Physics P1 Flashcards
What is a physical quantity?
A measurement of something and it’s unit
SI base units
(length, mass, time, current, temp, amount of substance)
Length - metre (m)
Mass - Kilogram (kg)
Time - second (s)
Current - Ampere (A)
Temp - Kelvin (K)
Amount of substance - Mole (mol)
How do you derive units?
Put together the units from the equation that make the physical quantity
What is a scalar of quantity?
A quantity that only has magnitude
What is a vector quantity?
A quantity that has magnitude and direction
Give 8 examples of scalar quantities
Distance, speed, time, area, volume, mass, energy, temperature, electrical charge, frequency
Give 5 examples of vector quantities
Displacement, velocity, acceleration, momentum, force, weight
How do you calculate the resultant force of a vector triangle?
Use Pythagoras theorem
What does resolving a vector mean?
Separating the horizontal and vertical velocities as mutually perpendicular components
Components of a vector
- Velocity in the x-direction = Fcos(0)
- Velocity in the y-direction = Fsin(0)
- Angle made by the vector = tan(0)=x/y
How to add non-perpendicular vectors
- Make sure/ rearrange the horizontal and vertical velocities head to tail
- Create a vector triangle
- Use the sine or cosine rule to find the resultant force
What is the cosine rule?
a^2 = b^2 + c^2 - 2bc x cos(A)
What a the sine rule?
a/sin(A) = b/sin(B) = c/sin(C)
How do you measure the distance travelled by an object?
A meter rule or a measuring tape
Define speed
The rate of change of distance
What is the gradient in a distance time graph?
The speed
Define displacement
The distance in a given direction
Define velocity
The rate of change of displacement
How do you calculate velocity?
Change in displacement/time taken
In a displacement-time graph, what is the gradient?
The velocity
In a displacement-time graph, what does a negative gradient represent?
Motion in the opposite direction
What does the gradient of a curve represent in a displacement time graph?
Drawing a tangent on a curve determines the instantaneous velocity
Define acceleration
The rate of change of velocity
How do you calculate acceleration?
Change in velocity/time taken
What does the gradient of a velocity – time graph represent?
The acceleration
What does the area under a velocity – time graph represent?
The displacement of the object
What does a straight line on a velocity – time graph represent?
Constant speed
What do the letters of SUVAT represent?
S - displacement/distance
U - initial velocity
V - final velocity
A - acceleration
T - time taken
What is stopping distance?
The sum of thinking distance and breaking distance
Define thinking distance
The distance travelled between the moment the driver sees a reason to stop and the moment the driver applies the brake
Define breaking distance
The distance travelled by the car from the instant the break is applied until the car stops
What are thinking distance and breaking distance proportional to?
- Thinking distance is directly proportional to u
- Breaking distance is directly proportional to u^2
Factors that affect thinking distance
Reaction time, drugs, alcohol, tiredness
Factors that affect breaking distance
Car conditions, road conditions, weather conditions
What is freefall?
All objects on the surface of the earth for the same acceleration as long as air resistance is negligible.
This acceleration of freefall is equal to 9.81 m s^-2
How do you determine the speed of a falling/moving object?
Light gates connected to either a timer or data locker
Projectile motion - vertically and horizontally
- If an object is thrown vertically there will be a straight line path
- If an object is thrown at an angle with a vertical force that will be a curved (parabolic) path
What is the acceleration of projectiles if air resistance is ignored?
All projectiles on earth will accelerate at 9.81 vertically downwards
Independent horizontal velocity of a projectile
The horizontal velocity remains constant
Independent vertical velocity of a projectile
The velocity changes and the motion can be described using the equations of motion
Horizontal velocity of a projectile at an angle
The velocity remains constant at vcos0
Vertical velocity of a projectile at an angle
The initial velocity is vsin0
How do you calculate resultant force?
F = ma
For a constant mass F is directly proportional to a
Define the Newton
The resultant force that will give a mass of 1 kg and acceleration of 1 ms^-2 in the direction of the force
Define weight
The gravitational force acting on the object
How do you calculate weight?
Mass x gravity
How do you calculate the vertical acceleration using resultant force?
Resultant force = thrust - weight
Define centre of gravity
The point where the entire weight of an object appears to act
Define centre of mass
A point through which any externally applied force produces straight line motion without any rotation
Centre of mass and centre of gravity on earth compared to space
- The centre of gravity and the centre of mass are at the same point on earth
- In deep space, there is no gravitational field, there is no sense of gravity but the object has a centre of mass
What is the resistance forces when objects move through air?
Drag and upthrust forces - which means an object will no longer accelerate at 9.81
What is drag?
