AQA Physics: A2 Unit 4

Pregunta	Respuesta
Define: conservation of momentum	In any collision or explosion, total momentum remains constant provided no external forces act.
Define: elastic collision	No kinetic energy is lost
How are force and momentum related?	Force = rate of change of momentum
What is meant by 'impulse'?	The product of force and time
Define the term 'radian'	1 radian is the angle subtended by a circle of radius r by an arc of length r
Define: angular speed	The angle in radians turned through in 1s
Give 3 factors that affect centripetal force	mass speed radius (of circle)
What conditions must be satisfied for an object to be in SHM?	1. The acceleration of the object is proportional to its displacement from the equilibrium position. 2. The acceleration is always directed back towards the equilibrium position.
Energy in SHM is the interchange between which two energy forms	Kinetic and potential
How is the oscillations of a simple pendulum and a mass-spring system affected if taken to the Moon?	simple pendulum - longer time period mass-spring - no effect
What is a free vibration?	System displaced and left to oscillate at its natural frequency
What is a forced vibration?	System driven at frequency other than natural frequency
What happens when a system is driven at its natural frequency?	Resonance Amplitude of oscillations increases
What is damping?	Resistive force that causes amplitude of oscillations to decrease
What is critical damping?	System returns to equilibrium as quickly as possible without 'overshooting'
State Newton's gravitational law	Gravity is an attractive force between point masses that is proportional to the product of the two masses and inversely proportional to the square of the distance between them
What do gravitational field lines show?	The direction a small test mass would move when placed in the gravitational field
Describe in one word the gravitational field around a planet	Radial (inwards)
What is an equipotential?	A region / surface of constant potential
Define: gravitational field strength	The force per unit mass acting on a small test mass placed in a gravitational field
Define: gravitational potential	The energy required to bring a unit mass from infinity to some point in a gravitational field, this is work done by gravity
Why is gravitational potential a negative value?	Work is done to reach infinity at which point gravitational potential is zero
What is the gradient of a gravitational potential vs. distance graph equal to?	Gravitational field strength
Define: electric field strength	The force per unit charge acting on a small positive charge at that point
What is the unit for electric field strength?	Newton per Coulomb or Volt per metre
What is the unit for gravitational field strength?	N / kg
Define: electric potential	The work done in bringing a unit positive charge from infinity to that point
How do you find the electric potential difference between two points in an electric field?	The work done in moving a unit positive charge from the point at the lower potential to that at the higher potential
What is the unit for electric potential difference?	Volt, V
What is the electric field strength equal to in a uniform field?	E = V / d
What is the force F on a charge q equal to in an electric field of field strength E?	F = qE = qV / d
Describe the motion of a charged particle entering a uniform electric field perpendicular to the field lines	parabolic
Give one difference between electric and gravitational fields	Act on charges instead of masses or Can be attractive or repulsive, g fields are only attractive
Define capacitance	charge stored / potential difference
Define the Farad, F	1 Coulomb stored when a p.d. of 1 Volt is applied
What does the area under of a graph of Q against V represent?	Energy stored
What does the gradient of a graph of Q against V represent?	Capacitance
What is the time constant of a capacitor equal to?	RC
After how many time constants is a capacitor considered to be charged or discharged?	5
Define: magnetic flux density	Force per unit current per unit length
Fleming's left hand rule: State what the 3 fingers represent	First finger: B field Second finger: Current Thumb: Force
Fleming's right hand rule: State what the 3 fingers represent	First finger: B field Second finger: Current Thumb: Force
Why does a particle moving in a circular path not spiral inwards?	Force is perpendicular to the velocity at any time The force causes a change in direction, but not a change in the magnitude of the velocity No work is done in the direction of the force
State Lenz's law	The direction of the induced EMF is such that the current it causes to flow opposes the change which causes it
Define: magnetic flux and state the unit	Product of B and A Webers, Wb
Define: magnetic flux linkage and state the unit	Product of the number of turns on a coil and the magnetic flux (NBA) Unit: Weber-turns
How is the induced EMF and the flux linkage related?	Rate of change of flux linkage = magnitude of induced EMF
Describe how a p.d. is induced across the secondary coil of a transformer	An alternating p.d. across the primary coil produces an alternating magnetic field in the iron core. This is linked to the secondary coil. An alternating p.d. is induced across the secondary coil.
Describe how the efficiency of a transformer is maximised.	The core is made from iron which can be magnetised and de-magnetised quickly. The core is laminated, reducing eddy currents. Low resistance wire is used for the coils, reducing the heating effect of the current.
Why is electricity transmitted at high potential differences?	High p.d. means low current. Low current means less energy transferred to the surroundings, increasing efficiency

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