mechanics, materials, electricity Flashcards
Current
- Movement of charge carriers
The greater the Current…
- The more carriers that flow
- The quicker the charge carriers flow
What is EMF?
The energy supplied by the power source to each unit of charge (1C)
- 1C = 1As
- EMF = P.D + Lost Volts
- lost volts = energy lost due to resistance.
What are lost volts?
Energy dissipated per unit charge
What is potential difference?
The energy transferred from the charge to the output of the electrical circuit is the P.D..
/
ability for the charge carriers to do work
Kirchoff’s 1st Law
Law 1: charge is conserved in an electrical circuit
- The sum of current into a junction = sum of current out of
a junction
- at a junction current splits in inverse to total resistance of
each path (e.g. see example below)
- Current entering a component is the same as current
leaving a component.
- The current passing through two or more components in series is the same through each.
Kirchoff’s 2nd Law
Law 2: energy is conserved in an electrical circuit.
- loop law/closed circuit law
- the algebraic sum of P.D.s in any loop must equal zero
- can find P.D. at component using the Directly Proportional relationship of P.D. and Resistance.
- Potential difference across components in parallel are the same (each loop is considered a closed system)
- For any complete loop of a circuit, the sum of the EMF is equal to the sum of the potential drop around the loop.
Principle of Moments
Anti-Clockwise Moments = Clockwise Moments
Magnitude of Moments
The moment is dependent on
- the magnitude of the force ( larger force, greater moment)
- perpendicular distance ( greater distance, greater moment)
M=fd
NM = N x m
Couples
A couple is a pair of forces acting on a body that are of equal magnitude and opposite direction, acting parallel to one another, but not along the same line
thus the resultant force is zero but the resultant moment is a non-zero value
A couple causes a turning effect but not by accelerating, instead by changing direction.
Centre of Mass & Weight
The point at which all the mass of an object is concentrated
The point at which all the weight of an object is concentrated
CM —> M
CM^2 —-> M^2
CM^3 —> M^2
%\x1000
%\x 10,000
%\x 1,000,000
Hooke’s Law
The force needed to stretch or compress a material is directly proportional to the extension or compression of the material from its natural length provided the limit of proportionality hasn’t need exceeded.
What is the UTS?
The ultimate tensile stress of a material is the greatest stress a material can handle.
_ for materials with high UTS’ we consider them to be stronger materials.
Brittle
A material that fractures without significant plastic deformation.
Ductile
The degree to which a material can withstand plastic deformation without fracturing.
Newton’s 1st Law
Object remains at rest or at constant velocity unless a resultant force is enacted upon it.
Inertia
- The tendency for an object to resist changes in velocity.
- ## Objects of greater mass have a greater inertia
Newton’s 2nd Law
The resultant force is directly proportional to the product of an object’s mass and acceleration
Why may an object moving upwards but slowing down experience a force downwards/ feel lighter?
Because an object decelerating is the same as on object accelerating downwards, thus the resultant force is downwards.
Newton’s Third law
If object A exerts a force on object B, then object B exerts an equal and opposite force of the same type on object A.
- The action and reaction forces must be the same and equal.
How does newton’s third law explain the motion of a vehicle?
The frictional force of the tire on the ground pushes it back whilst the frictional force of the ground on the tire pushes it forwards.
Conservation of Momentum
the momentum before = momentum after
given a CLOSED SYSTEM with NO external forces
Circuits in Parallel
- For components in parallel the potential difference is the same
- The total current in to a junction must equal the total current out of the junction
- Most of the current flows through the route with the smallest resistance.
- adding resistors in parallel decreases TOTAL resistance by providing an additional path for the current to flow through.
the definition of potential difference/ voltage
the work done per unit charge across a component
Circuits in series
- for components in series the current flowing is the same (current in = current out)
- the potential difference is shared in the same ratio as resistance
- adding resistors in series increases TOTAL resistance and therefore DECREASES current
What effect does adding a component to a circuit have if it’s in:
Series
Parallel
SERIES <—
- total resistance will increase because current will have to flow through all components/resistors
- this decreases current flow because V=IR (for a given p.d.)
