipates a power of 36W.
How much energy is dissipated in the 6Ω resistor in 1 minute?
magnetic materials
iron, cobalt and nickel.
Magnetic field lines
NORTH TI SIUTH
Soft magnetic materials
easy to magnetise but also easily lose their magnetisation.
Iron
Hard magnetic materials
are difficult to magnetise but once they are magnetised, they are difficult to demagnetise.
Steel
Factors affecting the magnetic field created by an electric current
Reversing the direction of the current
Increasing the current increases
Proof to show electric current create magnetic field
Demonstrate by placing a small magnetic compass close to a current carrying conductor and then switching the current on and off ; compass needle will point north when the current is off and deflect from north when the current is on (current has created a magnetic field)
What is magnetic field actually created by
by these moving charges and not by the material through which they are moving (e.g. a copper conductor).
A beam of charged particles (e.g. electrons or ions) moving through a vacuum will also create a magnetic field, just like an electric current in a wire.
magnetic field pattern around a long, straight current-carrying wire:
consists of concentric circles,
that become farther apart at greater distance from the wire,
and have a direction given that can be predicted using a right-hand grip rule.
Right Hand rule
Magnetic field through solenoid
The strength of the magnetic field around a wire depends on:
the current in the wire: increasing current increases magnetic field strength
the distance from the wire: farther from the wire the field is weaker
the medium surrounding the wire: magnetic media such as iron can increase the field strength.
Iron
ferromagnetic material.
Each iron atom acts like a tiny bar magnet being north at one end and south at the other.
Differences between electromagnets and permanent magnets
How to increase strength if solenoid
- increase number of coils
- increase current
Why can’t increase current to crazy
Strong heating effect = melt insulation
How do strong electromagnets avoid this heat problem
by using superconducting coils, that is coils of zero resistance.
Disadvantage of superconducting coils
coils only become superconducting when cooled to extremely low temperatures using liquid helium.
A permanent magnet can be made by
placing a hard magnetic material in a strong external magnetic field, usually from an electromagnet.
How can hard / Permanent magnets suddenly lose their magnetism
Heated above curie temp = specific to each material
Maximum motor force
when the current and magnetic field are at right angles to one another.
Reversing the direction of the current OR the magnetic field..
Reversing the direction of the current OR the magnetic field
Reversing the directions of both the current and the magnetic field
results in no change in the direction of the motor effect force.
If the magnetic field and current are not at 90°
then the direction to use is the part (component) of the magnetic field that is perpendicular to the current, as shown in the diagram:
motor effect force is directly out of the page.
ANGLE
angle between magnetic field and current: force is greatest at 90° and zero at 0°.
Investigating the strength of the motor effect force
When there is a current in the wire there is an upward motor effect force on the wire.
By Newton’s third law, there is an equal downward force on the magnets.
The magnets press down on the top pan balance and the reading increases.
This change can be used to measure the motor effect force (using W = mg).
Tesla
1T = 1 Nm-1A-1
Angle in motor effect
when is force at maximum + minimum
It is at its maximum when the two forces are farthest apart (coil in plane of field)
zero when the two forces are in the same vertical plane (coil perpendicular to field).
graphite brushes in split
make a low friction sliding contact with the surface of the commutator but mai
The material used to make the brushes must be a
CONDUCTOR
Electromagnetic indication
ONLY HAPPENS IF THEIR IS A CHANGE = cutting field lines
Will Electromagnetic induction always induce voltage
Yes - but only current in a closed system
The magnitude of an induced voltage is directly proportional to:
the rate at which a wire cuts magnetic field lines
or
the rate at which the magnetic field through a conductor (e.g. a coil) changes.
The induced voltage will increase if:
- the magnet is moved faster – more field lines are cut per second – the rate of cutting field lines increases
- a stronger magnet is used – there is a higher density of field lines, so more field lines are cut per second than with a weaker magnet at the same speed – the rate of cutting field lines increases
the ac frequency in coil A is increased – the rate of change of the field through coil B increases
- the ac amplitude in coil A is increased – the rate of change of the field through coil B increases*.
