Physics Y12 WACE Theory

What is a gravitational field?

A region in which a mass experiences a gravitational force.

How does gravitational force depend on mass?

It is directly proportional to the product of the two masses.

How does gravitational force depend on separation?

It is inversely proportional to the square of the separation between the masses.

What does the inverse-square law mean for gravity?

Doubling the distance makes the gravitational force one quarter as large.

What provides the centripetal force for an orbiting satellite?

Gravity.

What is the direction of centripetal acceleration?

Towards the centre of the circular path.

What is the direction of centripetal force?

Towards the centre of the circular path.

Is centripetal force a separate type of force?

No. It is the name for the net inward force; it may be gravity, tension, friction or a normal/reaction force.

State Kepler's third law.

For objects orbiting the same central mass, T^2/r^3 is constant.

What two motions are analysed separately in projectile motion?

Horizontal and vertical motion.

What is the horizontal acceleration of a projectile when air resistance is ignored?

Zero.

What happens to a projectile's horizontal velocity when air resistance is ignored?

It remains constant.

What is the vertical acceleration of a projectile near Earth when air resistance is ignored?

Constant downward acceleration due to gravity, g.

What quantity is shared by horizontal and vertical projectile calculations?

Time.

At the highest point of a projectile, what is its vertical velocity?

Zero.

At the highest point of a projectile, is its acceleration zero?

No. Its acceleration is still downward at g.

For a projectile landing at its launch height, how do time up and time down compare?

They are equal.

What launch angle gives maximum range on level ground without air resistance?

45 degrees.

What happens to a projectile's path when air resistance is included?

It has lower maximum height, shorter range and a non-symmetrical path.

Why is a projectile's path non-symmetrical with air resistance?

Air resistance opposes motion and varies with velocity, so descent is not a mirror of ascent.

What is torque?

The turning effect of a force about a pivot.

What increases torque?

A larger force, a larger perpendicular distance from the pivot, or a more perpendicular force.

Why does only the perpendicular component of a force produce torque?

Only that component causes rotation about the pivot.

What is translational equilibrium?

Net force is zero, so there is no linear acceleration.

What is rotational equilibrium?

Net torque is zero, so there is no angular acceleration.

What two conditions are needed for complete equilibrium?

Net force equals zero and net torque equals zero.

Can an object in translational equilibrium be moving?

Yes, it may move at constant velocity.

State the first postulate of special relativity.

The laws of physics are the same in all inertial reference frames.

What is an inertial reference frame?

A frame moving at constant velocity, with no acceleration.

State the second postulate of special relativity.

The speed of light in a vacuum is the same for all observers, regardless of their motion.

What is time dilation?

A moving clock is measured by a stationary observer to run more slowly; the observed time interval is longer.

What is proper time?

The time interval measured in the frame where the clock/events are at rest.

What is length contraction?

An object's length measured parallel to motion is shorter when it moves relative to the observer.

What is proper length?

The length measured in the frame where the object is at rest.

Does length contraction occur perpendicular to motion?

No. It applies only in the direction of motion.

What happens to relativistic effects as speed approaches c?

They become very large.

Why cannot an object with mass reach c?

Its relativistic momentum and required energy increase without limit as speed approaches c.

Why is classical velocity addition inaccurate near c?

It can predict speeds greater than c, violating special relativity.

What produces a magnetic field?

Moving charges, including an electric current in a wire.

What is the shape of the magnetic field around a straight current-carrying wire?

Concentric circles around the wire.

How is the magnetic-field direction around a straight wire found?

Use the right-hand curl rule with thumb in the direction of conventional current.

What is conventional current?

The direction positive charge would move, from positive to negative.

State Coulomb's law in words.

Electrostatic force is proportional to the product of the charges and inversely proportional to the square of their separation.

What is an electric field?

A region in which a charge experiences an electric force.

What does electric field strength represent?

Force per unit positive test charge; it is not energy.

What direction do magnetic-field lines point outside a bar magnet?

From north to south.

What does a dot represent in a 2D magnetic-field diagram?

A vector out of the page.

What does a cross represent in a 2D magnetic-field diagram?

A vector into the page.

How is the magnetic field of a solenoid made stronger?

Increase the current or increase the number of turns per unit length.

How is the magnetic-pole direction of a solenoid found?

Use the right-hand grip rule: curl fingers with conventional current; thumb points to the north pole/field direction inside.

What is the Lorentz force?

The force on a moving charge or a current-carrying conductor in a magnetic field.

When is magnetic force on a moving charge greatest?

When velocity is perpendicular to the magnetic field.

When is magnetic force on a moving charge zero?

When it moves parallel or antiparallel to the magnetic field.

