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Physics > Current Electricity > Cells, EMF and Internal Resistance
Current Electricity

Cells, EMF and Internal Resistance

Cells, EMF, Internal Resistance are the components which complete the circuit and help the flow of electricity within the circuit. Cells, emf and internal resistance are inter-related to one another. Batteries i.e. Cells are posses internal resistance and potential difference i.e. voltage. Know more about Cells, emf and internal resistance in this article, explore more below!

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Cells

An “electric power supply” is also an Electric cell. Cells generate electricity and also derives chemical reactions.  One or more electrochemical cells are batteries. Every cell has two terminals namely:

  • Anode: Anode is the terminal from where the current flows in from out i.e. it provides an incoming channel for the current to enter the circuit or the device.
  • Cathode: Cathode is the terminal from where the current flows out i.e. it provides an outgoing current flow from the circuit or the device.

Browse more Topics under Current Electricity

There are different types of cells available and some of them are as follows:

  • Electric Cells
  • Fuel Cells
  • Secondary Cells
  • Galvanic Cells
  • Photovoilatatie Cells
  • Solar Cells
  • Storage Cells
  • Primary Cells
Internal Resistance

[Source: Studytronics]

EMF

EMF is Electromotive Force, which is measured in coulombs of charge. It is pressure developed or an electric intensity from a electrical energy or a source. It is a device which converts any form of energy into electrical energy which is then measured with coulombs of charge. EMF i.e ElectroMotive Force id denoted by,  .

 emf = I (R + r)

Where I is the current in amperes; R is the resistance of load in the circuit in ohms; r is the internal resistance in ohms.

emf = E/ Q

Where E is the energy in joules;  Q is the charge in coulombs.

EMF

[Source: Energy Education]

Internal Resistance

When there is current present in the device or the electrical circuit and there’s a voltage drop in source voltage or source battery is internal resistance. It is caused due to electrolytic material in batteries or other voltage sources.

Internal Resistance (r) = (E – V)/I

Where E is the emf of the device; V is the potential difference between the device; I is the current in the device. Internal Resistance is the result of the resistance in the battery or the accumulation in the battery. the equation used to derive this is as follows:

V = (E – Ir)

Internal Resistance

[Source: Wikipedia]

Solved Examples for You

Question 1: The terminal voltage of a cell in an open circuit condition is

  1. Less than its emf
  2. More than its emf
  3. Equal to its emf
  4. Depends on its internal resistance

Solution: Option C. Equal to its emf. The terminal voltage of a cell in open circuit condition will be equal to the emf of the cell as the circuit is open there won’t be any drop across the internal resistance.

Question 2: What is the p.d. across the terminals (VT)  of a cell with emf E  for the open circuit?

  1. VT<E
  2. VT>E
  3. VT=0
  4. VT=E

Solution: Option D. VT=E, When the circuit is closed, the resulting current not only flows through the external circuit but through the source (battery, generator, transformer, etc.) itself. All sources have an internal resistance, which causes an internal voltage drop, slightly reducing the voltage across the terminals.

The larger the current, the larger the internal voltage drop, and the lower the terminal voltage. When the circuit is open, no current flows. So there is no internal voltage drop, and the full voltage appears across the source’s terminals. This is why the potential difference across the terminals of a cell when connected to a circuit is slightly lesser than the emf of the cell.

Question 3: The common dry cell produces a voltage of:

  1. 1.5V.
  2. 30V.
  3. 60V.
  4. 3V

Solution: Option A. 1.5V, A common dry cell is a type of electricity-producing chemical cell, commonly used today for many home and portable devices, often in form of batteries. By standards, a common dry cell has a constant voltage of 1.5

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