Movements of Ocean Water
NCERT Solutions • Class 11 Geography • Chapter 131. Multiple Choice Questions
(i) Upward and downward movement of ocean water is known as the:
(a) tide
Reason: Tides refer to the periodical vertical rise and fall of the sea level. Waves are horizontal movements, and currents are continuous flows.
Reason: Tides refer to the periodical vertical rise and fall of the sea level. Waves are horizontal movements, and currents are continuous flows.
(ii) Spring tides are caused:
(a) As result of the moon and the sun pulling the earth gravitationally in the same direction.
Reason: During the full moon and new moon, the sun, moon, and earth are in a straight line (Syzygy). Their combined gravitational pull causes higher high tides (Spring Tides).
Reason: During the full moon and new moon, the sun, moon, and earth are in a straight line (Syzygy). Their combined gravitational pull causes higher high tides (Spring Tides).
(iii) The distance between the earth and the moon is minimum when the moon is in:
(b) Perigee
Reason: Perigee is the point in the moon’s orbit where it is nearest to the earth. Apogee is when it is farthest. (Peri = Near, Apo = Away).
Reason: Perigee is the point in the moon’s orbit where it is nearest to the earth. Apogee is when it is farthest. (Peri = Near, Apo = Away).
(iv) The earth reaches its perihelion in:
(d) January
Reason: The earth is closest to the sun (Perihelion) around January 3rd each year. It is farthest (Aphelion) on July 4th.
Reason: The earth is closest to the sun (Perihelion) around January 3rd each year. It is farthest (Aphelion) on July 4th.
2. Short Answer Questions
(i) What are waves?
Waves are the oscillatory movements of water where the water particles move up and down in a circular motion, but the water itself does not travel forward. They act as energy carriers across the ocean surface.
(ii) Where do waves in the ocean get their energy from?
Waves derive their energy primarily from the wind. The friction between the wind blowing over the water surface transfers energy to the water, causing ripples that grow into waves.
(iii) What are tides?
Tides are the periodical rise and fall of the sea level, occurring once or twice a day. They are caused by the combined effects of the gravitational forces exerted by the moon and the sun and the rotation of the earth.
[Image of high tide and low tide diagram]
(iv) How are tides caused?
Tides are caused by:
- Gravitational Pull: The moon (and sun) pulls the water on the side of the earth facing it, causing a bulge (High Tide).
- Centrifugal Force: On the opposite side of the earth, the centrifugal force (due to earth’s rotation) balances the gravity, creating a second bulge (High Tide).
(v) How are tides related to navigation?
Tides are crucial for navigation, especially in harbours and estuaries.
- High tides raise the water level, allowing big ships to enter shallow harbours safely (e.g., Kandla, Kolkata).
- Tidal currents help ships move in and out with the flow, saving fuel.
3. Long Answer Questions
(i) How do currents affect the temperature? How does it affect the temperature of coastal areas in N. W. Europe?
Effect on Temperature:
Ocean currents act as conveyor belts of heat.
- Warm Currents: Flow from the equator to poles. They raise the temperature of the coastal areas they wash (e.g., Gulf Stream).
- Cold Currents: Flow from poles to equator. They lower the temperature of the adjacent coast (e.g., Labrador Current).
- Despite being in high latitudes (which should be freezing), this warm current keeps the ports ice-free throughout the winter.
- It creates a moderate, foggy climate distinct from the frozen interiors of the same latitude.
(ii) What are the causes of currents?
Ocean currents are generated and directed by two sets of forces:
1. Primary Forces (Initiate Movement):
1. Primary Forces (Initiate Movement):
- Heating by Solar Energy: Heated water expands (becomes lighter) and rises near the equator, causing a slight slope. Water flows down this slope.
- Wind: Friction between wind and water surface pushes the water in the direction of the wind (e.g., Trade Winds drive Equatorial currents).
- Gravity: Pulls water down the pile created by wind or heating.
- Coriolis Force: Deflects the moving water to the right in the Northern Hemisphere and left in the Southern Hemisphere.
- Temperature Difference: Cold water is dense and sinks; warm water is light and floats.
- Salinity Difference: High salinity water is denser and sinks, driving deep ocean currents.
Project Work
Topic: Observing Waves & Mapping Currents
(Guidance for Student Activity)
(i) Observing Waves: When you throw a stone in a pond, you see concentric circles (ripples) moving outward. However, if you place a floating leaf on the water, you will notice the leaf just bobs up and down; it doesn’t move to the shore. This proves waves move energy, not mass.
(ii) Mapping Currents (Discussion Points):
(i) Observing Waves: When you throw a stone in a pond, you see concentric circles (ripples) moving outward. However, if you place a floating leaf on the water, you will notice the leaf just bobs up and down; it doesn’t move to the shore. This proves waves move energy, not mass.
(ii) Mapping Currents (Discussion Points):
- Why Warm/Cold?
– Current from Equator $\to$ Pole = Warm (e.g., Kuroshio).
– Current from Pole $\to$ Equator = Cold (e.g., Oyashio). - Why Deflect?
– Coriolis Force: Causes clockwise circulation in the Northern Hemisphere (Atlantic/Pacific gyres) and anti-clockwise in the Southern Hemisphere.
– Coastline Shape: Currents like the Brazil current are deflected by the shape of the continent (Cape Sao Roque).