How does the force direction for a negative charge compare with a positive charge?

It is opposite.

Why does a charged particle move in a circle in a uniform magnetic field?

The magnetic Lorentz force is perpendicular to velocity and acts as the centripetal force.

Does a uniform magnetic field change a charged particle's speed?

No. The force is perpendicular to motion, so it changes direction but not speed.

Which particles curve more tightly in the same magnetic field?

Particles with lower momentum or greater charge magnitude.

What is the motor principle?

A current-carrying conductor in a magnetic field experiences a force.

How does a motor produce rotation?

Opposite forces act on opposite sides of a coil, forming a couple and producing torque.

What is the role of a split-ring commutator in a DC motor?

It reverses coil current every half-turn so the torque continues in one rotational direction.

What is electromagnetic induction?

The production of an emf when magnetic flux through a conductor or coil changes.

State Faraday's first law.

An induced emf is produced when magnetic flux linkage through a circuit changes.

State Faraday's second law.

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

State Lenz's law.

The induced current flows in a direction that opposes the change in magnetic flux that produced it.

Why does moving a magnet towards a coil induce current?

It changes the magnetic flux through the coil.

What happens to induced emf if magnetic flux changes more quickly?

It increases.

Does a stationary magnet beside a stationary coil induce an emf?

No, because the magnetic flux is not changing.

How does induced current oppose increasing flux?

It produces a magnetic field opposite to the increasing external field.

How does induced current oppose decreasing flux?

It produces a magnetic field in the same direction as the original external field.

What is the generator principle?

Moving a conductor/coil in a magnetic field induces an emf.

How does an AC generator produce electrical energy?

An external force turns a coil in a magnetic field, changing flux and inducing an alternating emf.

Why is a generator the opposite of a motor?

A motor converts electrical energy to mechanical rotation; a generator converts mechanical rotation to electrical energy.

What are slip rings used for in an AC generator?

They maintain electrical contact with the rotating coil without reversing the connections.

Why is an AC generator output alternating?

The coil sides reverse motion through the field every half-turn, reversing the induced emf.

What distinguishes DC generator output from AC generator output?

A split-ring commutator reverses external connections each half-turn, producing one-direction pulsating DC.

What causes a larger induced emf in a generator?

A faster flux change, such as faster rotation, stronger field, more turns or larger coil area.

Why do transformers require AC?

AC creates a continuously changing magnetic field, inducing emf in the secondary coil.

How does a transformer transfer energy?

Changing primary current creates changing core flux, inducing emf in the secondary.

What makes a transformer step-up?

The secondary coil has more turns than the primary, so voltage is higher.

What makes a transformer step-down?

The secondary coil has fewer turns than the primary, so voltage is lower.

How are electric fields used in particle accelerators?

They accelerate charged particles by exerting forces in the field direction.

How are magnetic fields used in particle accelerators?

They steer charged particles by changing their direction, often into curved paths.

Why do magnetic fields not increase a particle's kinetic energy?

Magnetic force is perpendicular to velocity, so it does no work.

What is relativistic momentum?

Momentum that increases by a relativistic factor at speeds close to c.

What is rest energy?

Energy a particle has because of its rest mass: E = mc^2.

What does mass-energy equivalence mean?

Mass and energy are interchangeable; a mass corresponds to energy E = mc^2.

How is total relativistic energy related to rest energy?

Total energy includes rest energy plus kinetic energy.

What is the wave nature of light?

Light behaves as a transverse electromagnetic wave produced by accelerating/vibrating charges.

What is the particle nature of light?

Light transfers energy in discrete massless packets called photons.

What is a photon?

A quantum, or discrete packet, of electromagnetic-radiation energy.

What is wave-particle duality?

Light shows both wave and particle behaviour depending on the observation.

What is the photoelectric effect?

Emission of electrons from certain metal surfaces when light of sufficient frequency is incident on them.

What is the threshold frequency?

The minimum light frequency required to eject electrons from a particular metal.

What happens below threshold frequency?

No photoelectrons are emitted, regardless of intensity.

What is the work function?

The minimum energy needed to remove an electron from a metal surface.

What happens when photon energy equals the work function?

An electron is just emitted with zero maximum kinetic energy.

What happens to excess photon energy in the photoelectric effect?

It becomes the maximum kinetic energy of the emitted electron.

How does increasing intensity above threshold affect the photoelectric effect?

It increases the number of emitted electrons/current, not maximum kinetic energy.

How does increasing frequency above threshold affect the photoelectric effect?

It increases the maximum kinetic energy of emitted electrons.

What is stopping potential?

The minimum reverse potential that stops the most energetic photoelectrons.