Flow Needs a Difference: Water, Heat and Air
Why does a current flow in a circuit at all? Your textbook answers this on pages 132–134 by first looking at three flows you already know: water, heat and air. Each of them flows only when there is a difference between two places – a difference in level, in temperature (താപനില) or in pressure (മർദം). When the difference disappears, the flow stops – unless something, like a pump, keeps the difference going. Electric charge behaves the same way. This page also revises the forms of energy that a cell, a bulb or a generator change into one another.
- Adding, subtracting and finding the average of two numbers.
- Charges and free electrons (page 1).
1. Water flows from a higher level to a lower level
Put two vessels of water side by side and join them with a bent tube full of water – a siphon (സൈഫൺ). If the water levels (ജലനിരപ്പ്) are the same, nothing happens. Raise one vessel, and water starts flowing from the higher level to the lower level. The flow is fast at first, slows down as the levels come closer, and stops when the two levels are equal.
Both beakers are identical and start with 15 cm of water. Make the stands equal and press ▶: no flow, because the levels are the same. Then raise A and try again. Finally switch the pump on: every second it lifts back from B to A exactly as much water as the siphon carries down, so the level difference stays and the flow never stops. The pump needs energy to do this – it is an external energy source (ബാഹ്യ ഊർജസ്രോതസ്സ്).
Worked example 1 · Two connected tanks
Q. Two identical tanks stand on the same floor. Tank A has water up to 30 cm and tank B up to 12 cm. They are joined by a pipe at the bottom and the valve is opened. (a) Which way does the water flow? (b) What is the water level in each tank when the flow stops?
Water flows from the higher level. The flow stops when the levels are equal. The tanks are identical, so whatever A loses, B gains – the final level is the average of 30 and 12.
2. Heat flows from a higher temperature to a lower temperature
Hold one end P of an iron rod and put the other end Q in a candle flame (textbook Fig 7.5). The temperature rises first at Q, the end in the flame. Then heat flows along the rod from Q to P – from the part at a higher temperature to the part at a lower temperature – and slowly P gets warm too. If the two ends were at the same temperature, no heat would flow.
Sample model: the end in the flame is kept at about 250 °C and the room is at 30 °C (real values depend on the rod and the flame). Watch the colour spread from Q towards P. Heat always flows down the temperature slope, from hot to cold.
3. Air flows from a higher pressure to a lower pressure
A tyre tube pumped full of air (textbook Fig 7.6) has a higher pressure inside than the air outside. Open the valve and the air rushes from inside to outside – from the higher pressure to the lower pressure – until the pressures are equal. The air around us presses with about 100 kPa (kilopascal, a unit of pressure).
Try a pressure below 100 kPa too (as if some air had been sucked out): now air flows into the tube. Either way it flows from higher to lower pressure and stops when they are equal.
Worked example 2 · Which way does the air go?
Q. A cycle tube has an air pressure of 250 kPa inside. The air outside is at 100 kPa. (a) Find the pressure difference. (b) Which way does air flow when the valve is opened, and when does it stop?
Difference = higher − lower. Air flows from the higher pressure to the lower pressure, until there is no difference left.
4. The pattern – and what keeps a flow going
Put the three cases side by side, as in your textbook's Table 7.1:
| What flows | Direction of flow | The flow stops when… |
|---|---|---|
| Water | From the higher water level to the lower water level | the levels are equal |
| Heat | From the higher temperature to the lower temperature | the temperatures are equal |
| Air | From the higher pressure to the lower pressure | the pressures are equal |
A flow needs a difference. A continuous flow needs something that keeps the difference going – and that something needs a supply of energy. For water it is a pump. In an electric circuit it is a cell or battery, which keeps a potential difference (പൊട്ടൻഷ്യൽ വ്യത്യാസം) between its two ends. You will meet this on page 5.
The comparison is only a guide: water needs a pipe and can spill, charge needs a closed loop of conductor. But “a pump keeps a level difference, so water keeps flowing” is exactly how “a cell keeps a potential difference, so charge keeps flowing” works.
5. Energy changes form – a recap
A pump or a cell does not create energy; it changes energy from one form to another. These forms appear again and again in this chapter:
- Chemical energy – stored in the substances inside a cell or battery (and in fuel and food).
- Electrical energy – carried by the current in a circuit.
- Light energy and heat energy – given out by a bulb or an LED (an LED gives much less heat).
- Mechanical energy – the energy of moving things: a fan, a motor, a bicycle wheel turning a dynamo.
- Solar (light) energy – turned into electrical energy by a solar cell.
Every source of electricity changes some other form of energy into electrical energy; every appliance changes electrical energy into other forms. Page 6 collects these sources in Table 7.2.
Worked example 3 · The bicycle dynamo lamp
Q. A bicycle has a dynamo that runs a small lamp when you pedal. Write the energy changes, starting from the food you ate.
Food stores chemical energy. Your legs turn the wheel. The dynamo turns motion into electricity. The lamp gives out light – and gets warm.
In most Kerala homes a motor pump lifts water from the well to the overhead tank. From there water flows down to every tap by itself, because the tank is higher than the taps. When the tank is empty the taps run dry – until the pump (using electrical energy) fills it again and restores the height difference. A battery does the same job for electric charge.