Physics A Level OCR A

2 forces are on an object, and the object is in equilibrium, what must be true?

What about 2+ forces?

The forces have equal magnitudes

The forces act in opposite direction

Their vectors must resolve to 0

What is a projecile?

An object with an initial velocity that moves freely under gravity.

Rules to help solve projectile motion problems

• Horizontal and Vertical components in motion are independent.

• The horizontal velocity will remain constant

• Vertical velocity will decelerate at 9.81ms^-2

• At the highest point the vertical velocity of the projectile is 0

The 2 types of friction.

Contant friction - This is between 2 surfaces in contact

Fluid friction / Drag - This is between a solid object and a fluid (liquid or gaseous)

Important stuff about drag (x3)

It increases as the object’s force increases

It is impacted by the shape and size of the object

Drag can never cause an object to accellerate

Experiment to investigate terminal velocity.

What is a moment?

Torque, turning effect generated when a force is applied at a distance from a pivot.

What is a couple and how do you calculate them?

They are 2 equal and opposite forces acting in the same direction, but seperated by a distance, creating torque and inducing rotation.

T (Nm) = F x d

F = magnitude of one of the forces

d = distance between the forces

T = torque

What is the pressure in a fluid?

gravity (N/kg) x density of fluid (kg/m³) x depth (m) = Pressure (N/m²)

What is upthrust?

When an object is partially or fully in a fluid, it experiences an upward force.

This results from the pressure difference between the top and the bottom of the object.

Archimides Principle?

Upthrust = Weight of the water displaced

= density of fluid x volume of fluid x gravity

Why is efficiency important?

Conserves resources

Reduced environmental impact

Reduces costs

What are newton’s 3 laws?

An object’s velocity will not change, unless a resultant force acts on it.

F = ma

Every action has an equal and opposite reaction.

What are the two types of collisions?

• Elastic: KE is conserved

  • Inelastic: KE is lost (heat or sound)

How to deal with two-dimensional collision?

Split into x and y components.

Apply the conservation of momentum law to each separately.

Plastic vs Elastic deformation

Elastic deformation: material returns to original size and shape, once the stress is removed

Plastic deformation: material does not return to original size and shape, once the stress is removed.

Equation for spring constant in series

\frac{1}{k_\text{eq}} = \frac{1}{k_1} + \frac{1}{k_2} + \frac{1}{k_3} + \cdots + \frac{1}{k_n}

Equation for spring constants in parallel

K₁ + K₂ + K₃ + … + K_n

Equation for stress?

Force (N) / Area (m²)

Stress’s Force Unit

Equation for strain?

Extension (m) / original length (m)

Strain’s Deformation Unit

What is Young’s Modulus?

What is it also called?

Stress / Strain

Stiffness

Facts about Young’s Modulus

This will remain directly proportional until the material reaches its breaking point.

What is UTS?

Ultimate tensile strength.

The maximum amount of load or stress a material can handle until it fractures and breaks.

What is breaking stress?

The stress level at which a material fractures.

Elastic potential energy equation (in relation to Young’s Modulus)

Work done = Average force x distance

EPE = Final force x 0.5 x extension

How to determine Young’s Modulus in a common experiment.

How will the stress/strain graph of a ductile material look?

There will not be a sudden change (a breaking point) in gradient, but it will be gradual, this signifies that the material is ductile.

What properties can you deduce from a Stress/Strain graph?

Max height is the strength of the material.

The gradient shows how stiff the material is.

What does the Stress/Strain graph for rubber and other polymers look like?

Name of these types of graphs?

Polymeric stress-strain graphs

The unloading is different due to the energy which is lost as heat.

If a wire was a pipe carrying water, what would each electrical property map to?

• Voltage (V) = pressure difference pushing water.

• Current (I) = flow rate of water (litres/s).

• Charge (C) = total water that flowed (litres).

• Mean drift velocity = average speed of the water drifting through the pipe.

• Resistance (R) = pipe roughness/narrowness, slowing the flow.

