Physics 9702 A2

Why is the max ke of an emitted electron independent of the intensity of incident radiation?

frequency of incident photon determines it energy; the ke of e depends upon the energy of the photon. the intensity of radiation only determines the number of photons incident on the metal surface per unit time.

what is the photoelectric effect?

the emission of electrons from a metal surface when electromagnetic radiation is incident on it.

work function energy

minimum energy required for an electron to leave the surface of the metal.

Define electric field.

a field of force in which charged objects experience a non-contact force.

Define electric field strength E at a point.

Electric field strength is the force per unit positive charge acting on a small stationary test charge placed at that point.

E = F / q

What is meant by a 'test charge' when defining electric field strength?

A small positive charge used to probe the field without disturbing it. It must be:

• Small — does not disturb the field being measured

• Positive — by convention, so E points in the direction of force on it

• Stationary — so no magnetic effects arise

Why can electric field lines never cross?

If two lines crossed at a point, there would be two different directions of force on a test charge at that point, which is physically impossible — a charge can only experience one unique net force at any location.

Why is the electric field uniform between parallel plates?

The potential decreases linearly (uniformly) from one plate to the other, so the potential gradient ΔV/Δd is constant throughout the gap. This means every point has the same field strength and direction.

Describe the motion of a positive charge projected horizontally into a uniform electric field directed vertically downward.

Horizontal: uniform velocity (no force component) → constant horizontal displacement.

Vertical: constant force F = qE downward → uniform acceleration a = qE/m downward.

Combined result: parabolic path curving downward (analogous to projectile motion in gravity).

A negative charge in the same field would curve upward.

State Coulomb's Law and write the equation

The force between two point charges is:

• Proportional to the product of the charges

• Inversely proportional to the square of their separation

• Directed along the line joining them

F = Q₁Q₂ / (4πε₀r²)

ε₀ = 8.85 × 10⁻¹² F m⁻¹, r = separation (m)

Like charges → repulsion; unlike → attraction

Under what condition can a spherical conductor be treated as a point charge? What happens inside?

For any point outside the sphere, the entire charge may be considered as a point charge located at the centre of the sphere.

Inside the sphere the electric field is zero (the potential is constant throughout the conductor).

Compare Coulomb’s Law with Newton’s Law of Gravitation.

Similarities:

• Both are inverse-square laws

• Both act along the line joining the objects

• Both are non-contact forces

• Both have potential ∝ 1/r and field strength ∝ 1/r²

Differences:

• Gravity is always attractive; electric force can be repulsive

• Electric force is far stronger than gravitational force

• Gravity depends on mass; electric on charge

What is an equipotential surface? What is its relationship to field lines?

An equipotential surface is a surface on which the electric potential is the same at every point. No work is done moving a charge along an equipotential.

Field lines are always perpendicular to equipotential surfaces.

• Point charge → equipotentials are concentric spheres

• Uniform field → equipotentials are parallel planes perpendicular to the field

Define electric potential V at a point.

Electric potential at a point is the work done per unit positive charge in bringing a small test charge from infinity to that point.

V = W / q

What is meant by saying the electric potential at infinity is zero?

It is a reference convention. We define V = 0 at infinity because at infinite separation from all charges there is no electric influence. The potential at any other point is then the work done bringing unit positive charge from infinity to that point, giving a consistent, unique value everywhere.

State two similarities and two differences between electric fields and gravitational fields.

Similarities:

• Both have field strength defined as force per unit (charge / mass)

• Both obey inverse-square laws for point sources; potential ∝ 1/r

Differences:

• Gravity is always attractive; electric force can repel or attract

• Conducting materials can shield electric fields; gravity cannot be shielded

State all six key electric field equations and the context for each.

F = qE — force on any charge in any field

E = ΔV/Δd — uniform field between parallel plates

F = Q₁Q₂/(4πε₀r²) — Coulomb’s law (force between point charges)

E = Q/(4πε₀r²) — field due to a point charge

V = Q/(4πε₀r) — potential due to a point charge

Eₚ = Qq/(4πε₀r) — potential energy of two point charges

A positive charge is released from rest in an electric field. Describe its motion.

• Accelerates in the direction of the electric field (F = qE)

• Moves from high potential to low potential

• Kinetic energy increases; electric potential energy decreases

• Energy is conserved: gain in KE = loss in PE = qΔV

How does a negative charge move when released in an electric field?