A resistance force experienced by an object moving through a fluid
Factors that affect drag
- The speed of an object (the greatest speed, the greater the drag)
- The cross-sectional area of an object (more air particles hitting, the great of the drag)
- The type of fluid or its viscosity
For most objects, what is drag force directly proportional to?
Speed squared
What is terminal velocity?
Where the speed of an object is constant and acceleration is zero
Therefore when net force = zero
A skydivers speed, drag, resultant force and acceleration at terminal velocity
Speed = constant
Drag = weight
Resultant force = 0
Acceleration = 0
What is equilibrium?
When the forces acting on an object do not produce any rotation or any acceleration
What is a moment of force?
The product of a force applied and the perpendicular distance of a line of action of the force from the X axis or point of rotation
Moment = Fx
- The turning effect of a force acting on an object is called a moment
2 conditions, of moments/objects and equilibrium
- resultant force = 0
- Resultant moment = 0
Principle of moments
The sum of the clockwise moments about a point or an axis is equal to the sum of anticlockwise moment about the same point or axis
What is a couple?
Two equal but opposite forces acting on an object (the forces must be parallel and long different lines)
What is a torque of a couple?
The product of one of the forces and the perpendicular separation between the forces
What is a torque measured in?
N m
How do you calculate a torque?
Torque = Fd
What is a triangle of forces?
Three forces acting on a point object in equilibrium will produce a closed factor triangle or a triangle of forces
Resolving forces in the X direction
Total force = 0 therefore Fcos0=x
Resolving forces in the y direction
The total force = 0 therefore Fsin0 = y
Define density
Mass per unit volume
How do you calculate density?
Mass divided by volume
How do you measure the mass of a substance?
A balance
How do you calculate the volume of a regular solid?
Measure its dimensions using a ruler, vernier calliper or micrometer
B x W x H
How do you calculate the volume of a irregular solid?
Determine the volume of water displaced when the object is completely submerged. The volume of displaced water can be measured directly using a measuring cylinder.
Define pressure
The normal contact force exerted on a surface per unit cross-sectional area
How do you calculate pressure?
Force divided by cross-sectional area
What is Archimedes’ principle?
The upthrust exerted on a body immersed in a fluid whether fully or partially submerged, is equal to the weight of the fluid that the body displaces
Why does an object submerged in fluid experience and output force?
The pressure at the bottom surface of the object is greater than the pressure at its top surface
Define work done
The product of force and the distance moved in the direction of the force
Define a joule
1J is defined as the work done by a force of one 1N when the point of application of the force moves a distance of 1m in the direction of the force
How do you calculate work done?
W = Fxcos0
What is conservation of energy?
Energy in a closed system cannot be created or destroyed, but it can take many different forms and be transferred from one to another
What is energy?
The capacity to do work
Work done = energy transferred
What is a closed system?
Where there are no external forces acting
What is kinetic energy and gravitational potential energy?
KE is associated with a moving object and GPE is linked to the position of an object in the gravitational field. GPE is energy stored by an object.
How do you calculate kinetic energy?
E = 1/2mv*2
How do you calculate gravitational potential energy?
E = mgh
Relationship between KE and GPE if the system is closed
GPE = KE
mgh = 1/2mv2
(must cancels out)
v2 = 2gh
Define power
The rate of work done or the rate of energy transfer
How do you calculate power?
P = W/t or P = Fv
Define 1 Watt
1 Joule of work done per second
What does rate mean?
Per unit time or divided by time
How do you calculate efficiency?
Useful output energy/total input energy x 100
Rules of efficiency
- The efficiency of a system cannot exceed 100% because this would violate the principle of conservation of energy
- The efficiency of most mechanical systems is less than 100% because of thermal losses caused from friction
What is Hooke’s law?
The extension of a spring is directly proportional to the force applied as long as the elastic limit of the spring is not exceeded
What are tensile and compressive forces?
Forces that will cause a material to deform
- Equal and opposite forces that stretch an object/spring = tensile forces
- Equal and opposite forces that squash a spring/object = compression forces
Equation of Hookes law
F = kx
What does the gradient represent in a linear force against extension graph
A constant force
What does the area under a force-extension graph represent?
The work done
How do you calculate the energy of a spring?
E = 1/2Fx
What is Newton’s first law?
An object will remain at rest or continue at a constant velocity and less acted upon by a resultant force
What is Newton’s third law?
When two objects interact, they will exert equal and opposite forces on each other
Principles of Newton‘s third law?
- The forces must act on the same object
- Forces must have an equal magnitude
- Forces are in opposite directions
- They must be of the same type/nature
What is linear momentum?
