♣Define Terminal potential difference of a cell. Find its expression.
♣Why terminal potential difference decreases when current is drawn from cell.
Answers
Answer:
Terminal voltage is the voltage output of a device is measured across its terminals.Terminal voltage is calculated by V = emf - Ir. ... It is just the potential differenceacross a cell in a circuit. When no current is flowing the terminal potential difference is equal to EMF of the cell.
I'd guess that you have in mind the pd between the terminals of a cell, battery or electrical supply. The terminal pd, Vterm is simply the pd that a voltmeter reads if connected across the cell's terminals.
Suppose you connect a 'load' resistor Rload across the cell's terminals. Then Vterm is also the pd across the load resistor. The cell behaves as a source of emf in series with an internal resistance r, which (as a first approximation) is constant. So Vterm is the pd across the source of emf in series with an internal resistance, which is the same pd as the pd across the load resistance. We have a simple series circuit (source of emf, r and Rload).
Consider the circuit given below. The cell can be modified with an emf ε and the internal resistor with resistance r which is connected in series. An external load resistor with resistance R is also connected across the circuit. The terminal potential difference represented as V is defined as the potential difference developed between the positive and negative terminals of the cell when current flows through the circuit.
V = V+ + V- – Ir. This is the voltage drop accomplished due to the internal resistance.
We know that ε = V+ + V-. = I (R + r).
ε = IR + Ir.
= V + Ir
V = ε – Ir.
So, V = ε – Ir, where V is the potential difference across the circuit, ε is the emf, I is the current flowing through the circuit, r is internal resistance.
Usually internal resistance of a cell is not considered because ε >> Ir. The value of internal resistance changes from cell to cell.
Combination of Cells in Series Connection
Consider two cells which is connected in series. The positive terminal of one cell is connected to negative terminal of the next cell. Here one terminal of two cells are free and the other terminal of two cells are joined together. ε1 and ε2 are the emfs of the cells and r1 and r2 are the internal resistance of the cells respectively. Let I be the current flowing through the cells.
When the electric cell is in a closed circuit the current flows through the circuit. There is a fall of potential across the internal resistance of the cell. So, the p.d. across the terminals in a closed circuit is less than the p.d. across the terminals in an open circuit by an amount equal to the potential drop across the internal resistance of the cell.