Lenzes law
induced voltage is always in a direction that opposes the change that caused it.
The direction of an induced voltage reverses when:
the direction of the cutting of magnetic field lines reverses
an increasing magnetic field in a coil changes to one that is decreasing
a decreasing magnetic field in a coil changes to one that is increasing.
The amplitude of the output ac voltage increases if:
the coil is rotated more rapidly – greater rate of change of magnetic field through the coil (or of cutting magnetic field lines)
the magnetic field is stronger – greater rate of change of magnetic field through the coil (or of cutting magnetic field lines)
the coil has greater area –greater rate of change of magnetic field through the coil (or of cutting magnetic field lines)
there are more turns on the coil – each coil has the same induced voltage and these voltages add together because the turns are in series.
The frequency of the output ac voltage is equal to
the coil’s rotation frequency.
If the time for one rotation of the coil is doubled (generator)
the coil is rotating more slowly and it cuts the field at a lower rate inducing a smaller voltage
FREQUENCE HALFS USING f=1/time periods
Do the slip rings change polarity
Yes
For half of the rotation, one slip ring is positive and the other negative but in the other half of the rotation, the wires are moving through the field in the opposite direction, so the slip rings both change polarity, becoming negative and positive respectively.
Angle positions
Draw a graph of voltage output for one cycle
Increasing the frequency of rotation of the coil has two effects:
it increases the frequency of the output ac voltage because the direction of cutting of the field lines changes more rapidly
it increases the amplitude of the output ac voltage because the rate at which the field lines are cut increases.
What’s peak + what’s zero
Position 1 = peak
Position 2 = zero
Generators transfer
mechanical work (to rotate the generator) to electrical energy in the form of ac electricity
A step-up transformer
increases the voltage
Mains voltage
230V
Transformer equation
Vp = (ac) voltage across the primary coil
Vs = (ac) voltage across the secondary coil
np = number of turns on the primary coil
ns = number of turns on the secondary coil
Transform equation thing
only valid for an ideal transformer, i.e. one that is 100% efficient
Other transformer equation
the current ratio is the inverse of the voltage ratio
Put two transformer equations together
the current ratio is equal to the inverse ratio of the turns:
Why not 100% Effiecient
the resistance in the wires on the coils
heating effects in the core as it magnetises and demagnetises
currents induced in the core (eddy currents) by the changing magnetic field.
What is used in transmission lines
High voltage low current
- reduce losses due to heating of the cables.
The higher the voltage
the harder it is to insulate from other conductors.
Cause of weight
mass in a gravitational field
Cause of normal contact
two solid objects in contact with each other
Cause of drag
movement of an object through a fluid
Cause of friction
relative sliding motion between two solid surfaces
Magnetic
two magnets or a current in a magnetic field
Electrostatic
two charges or a charge in an electric field
upthrust
solid immersed in a fluid
thrust
driving force from an engine
lift
aerofoil (wing) moving through a fluid
non-contact forces
weight associated with a gravitational field,
magnetic force associated with a magnetic field
electrostatic force associated with an electric field
force-extension graph: steeper
steeper this graph, the more force is required to produce a given extension. = more rigid
The shallower this graph, the greater the extension for a given force.= less rigid
Elastic
if the spring / wire returns to its original length when the tension force is removed.
Hooke’s law.
Springs and wires that are being extended within their elastic limits experience an extension that is proportional to the tension force
F=kx
spring constant
force per unit extension
Spring constant is a measure of
Rigidity
Materials with high spring constants….
require large forces to produce small extensions
(Gradient)
Cross-sectional area + K
the greater the cross-sectional area, the greater the spring constant
Length + K
longer the wire, the smaller the spring constant