Physics paper 2 Flashcards
what is a system
a single object or group of objects that your interested in
what happens when a system changes
energy is transferred into or away from the system, between different objects in the system or between different types of energy stores
some energy is dissipated and stored in less useful ways
what is a closed system
the net change in energy is zero
define work done
the energy transferred by a force acting over a distance
the force does work to move the object and energy is transferred mechanically from one store to another
when energy is wasted what can you still say
work has been done
what is the equation for work done
E = f x d
work done = force distance
(J) = (N) x (m)
what does a force doing work often cause
a rise in temperature as energy is dissipated to the thermal stores of the moving object and its surroundings. this means that the process is wasteful and the efficiency of the process in reduced
what is the equation for efficiency
useful / total
what happens when you push something along a rough surface
you are doing work against frictional forces
energy is been transferred to the kinetic energy store of the object because it starts moving but some is also being transferred to the thermal energy stores due to friction
this causes the overall temperature of the object to increase
what is power
the rate of energy transfer (how much work is been done every second)
what is power measured in
watts
what is the formula for power
work done / time taken
what is force
a push or a pull on an object that is caused by it interacting with something
what are non-contact forces caused by
interacting fields
what are contact forces caused by
friction
why is there a gravitational attraction between the Earth and the Sun
because their gravitational fields are interacting
what can interacting magnetic fields cause
attraction or repulsion between magnetic objects
what does interaction between electric fields cause
attraction or repulsion between electrical charges
what is Newtons third law
whenever two object interact both objects feel an equal but opposite force - an interaction pair
what does a free body force diagram show
an isolated body and all the forces acting on it
what do objects in equilibrium have
a resultant force of zero
either stationary or moving at a steady speed
what is moment
a force, or several forces can cause an object to rotate, the turning effect of a force is called its moment
what is the equation for the moment of a force
force x distance
when will a current only flow through an electrical component
a current will only flow through an electrical component if there is a potential difference across that component and if the circuit is closed/complete
the higher the potential difference across a given component
the higher the current will be
calculate charge
current x time
how would you investigate the relationship between current, pd and a chosen component
the standard test circuit
- set up circuit with d.c source, voltmeter, ammeter and component of choice
- change the output pd of the power supply
- this alters current flowing through the circuit and the pd across the component
- take several pairs of readings from the ammeter and voltmeter to see how the current through the component varies as the pd across it changes
- plot current against pd to get current, pd graphs
- use data to work out resistance for each measurement
- you can see how resistance changes as I and V change
don’t let circuit get too hot or readings will mess up
disconnect between readings to cool down
repeat
work out means
how can you investigate thermistors
keep the pd supply constant and gradually heat the thermistor
place thermistor is beaker of hot water
as temperature increases, current in thermistor increases and resistance decreases
how can you investigate LDR’s
conduct in a dim room
keep pd constant and slowly adjust the light level near to LDR using lam with dimmer switch
light level gets brighter, current through LDR increases and resistance decreases
what is current and pd like in a series circuit
current is the same everywhere
pd is shared between components
what is current and pd like in a parallel circuit
current is shared between the branches
- total current flowing around the circuit is equal to the total of all the currents through sperate components
pd is the same across all components
what happens if you add two resistors in a series circuit
the total resistance of the two components is just the sum of the resistances because they have to share the total potential difference. The bigger a components resistance, the bigger its share of the total potential difference
what happens if you add two resistors in a parallel circuit
if you have two resistors in a parallel circuit
their total resistance is less than the resistance of the smallest of the two resistors
- in parallel both resistors have the same pd across them as the source
- this means the pushing force making the current flow is the same for each resistor you add
- but by adding another loop the current has more than one direction to go in
- this increases the total current that can flow around the circuit
- using V=IR an increase in current means a decrease in the total resistance of the circuit
what does the power transferred by an appliance depend on
potential difference and the current flowing through it
what is the neutral wire
blue
completes the circuit
electricity flows out through the blue wire which is always at 0V
what is the live wire
red or brown
carries the voltage at 230V
what is the earth wire
green and yellow
for safety and protecting the wiring
it carries the current away if something goes wrong and stops the appliance becoming live
always at 0V
why does touching the live wire give you an electric shock
your body is at 0V so when you touch it a large pd is produced across your body and current flows through you
what would happen if you made contact with the live wire in a plug socket that is turned off
still a danger of an electric shock
a current isn’t flowing but there is still a pd in the live wire
if you made contact with the live wire your body would provide a link between the supply and the earth so current would flow through you
give one advantage and disadvantage of using circuit breakers instead of fuses
- they turn off quicker than a fuse melting
- they can be reset instead of replaced
- but more expensive
when do you not need an earth wire
when an appliance is double insulated
the changing direction of the current…………….
