ELECTRIC CELLS
These are cells that convert chemical energy to electrical energy. They can be classified into two. They are
- Primary cells
- Secondary cells
These are cells whose chemical reactions are irreversible, that is they can not be recharged after exhaustion. They have high internal resistance. Examples of primary cells are Daniel cell and leclanche cell.
These are cells whose chemical reactions are reversible, that is they can be recharged after exhaustion. They have low internal resistance. Secondary cells are also called accumulators. Examples of secondary cells are lead-acid accumulator and alkaline accumulator (Ni-Fe accumulator).
- Local action: This is caused by the presence of impurities on the zinc plate electrode. The impurities will increase the resistance of the electrode, hence preventing the direct flow of current. Local action can be prevented by amalgamation i.e. coating the zinc plate with mercury or by using pure zinc plate.
- Polarization: This is caused by the presence of hydrogen bubbles around the copper plate. The hydrogen bubbles create what is known as hydrogen electrode which prevents the direct flow of current creating what is known as back EMF. Polarization can be prevented by using substances called depolarizer. Examples of depolarizers include CuSO4 and MnO.
Leclanche cells are used in transistor radios and in torches. Alkaline accumulator is more durable than lead-acid accumulator. A battery should be recharged if the value of its relative density has reduced drastically.
- Cells should be left in a cool and dry place.
- Batteries should not be left unused after initial usage for a long period of time as this could cause deterioration.
- Batteries should not be exposed to sunlight or place close to fire.
Cells can be arranged in two ways
- Series arrangement: The major reason for arranging cells in series is to increase their effective E.M.F
Effective E.M.F = E1 + E2 + E3
Effective internal resistance = r1 + r2 + r3
- Parallel arrangement: Cells are arranged in parallel in order to reduce their effective internal resistance, hence increasing the current production.
For cells arranged in parallel, the Effective E.M.F = E
Effective internal resistance = r/n, where ‘n’ is the number of cells .
An Electric circuit is a continuous conducting path consisting of wires and other devices between two terminals of a cell or battery along which an electric current flow.
- Closed circuit: T is a continuous conducting path consisting of wires and other devices between two terminals of a cell or battery along which an electric current flow. his is a type of circuit in which current is flowing through it.
- Open circuit: This is a type of circuit in which current is not flowing through it.
- Three cells each of EMF 1.5V and internal resistance 2.5Ω are connected as shown in the diagram. Find the net EMF and internal resistance
Since the cells are arranged in parallel,
- A radio is operated by 8 cells each of EMF 2V connected in series. If two of its cells are wrongly connected, the net EMF of the radio is?
Solution
- The most suitable cell use for short interval switches in electric bell is (a) Nickel iron accumulator (b) lead acid accumulator(c) Daniel cell (d)leclanche cell.