PARALLEL <—
- total resistance will decrease because the current now has additional paths to flow through
- increases the current that can flow for a given pd
Ohm’s Law
The potential difference across a metallic conductor is directly proportional to the current through it, provided the physical conditions do not change.
Negative Temperature Coefficient Thermistor
- A thermistor where when the temperature goes up the resistance goes down
superconductor
A material which has zero resistivity at or below a critical temperature
applications:
strong electormagnets
to produce large currents
transmission cables
difficulty of superconductors
- the conductor will need to be cooled below a critical temperature which can be very low and may require liquid nitrogen or helium
internal resistance definition
the lost volts
the work done per unit charge across the internal resistance of the circuit.
Emf definition
The energy supplied per unit charge by the power source
terminal voltage
the work done per unit charge across the load.
Explain why the terminal voltage decreases as current increases on a terminal voltage-current graph
- As current increases, more energy lost inside the cell due to internal resistance increases.
- Since the lost p.d. larger and the terminal voltage decreases
or
- To increase the current the load’s resistance must be decreased. Internal resistance is unchanged.
When is power at a maximum in a circuit?
power delivered to the load is maximum when the resistance of the load = internal resistance of the power supply.
Displacement definition
Displacement is the shortest distance from the initial position to the final position of an object in motion.
/ a change in position
Angular displacement, angular velocity & centripetal acceleration
A change in position of an object taking a circular path defined by its change in angle.
- Δθ
The rate of change of angular displacement
the rate of change of angular velocity directed to the centre of an objects circular path caused by a centripetal force
( TIF: do you remember how we derive linear displacement, acceleration and velocity from khan academy?)
Why can we ignore the internal composition of an object when looking at the external forces applied on it?
Due to the force pairs ( ie the electromagnetic forces between the particles of the object holding it together) that naturally arise due to the nature of forces arising from the interaction of two objects.
According to Newton 3 those force pairs are equal in magnitude and opposite in direction so they cancel each other out overall in the system so it doesn’t impact the macroscopic motion of an object.
Why can a system not exert a net force on itself?
Each intern push or pull force on one part of a system is countered by an opposite push or pull force on another part of the system.
Explain why this is an incorrect free-body diagram for the forces acting on a bucket of balls being lifted at a constant velocity
- The force of one bucket on the ball and the balls on the bucket are valid newton 3 pairs but their interactions are internal to the system
- free body diagrams only model external forces
Explain why this free-body diagram that models the forces applied to a bucket full of tennis balls is incorrect
- It’s true that there are newton 3 pairs for the hand and system + the system and earth
- HOWEVER, only the forces in each pair that acts on the system should be modeled on the system’s free-body diagram.
- the other force in each pair acts on a different object
Law of Inertia
If the net force acting on a system is zero, the velocity of the system will remain constant.
Where Inertia - the tendency of objects to maintain their present state of motion (ie their present velocity)
Newton’s 2nd Law
F = ma
Force is proportional to acceleration- the greater the net force the more
rapidly the system’s velocity changes
Acceleration is inversely proportional to mass - the more massive an object/system is, the more inertia and thus resistance to change in velocity it has.
Thus the greater the mass the lower the acceleration
Why may an elevator still be moving upwards if it’s experiencing a resultant force and an acceleration downwards?
- The net force of an object does not affect the direction of motion (velocity£ on an object, it only dictates the direction of acceleration.
- a negative acceleration with a positive velocity means an object/system is slowing down
- thus the elevator is slowing down.
How does work done relate to the kinetic energy of an object in motion.
the net work done on an object during a displacement is equal to the change in the object’s kinetic energy during that displacement:
NET Work Done
The net work done on an object during a displacement is the sum of the work done by all individual forces acting on the object. (This is equivalent to first finding the net force, and then finding the work done by the net force.)
