Final answer:
To create a 9-V battery, 6 carbon-zinc dry cells with an emf of 1.54 V each are needed. The total emf will be 9.24 V but the terminal voltage may be less due to internal resistance affecting the battery under load.
Step-by-step explanation:
To determine the number of 1.54-V carbon-zinc dry cells needed to make a common 9-V battery, we divide the desired voltage by the voltage of one cell. So, you would calculate 9 V divided by 1.54 V per cell, which equals approximately 5.84 cells. Since we can't have a fraction of a cell, we round up to the nearest whole number, which is 6 cells. Therefore, option (a) 6 cells is required to make a common 9-V battery.
The actual electromotive force (emf) of the approximately 9-V battery would be the sum of the emf of the individual cells. With 6 cells, each having an emf of 1.54 V, the total would be 6 x 1.54 V = 9.24 V. However, due to internal resistance, the terminal voltage can be lower than the actual emf when the battery is under load.
The series connection of cells with internal resistance will result in a decrease in terminal voltage when the battery is supplying current to a circuit. This drop in terminal voltage is due to the voltage drop across the internal resistance of the cells while current flows through them. The larger the internal resistance or the larger the current drawn from the battery, the more significant the effect on the terminal voltage.