Cells, Electromotive Force and Internal Resistance Questions

Cells

An electric cell is basically an electrochemical device that generates electrical energy from chemical reactions, it also derives chemical reactions to generate electric power which means that a cell or a battery converts chemical energy into electrical energy by making use of chemical energy stored in the chemicals. Combinations of one or more electrochemical cells are known as batteries. By the means of electromotive force in an electrical circuit, a battery delivers a continuous current source, which is steady in nature.

Every cell consists of two major terminals, which are connected in series by a conductive electrolyte

Anode: Anode is the terminal to which the negatively charged ions, also known as Anions, migrate, that is the current flows in from out.

Cathode: Cathode is the terminal to which the positively charged ions (cations) migrate, that is the current flows out from here.

Redox reactions, that is reduction and oxidation, is the process that powers the battery as both these reactions occur at the same time. Cations gain electrons at the cathode throughout the charging, while anions lose electrons at the anode throughout charging, wherein gaining of electrons is reduction and losing of electrons is oxidation.

EMF

No current flows through a cell when no electrical apparatus is attached to it; this no-current condition is also known as open circuit. An open circuit results in an emf of the cell which is equal to the difference in emf of the electrodes. This potential difference is known as the Electromotive Force (EMF) where Anode has a positive potential (V+), whereas Cathode has a negative potential (-V). EMF is directly related to the source of potential difference, such as the particular combination of chemicals in a battery. Still, when current flows, the emf fluctuates from the voltage output of the device.

To be precise, the electromotive force (emf) is the potential difference of a source when no current is flowing. Units of emf are volts. Hence, a potential difference can be created by all such devices that can also supply current if connected to a resistance. Emf is not a force at all, actually it is a special type of potential difference.

Internal Resistance

When there is current present in the device or the electrical circuit and there’s a voltage drop in source voltage or the source battery is internal resistance. The reason behind it is the electrolytic material that is present in the batteries or other voltage sources. Internal resistance is the resistance produced inside a battery to the flow of current, it is denoted by (r). When a source of power carries current, the measured output voltage is less than the voltage when no current is flowing through the circuit. This voltage drop to the flow of current is caused by the internal resistance of the battery.

V = V+ + V =- Ir

Where, Anode has a positive potential (V+) whereas Cathode has a negative potential (-V), and internal resistance of a battery is (r)

V = ξ – Ir

In real life the internal resistance is usually neglected.

Since, V = IR

= ξ – Ir = IR

= I = ξ/(R+r)

Important Questions on Cells Electromotive Force and Internal Resistance

1) Explain how redox reactions occur.

The reduction and oxidation process that happens simultaneously is known as redox reaction. Cations gain electrons at the cathode throughout the charging, while anions lose electrons at the anode throughout charging, wherein gaining of electrons is reduction and the loss of electrons is oxidation. The process is reversed during this entire process of discharge within a battery, where the electrodes are electrically connected by the electrolyte.

2) Define Anode.

Anode is said to be a terminal or an electrode of an electrochemical cell at which the oxidation process happens. Generally at an anode, negative ions or anions tend to react and give off electrons. The flow of current is in from out from anode, that is, an incoming channel is provided by the anode for the current to pass in the circuit or the device.

3) Define Cathode.

The cathode is a terminal or an electrode of an electrochemical cell at which the reduction process happens; this is where it gains the electrons. It is also known as a negatively charged electrode.

4) What is the difference between a cell and a battery?

An electric cell is basically an electrochemical device that converts chemical energy into electrical energy, whereas a battery can be termed as a group of cells.

5) What is the internal resistance of a cell? Explain.

Internal resistance is basically known as the obstruction to the current flow offered by the batteries and cells themselves, which results in the generation of heat. Internal resistance can be measured using the ohms laws. The relationship between internal resistance (r) and emf (e) of the cell is given by e = I (r + R)

6) What will be the terminal voltage of the battery when the circuit is closed, if a battery of emf 20V and internal resistance 4Ω is connected to a resistor, and the current in the circuit is 0.5A?

Given: A battery of emf 20V and internal resistance 4Ω is connected to a resistor. The current in the circuit is 0.5A

Therefore, to calculate the terminal voltage of the battery when the circuit is closed

Emf of the battery (E) = 20V

Internal resistance (r) = 4Ω

Current in the circuit (I) = 0.5A

Resistance of the resistor = R

Using Ohm’s law I= E/R+r

= R+r = E/I

= R+4 = 20/0.5

= R = 36 Ω

Therefore, the terminal voltage of the resistor = V

According to Ohm’s Law,

V=IR⟹V=0.5×36⟹V=18V

Therefore, the resistance of the resistor is 36Ω and the terminal voltage is 18V

7) What is the difference between potential difference and electromotive force?

An electromotive force is termed as the total voltage in an electric circuit produced by the battery or source, whereas the potential difference is known as the work done between two specific points in the circuit for moving a unit charge against the electric field.

8) What is terminal voltage?

Terminal voltage is basically the potential difference which is measured across the terminals of the battery. Terminal voltage is equal to the EMF value of the battery when no current is drawn from the battery. But when current is drawn, then the resultant potential difference will appear across terminal voltage as the difference between the EMF and the potential difference across the internal resistance (due to current flow).

9) What will be the value of EMF for a dry cell?

The electromotive force or EMF is termed as the total voltage in an electric circuit produced by the battery or source per coulomb of charge passing through it. It is measured in volts (V).

The emf value of a Dry cell (Leclanche cell) ranges between 1.2 and 1.5 V.

10) A current of 0.5A is provided by a cell over a coil of 3-ohm and a current of 0.4A through a coil of 9-ohm, what will be the emf and internal resistance of the cell?

Let the emf of the cell be ε and internal resistance of the cell is r.

for the 3Ω coil, 0.5×(3+r) = ε, 1.5+0.5r = ε ………………….. (1)

for the 9Ω coil, 0.4×(9+r) = ε, 3.6+0.4r = ε ……………………(2)

by equating (1) and (2) and solving for r, we get r = 21 Ω.

then from eqn. (1) substituting r=21, we get emf ε = 12V

Practice Questions

  1. Differentiate between EMF, potential difference and terminal voltage.
  2. What is an electrochemical cell?
  3. Define electromotive force.
  4. Define 1 Coulomb.
  5. State and explain Ohm’s law.

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