changes the direction of the magnetic field
what happens when a current, carrying conductor ( a wire) is put between magnetic poles
the two magnetic fields interact and the result is a force on the wire
how can you find the size of the force acting on a conductor in a magnetic field
F = B x I x L force = magnetic flux density x current x length of wire (N) = (T) x (A) x (m)
how is a build up of static caused by friction
- when insulating materials are rubbed together electrons will be transferred from one material to the other
- as the materials are insulators these electrons are not free to move and this build up of charge is called static electricity
- the materials become electrically charged with a + static charge on one that has lost electrons and an equal - charge on the other
which becomes negatively charged out of a polythene rod and an acetate rod
acetate = + polythene = -
electrically charged objects……………..
exert a force on each other
how can you see the forces between two electrically charged objects
suspend a rod with a known charge from a piece of string so its free to move and place charged objects next to it to see repulsion and attraction
what happens if you run a comb through your hair
electrons will be transferred to the comb making it negatively charged. It can then be used to pick up little pieces of uncharged paper - holding it near the little pieces of paper causes induction in the paper which means they jump and stick to the rod
how does to much static cause a spark
- as electric charge builds on an object, pd between earth and object which is 0V increases
- if the pd gap gets large enough electrons can jump across the gap between the charged object and the earth - this is the spark
- they can also jump to any earthed conductor that is near by which is why you can get static shocks from clothes or getting out of a car
- usually happens when the gap is fairly small but lightning is just a massive phat spark
how do photocopiers use static electricity
copy images onto a charged plate before printing them
how can static electricity in industrial chimneys be used
to reduce the dust and smoke that rises
why is there a static danger when refuelling cars
as fuel flows out of a filler pipe then static can build up which can lead to a spark causing an explosion in dusty or fumy places like petrol stations
why is there a static danger when on airplanes
as planes fly through the air friction between the air and the plane causes the plane to become charged
this build of charge can interfere with communication equipment
why is there a static danger in lightning
raindrops and ice bump together inside storm clouds leaving the top of the cloud positively charged and the bottom of the cloud negatively charged. This creates a huge voltage and a big spark which can damage homes or start fires when it strikes the ground
how can you stop electrostatic charge building up
- dangerous sparks can be prevented by connecting a charged object to the ground using a conductor (earthing)
- earthing provides an easy route for the static charges to travel into the ground
- this means no charge build up to give you a shock or make a spark
- the electrons flow down the conductor to the ground if the charge is negative and flow up the conductor from the ground if the charge is positive
- fuel tankers must be earthed to prevent any sparks that might cause the fuel to explode
discuss electric field lines
- always at right angles to the surface
- positive to negative
- closer together the lines are the stronger the field is, the further you go from a charge, the weaker the field is so the lines are further apart
- it is the region around a charged object where if a second charged object was placed inside it, a force would be exerted on both the objects
how can sparking be explained by electric fields
when an object becomes statically charges it develops an electric field
interactions between this field and other objects are the cause of events like sparking
sparks are caused when there is enough pd between a charged object and the earthed object
a high pd causes a strong electrostatic field between the charged and earthed object
strong field causes electrons in the air particles to be removed (ionisation)
air is normally an insulator but when it is ionised it is much more conductive so a current can flow through it - the spark
why is steel magnetic
because it contains iron
what do permanent magnets always do
produce their own magnetic field
what does magnetically soft mean
it will lose its magnetism quickly
- pure iorn
- nickel iron alloys
what happens when current flows through a long straight conductor
a magnetic field is created around it
what happens when a current carrying conductor is placed between two magnetic poles
a force is exerted on the wire
to experience the full force the wire has to be at 90 degrees to the magnetic field
if the wire runs along the field it won’t feel any force at all
the changing direction of the magnetic field or current will change the direction of the force because the force always acts in the same direction relative to the magnetic field and direction of the current