The product of the mass of an object and the velocity
Define momentum
The product of mass (scalar) and velocity (vector)
Conservation of momentum
In a closed system, the total initial momentum is equal to the total final momentum
Kinetic energy in imperfect and perfectly elastic collisions
- In all collisions the total energy and total momentum remain constant
- In perfectly elastic collisions the total kinetic energy remains constant
- In an imperfect elastic collision there is a change in the kinetic energy with transformation of energy from other forms (heat, sound etc)
Newton second law
The resultant force acting on an object is directly proportional to the rate of change of momentum
F = ma
What is an impulse?
The product of force and time
Change in momentum = impulse
In a force – time graph, what does the area underneath the graph represent?
The impulse
Collisions in two dimensions
- Total momentum in the X – direction remains constant
- Total momentum in the Y – direction remains constant
What is an electrolyte?
A conducting liquid that contains negative and positive ions
Define electric current
Rate of flow of charge
Equation for current
Change in charge / change in time
Define the charge of 1 coulomb
The flow of charge in 1 second when the current is 1 ampere
Current in metals vs electrolytes
Metals - the current is due to the movement of electrons
Electrolyte - the current is due to the simultaneous movement of positive and negative ions
Direction through a metal wire in a circuit
- Conventional current move from positive to negative
- The flow of electrons moves from negative to positive
What is Kirchhoff’s 1st law?
The sum of current into a junction is equal to the sum of current out of a junction
- Conservation of charge
Kirchhoff’s 2nd law?
The sum of EMFs in a closed loop is equal to the sum of p.ds
- Conservation of energy
What is mean drift velocity?
- The electrons in a metal move around at high speeds and make frequent collisions with the vibrating fixed positive metal ions
- When the wire is connected to a power supply the electrons gain an additional velocity towards the positive electrode
- The mean drift velocity is the average velocity gained by the electrons along the length of the metal wire
What is number density?
The number of charge carriers per unit volume of material
Conduction properties of metals semi-conductors and insulators
Metals - high number density
Semiconductors - mid
Insulators - small number densities
What affects the mean drift velocity?
The cross-sectional area, the number density and the elementary charge
Equation for current
I = Anev
Define potential difference
The energy transferred from electrical energy to other forms (heat, light, etc) per unit charge
What is electromotive force (EMF)?
The energy transferred from chemical energy to electrical energy per unit charged
What is 1volt ?
1 Joule per coulomb
What is an electron gun?
A device used to accelerate electrons to high speeds
Define resistance
Potential difference across a component / current in the component
Ohms law
At a constant temperature, the current in the wire is directly proportional to the p.d across it ends
Graph of a resistor
- I is directly proportional to V
- straight line through the origin
- Resistance remains constant
(resistor is an ohmic component)
Graph of filament lamp
- Resistance is not constant
- A curved line through the origin
- Resistance increases as current or p.d increases
- Not an ohmic component
How does a filament lamp work?
- Metal ions of the filament vibrate more quickly and with increased amplitude
- The conducting electrons make more frequent collisions with these vibrating ions
- They transfer greater amounts of energy to these ions.
- Therefore resistance increases as current p.d increases
What is a diode?
A component made from semiconductors that conduct in 1 direction
- The diet does not conduct when it is reversed biased
- The resistance is infinite of a negative p.ds
- The diode starts to conduct when it is forward biased and the p.d is greater than the threshold p.d
- The resistance of the diode decreases significantly as the p.d increases beyond the threshold because of the increase in the number density of charge carriers
Resistivity (p) equation
R = pL / A
What is the difference between resistivity and resistance?
Resistivity - a property of a material
Resistance - depends on the physical dimensions of the material
What is the relationship between resistivity and temperature in a metal?
Increasing the temperature of a metal increases the frequency and amplitude of vibration of the fixed metal ions, electrons collide more frequently with vibrating ions and subsequently lead to greater resistance
What is the relationship between resistivity and temperature in a semiconductor?
Temperature affect the number density of charge carriers in a semiconductor
The number density increases with temperature and this leads to a dramatic increase in resistance
Resistance of a thermistor
Dependent on its temperature
As temperature increases resistance decreases
NTC - negative temperature coefficient
Resistance of an LDR
It’s resistance decreases and intensity of light increases
Define power
The rate of energy transfer
What is kWh?
The Kilowatt-hour is defined as the energy transferred by a device with a power of 1kW operating for 1 hour
Principles of a series circuit
- The current in each component is the same
- Total p.d is split between components
Principles of a parallel circuit
- The p.d across each component is the same
- Current is split between each branch
What is internal resistance?
The resistance of the materials within the battery (lost volts)
