Earth & Space (OCR GCSE Physics A (Gateway)): Flashcards

False.

A main sequence star is in equilibrium — the inward gravitational force is equal to the outward pressure force from fusion reactions, making the star stable.

Hydrogen nuclei fuse together to form helium nuclei. Each reaction releases heat and light energy, which keeps the core hot and maintains the outward pressure that balances gravity.

As a nebula collapses under gravity, particles are pulled closer together, increasing the density and causing more frequent collisions, which raises the temperature until nuclear fusion can begin.

A gigantic explosion that occurs when a red supergiant's fusion reactions stop and its core collapses suddenly. It ejects the outer layers as a planetary nebula and leaves a neutron star (or black hole) at the centre.

False.

Massive stars have shorter lifespans — hundreds of millions of years rather than billions — because they burn through their fuel in nuclear fusion much faster.

A solar-mass star expands into a red giant, sheds its outer layers as a planetary nebula, then collapses into a white dwarf and eventually a black dwarf. A massive star becomes a red supergiant, explodes as a supernova, and leaves a neutron star or black hole.

An extremely dense point in space formed from the collapsed core of the biggest stars after a supernova. Its gravity is so strong that not even light can escape from it.

An object that absorbs all radiation incident on it, reflecting or transmitting none. Because a good absorber is also a good emitter, it is also the best possible emitter of thermal radiation.

False.

As temperature increases, the peak of the black body curve moves to a lower (shorter) wavelength and a higher intensity. Hotter objects emit higher-energy, shorter-wavelength radiation.

At very high temperatures the peak of the emitted radiation shifts to shorter wavelengths. At around 6000 °C an object emits mainly white or blue light, whereas at lower temperatures (~1000 °C) it emits predominantly red light.

All objects emit black body radiation in the form of electromagnetic waves regardless of their temperature, but hotter objects radiate more energy per second and at shorter peak wavelengths.

The thermal radiation emitted by all bodies (objects). At room temperature it is mainly infrared; at higher temperatures it can include visible light, ultraviolet or even X-rays.

True.

At room temperature the peak wavelength of emission lies in the infrared region. Objects only begin to glow visibly red at temperatures around 1000 °C.

The peak of the curve shifts to a lower wavelength (higher energy) and the intensity at all wavelengths increases. This means a hotter object emits more radiation and the radiation carries more energy per photon.

A small rocky object that orbits the Sun. Most asteroids are found in the asteroid belt between Mars and Jupiter.

True.

All planets orbit the Sun in the same direction, in approximately the same plane, following slightly elliptical orbits with the Sun near the centre.

As a comet approaches the Sun, its ice melts and vaporises. The solar wind pushes the ejected dust and gas away from the Sun, forming the tail. The tail therefore always points away from the Sun regardless of the comet's direction of travel.

A smaller body orbits a larger body because the larger body's gravity acts as the centripetal force, pulling the smaller body towards its centre and keeping it in a circular path.

An artificial satellite that orbits above the Earth's equator with a period of 24 hours, keeping it stationary relative to a point on Earth's surface. Used for radio and telecommunication broadcasting.

False.

A minor planet's gravitational field is not strong enough to clear nearby objects from its orbit, which is why other bodies may share its orbital path — this distinguishes it from a full planet.

If the satellite moves too fast, the gravitational force cannot provide enough centripetal force to maintain the orbit. The orbital radius increases and the satellite spirals outward into a higher orbit or into space.

The closer a moon is to its planet, the greater the gravitational pull on it. To maintain a stable circular orbit, it must travel at a higher speed and completes one orbit in a shorter time.

False.

Polar satellites orbit at a much lower altitude (_{200 km) than geostationary satellites (}36 000 km). Their lower orbit gives clearer signals and images but requires more than one satellite for continuous coverage.

A path in which an object moves around a central body at constant speed, with gravity acting at right angles to its instantaneous velocity towards the centre of the orbit.

False.

Although the speed is constant, the direction changes continuously, so the velocity is always changing.

The object's direction changes continuously, so its velocity changes, meaning it is accelerating. A resultant force — gravity — is needed to produce this acceleration, directed towards the centre of the orbit.

In a circular orbit, gravity always acts towards the centre of the orbit, at right angles to the object's instantaneous velocity.

A highly stretched, non-circular orbit typical of comets, in which the object's speed changes significantly as its distance from the Sun changes.

As the comet moves closer to the Sun it loses gravitational potential energy, which is converted into kinetic energy, causing it to speed up.

False.

Unlike planets, comets can orbit in different planes and some even orbit in the opposite direction to the planets.

The state reached when an object absorbs radiation at the same rate as it emits radiation, resulting in a constant temperature.

True.

When the rate of absorption exceeds the rate of emission, the object gains thermal energy and its temperature increases.

A light, shiny surface emits far less infrared radiation than a dark, matt surface. This reduces heat loss from the patient, keeping them warmer than an ordinary blanket would.

Without its atmosphere, the Earth's surface temperature at night would drop to about −180 °C, similar to the surface of the Moon at night.

The natural process by which greenhouse gases (e.g. water vapour, methane, carbon dioxide) absorb longer-wavelength infrared radiation emitted by the Earth and re-emit it back to the surface, keeping the Earth warmer than it would otherwise be.

1. The rate at which radiation is reflected back into space.

2. The rate at which radiation is absorbed by the atmosphere or the Earth's surface.

3. The rate at which radiation is emitted from the Earth's surface and atmosphere into space.

False.

Greenhouse gases absorb longer-wavelength infrared radiation emitted by the Earth's surface, not the shorter-wavelength radiation arriving from the Sun.

A longitudinal seismic wave that can pass through solids and liquids. P-waves are faster than S-waves and are very low-frequency sound waves (infrasound, below 20 Hz).

False.

S-waves are transverse waves and cannot pass through liquids. The fact that S-waves do not reach the far side of the Earth provides evidence that the outer core is liquid.

A transverse seismic wave that can only pass through solids. S-waves are slower than P-waves and cannot travel through the Earth's liquid outer core.

It tells us the mantle is solid (both wave types pass through it) and the outer core is liquid (S-waves cannot pass through liquids, so they are blocked by the outer core).

P-waves are refracted as they pass between the liquid outer core and the inner core. The size and position of the shadow zones where no P-waves are detected indicate large-scale refraction, consistent with a solid inner core.

A technique that uses ultrasound waves to detect objects underwater. The wave is reflected off the ocean floor and the time taken for it to return is used to calculate the depth of the water.

False.

The wave travels down and back, so the total distance is twice the depth. You must halve the calculated distance (or halve the time first) to find the actual depth.

Total distance = 1500 × 0.12 = 180 m. The pulse travels down and back, so depth = 180 ÷ 2 = 90 m.