What is Kinetic Friction?
Kinetic friction refers to the force that acts on an object in motion, opposing its movement.
Kinetic friction is always present between two surfaces that slide against each other
Ff = coefficient of friction x normal force
Mechanical Energy
E(mech) = KE(tot) + PE(tot)
where E(mech) arises from a conservative force
- when the objects in a system only interact via conservative forces, and there are NO unbalanced external forces doing work on the system, the system’s mechanical energy is conserved.
therefore the ΔE(mech) = 0
because the system’s total KE and PE energies will not change over time
when a system has non-conservative forces ( e.g. friction and air resistance ) these forces do not have associated potential energies, when they do negative work on an object, the KE transferred is instead dissipated as thermal or sound energy.
therefore the work done by a nonconservative force is lost from a systems mechanical energy
(beyond A-level) Path-independency of conservative forces
work is independent of the path taken from the initial to final configuration
Carbon Dating
- plants and trees contain a small percentage of radioactive isotope, carbon-14
- due to carbon being taken in by living plants due to photosynthesis a small percentage of the carbon content of any plant is carbon-14.
- the isotope has a half-life of 5570 years ( thus there is negligible decay during the lifetime of a plant)
- after a tree is dead the proportion of carbon-14 decreases due to nuclei decay.
- by measuring the activity of the dead sample its age can be calculated
( carbon-14 is formed in the atmosphere as a result of comics rays knocking out neutrons from nuclei )
Why does a person experience a greater pain when catching a heavier ball than a lighter ball provided both are travelling at the same velocity?
- Due to the larger ball being more massive the momentum of the ball is greater than the smaller ball at the same velocity.
- when caught both of the momentum’s of the ball reduce to zero
- the larger ball will thus have a greater change in momentum over time than the smaller ball
- Fnet = Δp/t so the larger ball will apply a greater force on the hand of catcher.
How does pulling a hand back or downwards when catching an object reduce the amount of force experienced?
- increased time over which momentum is reduced to zero ( change in momentum)
- Fnet = Δp/t
- therefore there is a reduction in the rate of change of momentum
- causing a reduced force on impact
Two Cars, A and B beginning from rest start their vehicles. Car A’s engine applies a thrust force of 25,000N in 0.1s, Car B’s engine applies a thrust force of 50N for over a minute, which vehicle will have the greatest velocity upon starting?
Impulse = change in momentum
F*t = Δp
F = 25000N t = 0.1s IMP = 2500Ns
F = 50N t = 60s IMP = 3600Ns
both vehicles started from rest, therefore car B has a greater velocity.
An egg dropped from the same height on to a stone breaks, but on to a pillow it doesn’t, why?
- The egg will have the same change in momentum in both scenarios
- ∴ The impulse for both scenarios of the egg also remain the same
- however, the pillow is able to deform when egg makes contact, meaning the egg is falling/in motion for a greater time period
- for impulse to remain the same in both scenarios the force on the egg from the pillows upon impact must be smaller
- causing the egg to not crack
- F*Δt = impulse
What is the purpose of a crumple zone on a vehicle? FINISH
Elastic vs Inelastic collisions
Elastic: Total kinetic energy is conserved (of a macroscopic object)
Inelastic: Total kinetic energy is NOT conserved
A perfectly elastic collision has 0 transfer of KE to waste so the two interacting objects would definitely rebound
A perfectly inelastic collision will cause the two objects to stick together and move off as a single unit.
regardless of whether a collision is elastic or plastic, momentum WILL be conserved.
Why is kinetic energy not conserved in a collision but momentum is?
Kinetic energy is a scalar quantity so when kinetic energy is lost it is not cancelled out due to direction
momentum is a vector quantity so any momentum in one direction will be cancelled to zero by momentum in the opposing direction, therefore momentum is always conserved in a collision
