A2 Physics Mistakes

formula for gravitational potential and what each letter stands for

G is gravitational constant, M is mass of the planet/sphere, r is distance of of the objects from both centers

formula for gravitational field strength

why does a liquid in glass thermometer not give a measurement of thermodynamic temperature

measurement depends on the properties of the liquid

Use one of the basic assumptions of the kinetic theory to explain what can be deduced about the potential energy associated with the random motion of molecules in an ideal gas

There are no intermolecular forces so potential energy is zero

Explain the decrease with time of the amplitude of the oscillations of the ball

oscillations of the ball lose energy due to resistive forces

Explain the exponential shape of current in a capacitor

• potential difference across capacitor = potential difference across resistor

• current in resistor is proportional to potential difference in resistor

  • charge is proportional to current so rate of decrease in current decreases as current decreases

Formula for maximum photon energy

E = qV (q is charge and V is voltage)

Explain why X-rays can be used to produce images of internal body structures that have good contrast

Bones and tissues have different attenuation coefficients meaning their transmitted intensities are very different thus making good contrast

What is half-life of an isotope

time taken for activity to halve

define radian

the angle subtended at the center of a circle where arc length = radius

how would angular speed, linear speed, and acceleration change if the object comes closer to the center

• angular speed would stay the same

• linear speed would decrease

  • acceleration would decrease

The gas in X consists of molecules that each have a mass that is four times the mass of a molecule of the gas in Y.

Explain how r.m.s speed of the molecules in X compares with the r.m.s. speed of the molecules in Y

Explain how internal energy of the system changes when:

a wire is stretched within its elastic limit at constant temperature

At constant temperature, kinetic energy stays constant since speed of molecules is the same. When the wire is stretched, the change in separation causes an increase in potential energy. With a constant kinetic energy and increasing potential energy, internal energy also increases

formula for speed in simple harmonic motion

V = ωx

Why does alpha-decay of a nucleus result in a release of energy

• fission occurs and as nucleons decrease it has a higher binding energy

  • nucleus formed has a lower nucleon number

Describe how reflected ultrasound pulses may be used to obtain diagnostic information about internal structures

• time taken for the wave to come back gives information about the depth of the structure

• intensity gives information about the nature of the boundary

Define specific acoustic impedance of a medium

product of density and speed of ultrasound in the medium

State what is indicated by the direction of the gravitational field line at a point in a gravitational field

direction of force acting on the mass at that point

Explain why, at the surface of the sphere, g always has the opposite sign of x

g is always acting towards the center of the sphere where force is opposite in direction of displacement

The gas at the surface of a star has a very high pressure.

Use the basic assumptions of the kinetic theory to suggest why, in practice, a gas at the surface of a star is unlikely to behave as an ideal gas.

At a very high pressure means molecules are very close together so volume of molecules is not negligible in comparison to volume of gas

formula for mean power

<P> = I_r.m.s ² R

What is the method of production for ultrasound and X-rays

• ultasound - vibrating quartz crystal

• X-rays - electrons hitting metal target

Explain why the variation with time of the magnitude of the force exerted by the minute hand on the piece of modelling clay is negligible as the minute hand undergoes one full revolution

centripetal acceleration is negligible to acceleration of freefall, so resultant force is negligible compared with weight

State Wien’s displacement law. Identify any symbols thar you use

Wavelength at which maximum emission rate occurs is inversely proportional to the thermodynamic temperature