• Internal resistance = resistance inside the pump itself.

• Number density (n) = how many molecules per volume of water.

• Charge per carrier (q) = "size" of each water parcel (same for all).

• Cross-sectional area (A) = thickness of the pipe.

Equation for mean drift velocity?

v=\frac{I}{A\cdot n\cdot q}

Important stuff about charges.

They move randomly, but tend to drift in one direction.

The mean drift velocity is the average of these velocities.

Compare charge carriers in metals

In metals, there are many delocalised electrons, this means that there is a high density of charge carries, so metals conduct electricity very well.

Compare charge carriers in semiconductors

In semiconductors, there are fewer charge carriers, so a higher drift velocity is needed to reach the same current.

Compare charge carriers in insulators

In insulators, there are very few or no free carries, meaning that they are poor conductors of electricity.`

Compare charge carriers in ionic solutions, or molten ionic compounds.

In ionic solutions or molten ionic compounds, both the negative and positive ions act as charge carriers, so electricity is conducted decently well.

Compare charge carriers in gases

In gases, the lack of carrier ions means that they cannot usually carry charge, but they can become conductive when they get ionised.

What is p.d.?

Work done per unit of charge.

What is the charge of an electron?

1.6 × 10^-19 C

What is the mass of an electron?

9.11 × 10^-31 kg

Why is saving energy important?

Examples of how to save energy

An ohmic resistor.

A filament lamp

A diode

What is resistivity?

This is the resistance encountered in a material that is 1m long, and has a cross-sectional area of 1m².

Its units are ohm-metres (Ωm).

Equation for resistivity?

\rho=\frac{RA}{L}

Experiment to determine resistivity of a wire.

Kirchhoff’s First Law

‘The sum of currents entering a junction is equal to the sum of currents leaving that junction.'

Kirchhoff’s Second Law

'The total e.m.f. in a closed loop is equal to the sum of potential differences across each component in that loop.'

Equation for resistance of components in series.

R₁ + R₂ + R₃ + … + R_n = R_total

Equation for resistance in parallel.

\frac{1}{R_\text{total}} = \frac{1}{R_1} + \frac{1}{R_2} + \frac{1}{R_3} + \cdots + \frac{1}{R_n}

What is internal resistance?

Very battery has its own resistance, this is caused mainly by the collisions of electrons as they move through the material of the battery.
This can cause the battery to heat up.

The symbol is often “r”

What is load resistance?

This is the total resistance outside the battery.

The symbol is often “R”

What is EMF?

Electromotive Force (E)

This is the total energy per unit charge of a battery, measured in volts.

What is Terminal Potential difference?

The actual potential difference across the terminals of a power source, measured in volts.

What are lost volts?

Without any internal resistance, the EMF would equal the load potential difference.

But the presence of internal resistance, means there is a loss of energy per unit charge.

The symbol is often “v”

EMF equation.

EMF = I(R + r)

EMF = V + v

Special note with EMF?

With not current flowing, there will be no lost volts, so just a voltmeter in parallel will read the battery’s EMF.

EMF in series?

The EMF of each cell adds up.

This is because each cell contributes to the pushing of the charge.

EMF in parallel?

The EMF will not change, but the current will increase.

Experiment to determine EMF and internal resistance?

How does this experiment work?

EMF = V + Ir

So as current increases, the contribution of Ir (lost volts) increases.

Rearranging into y = mx + c gives

V = -rI + EMF

So the y-intercept (when current is 0) will be the EMF, and the gradient will be -r.

A key point about semiconductors?

They have greater resistivity than metals, and become more conductive as energy is added.

This is because the density of charge carries increases, as more energy means more electrons are released, allowing charge to be better conducted: reducing resistance.

What is an NTC?

Key points?

Type of thermistor (Negative Temperature Coefficient).

How do LDRs work?

Light causes resistance to decrease because energy is provided to release more electrons to carry the charge.

How does a sensing circuit work?