• Force is opposite to the field direction (F = qE, q < 0)

• Accelerates toward higher potential

• Kinetic energy increases; potential energy decreases (becomes more negative)

• Trajectory in a uniform perpendicular field: parabolic, curving against the field direction

State the superposition principle for electric fields and potentials.

The total electric field at a point is the vector sum of the individual fields from each charge:

E_total = E₁ + E₂ + E₃ + ...

The total potential is the scalar (algebraic) sum:

V_total = V₁ + V₂ + V₃ + ...

Because V is a scalar, its sum is simpler than the vector sum for E.

what is a radian?

angle (subtended) at centre of circle

by an arc equal in length to the radius (of the circle)

what is meant buy angular speed?

rate of change of the angle by the string

why is gravitational potential always negative?

Gravitational potential is defined as the work done per unit mass moving from infinity to that point. Since gravity is attractive, the gravitational force acts in the same direction as the movement, so the work done by an external agent is negative. Therefore gravitational potential is always negative.

uses of capacitors in electrical circuits

• smoothing dc

• storing energy

• in timing circuits

• in tuning circuits

• blocking dc

Why does a capacitor store energy, not charge?

The total charge in the circuit is conserved. The capacitor takes positive charge on one plate and the same amount of negative charge on the other plate. So the net charge stored is zero.

However, work is done by the battery to separate these charges against the electric field — this work is stored as electrical potential energy.

Define capacitance:

Capacitance is the charge stored per unit potential difference across the capacitor.

C = Q/V

define rectification

Rectification is the process of converting an alternating current/voltage into a direct current/voltage, such that current flows in one direction only.

Why does the copper core heat up when AC flows through the coil wound around it?

AC creates a changing magnetic field → changing flux in the core → EMF induced in the core (Faraday's law) → eddy currents flow in the core → I²R heating → temperature rises.

What is an eddy current?

A loop of electric current induced within a bulk conductor when it experiences a changing magnetic flux. The currents swirl in closed loops throughout the material rather than along a defined path.

Why are eddy currents harmful in transformers, and how is this reduced?

They waste energy as heat. Reduced by using laminated cores — thin sheets insulated from each other break up the current paths and reduce eddy currents.

Why does magnet B (aluminium tube) fall slower than magnet A (plastic tube)?

Falling magnet causes changing flux in aluminium → eddy currents induced → eddy currents produce a magnetic field that opposes the magnet's motion (Lenz's law) → retarding force acts upward → acceleration is less than g.

why the path of the electron in the magnetic field is the arc of a circle?

magnetic force is always normal to the direction of motion, it is always constant so it provides the centripetal force

why do transformers have an iron core, rather than no core?

core reduces loss of (magnetic) flux linkage

state Faraday’s law of electromagnetic radiation

induced emf is proportional to the rate of change of magnetic flux linkage

state Lenz’s law

the direction of an induced electric current always opposes the change in magnetic flux that produced it

what is meant by magnetic flux linkage

product of the magnetic flux passing through a coil and the number of turns in that coil. It represents the total magnetic flux linking all turns of a coil

Why Liquid-in-Glass thermometer Doesn't Measure Thermodynamic Temperature

The measurement depends on the physical property of the liquid (thermal expansion), which varies from liquid to liquid. Two different thermometers may agree at the fixed points but give different readings in between. Thermodynamic temperature is independent of any material — it is defined by the ideal gas, not the behaviour of a specific substance.

How to decide if a material is suitable for a thermometer

The property (e.g. resistance, volume) varies linearly with temperature; and there is a unique value of the property for each temperature (one-to-one relationship).

what thermometer is best for measuring a rapidly changing temperature?

thermocouple

why is a thermistor not a good choice for rapidly changing temperature?

The thermistor has a large heat capacity, so it takes time to reach thermal equilibrium with its surroundings — meaning it responds slowly to temperature changes.

what is resonance?

oscillation (of object) at maximum amplitude when driving frequency = natural frequency (of system)

what causes centripetal acceleration?

a force of constant magnitude that is always perpendicular to the direction of motion

define gravitational field

force per unit mass

state newtons law of gravitation

State the basic assumptions of kinetic theory and what each implies:

• Molecules have negligible volume compared to the container → the gas is mostly empty space

• No intermolecular forces act between molecules → potential energy is zero, so all internal energy is kinetic

• Collisions are perfectly elastic → kinetic energy is conserved, so temperature stays constant

• Molecules move in random directions with a range of speeds → pressure is exerted equally in all directions

• Time of collision is negligible compared to time between collisions → forces only act during brief impacts