Classical mechanics

Does a positive force between two charges act inwards or outwards?

Outwards

When calculating the potential energy of two charged species, if bringing them together requires energy, is the energy positive or negative? So would the potential energy between two like charges be positive or negative?

Positive. And positive.

What is thermal energy?

The amount of energy causing random motion

What happens when the potential energy between two particles is less than the thermal energy?

The interaction between the particles will not take place, and random motion will prevail. This means that if there are two like charges and E < t.e., random motion could still push the two particles together.

Two types of thing in classical mechanics, and their possible interconversions

Matter and energy. One form of matter can be converted into another, and one form of energy can be converted into another, but matter and energy are fundamentally different and cannot interconvert.

When does interference occur

When two waves occupy the same space

What is the relationship between energy and amplitude of a wave

Energy is given by intensity, which is proportional to amplitude squared

When does pure constructive interference happen, and when does pure destructive interference happen

When the wavelengths and amplitudes are equal, and the peaks align (they are in phase): constructive interference, and when they are 180* out of phase, and a peak aligns with a trough: destructive interference.

How to calculate the energy of a particle?

Kinetic energy + potential energy

When does the potential energy increase for atoms forming a covalent bond

When the actual bond length is shorter or longer than the equilibrium bond length (because there is a force trying to return the atoms to their equilibrium positions)

What makes the instantaneous bond length different to the equilibrium bond length

Vibrations

How does the total energy of atoms forming a covalent bond change

The total energy does not change; it is constant. The kinetic energy and the potential energy of the nuclei interconvert - when one increases, the other decreases. E.g. when the actual bond length increases or decreases from the equilibrium, the kinetic energy of the nuclei decreases, but their potential energy increases. Max kinetic energy is at the equilibrium bond length (so potential energy = 0), and max potential energy is at the max or min bond length (where KE = 0).

What do you add to the equation when calculating the force or energy between two charged particles in a solution

A “fudge factor” - the dielectric constant of the medium. This takes into account the damping effect of the solvent on the interactions between the particles, and is a measure of how much the surroundings shield one charge from another.

What is a molecular force field

All the forms of potential energy a molecule possesses

How to computationally determine the most probably structure for a molecule

Energy minimisation - calculate the potential energy for many different possible arrangements of atoms (bond lengths, bond angles, conformations (rotations of sigma bonds)) for the molecule and choose the one with the lowest energy - this is the most stable.

How to use energy minimisation to model molecular dynamics

Add kinetic energy.

Why does a positive charge move towards the negative part of a field gradient

A positive charge is lower in energy in a negative electric field - the lower the energy, the more stable

Describe and explain the movement of a charged particle in an electric field

The particle will initially accelerate in the direction of the electrostatic force, which is in the direction of the field gradient. As acceleration increases, friction increases, until the friction = force of electric field. Then, the particle will travel at a constant velocity.

What happens to a dipole in an electric field. Real life example?

It won’t move, but it will align itself so that the d- points towards the positive part of the electric field, and the d+ points towards the negative part. E.g. membrane proteins aligning with membrane potential