CELLS & Internal Resistance

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Calvin Williamson
6 years ago
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1 CELLS & Internal Resistance Cells A Cell is a source of Electrical Energy and hence we can obtain a current from it. An electric current is made when a flow of electrons are passed through some medium. In Electronics, we obtain an electric current when electrons flow through a conductor / wire. Below is the schematic diagram of a IDEAL cell: 1

2 Types of Cells Cells are classified according to their ability to be recharged or not. A non rechargeable cell is referred to as a Primary Cell. Primary cells are made to be used once and then discarded. Cells that can be recharged are referred to as a secondary cell. These cells are recharged by passing current through the circuit in the opposite direction to the current during discharge. Secondary cells come in two types: Dry Cell: The electrolyte is a paste. Wet Cell : The electrolyte is a liquid Advantages and Disadvantages of Secondary Cells & Primary Cells Type of Cell Advantage Disadvantage Primary Cell 1. Cheaper to buy than rechargeable cells Mercury Cell 2. Can be stored for long Alkaline Cell periods without use Lithium cell 3. Can be designed to have a slow discharge rate (remote controllers and smoke detectors) 4. Disposable Secondary Cell Nickel cadmium Lithium Ion (Li-ion) [laptop] Lead Acid [Car Battery] 1. Rechargeable. So it can be used over and over again 2. Lesser environmental impact since cell is reusable 3. Can handle large power demands 1. Cannot be recharged 2. Used once and should be thrown away upon depletion. This negatively impacts the environment as toxins inside the cell are non recyclable 3. Maintenance necessary 1. High initial cost 2. Faster discharge rate than primary cells 3. Recharging required Potential Difference This is the difference in voltages between two points in a circuit. This is also referred to as a Voltage Drop. Electro Motive Force (EMF) This is the maximum voltage the cell can deliver. 2

3 Difference between Emf and potential difference: Emf 1. It is the potential difference between two electrodes when no current is flowing in the circuit. 2. It is the maximum voltage that the cell can deliver. 3. It is responsible for the steady flow of current in the cell. Potential difference 1. It is the difference of the electrode potentials of the two electrons two electrodes when the cell is under operation. 2. It is always less than the maximum the cell can deliver. 3. It is not responsible for the steady flow of current in the cell. Internal Resistance No cell or battery is a true 100% efficient energy source. There will always be associated with a cell some loss in energy (current) due to the materials from which the cell is made. We quantify these losses in energy as Internal Resistance. We represent the internal resistance as a resistor (R I ) in series with the power supply: In Diagram B, the True Voltage (V T ) is given by the formula: 3

4 Example: Step 1: Internal voltage drop: V RI = I T R Internal V RI = 100mA x 20 Step 2: Find True Voltage V T = V S - V RI V T = 6 2 = 4.0V V RI = 2000mV = 2.0V Note If we have a cell connected into a circuit and you are asked to calculate the true voltage, you must take into account all the resistors and ALSO the Internal resistor for the cell. Cells in series When cells are arranged in series the following characteristics are always true 1. The current is unchanged. Therefore, if one cell is rated at 3A, the Current flowing through all of the cells is 3A. 2. The voltage of each cell is added to arrive at the algebraic sum of the voltages. If one cell is 1.5V and 100mA 4

5 Example V T = V 1 + V 2 + V 3 + V 4 V T = = 6.0V And current is unchanged: I T = 100mA Draw some cells here ->and try it for yourself: Cells in Parallel Cells connected in parallel behave in the opposite manner to cells connected in series. The following characteristics are true for cells in parallel: 1. The total current for cells connected in parallel is the algebraic sum of each cell current. 2. The voltage is unchanged Example: If one cell is 1.5V and 100mA 5

6 Diagram A: V T = V 1 = V 2 = V 3 = V 4 I t = I 1 + I 2 + I 3 + I 4 = 100mA + 100mA +100mA +100mA = 400mA What is the Voltage and Current from Diagram b? 6

12-1: Introduction to Batteries

Chapter 12 Batteries Topics Covered in Chapter 12 12-1: Introduction to Batteries 12-6: Series and Parallel Connected Cells 12-7: Current Drain Depends on Load Resistance 12-8: Internal Resistance of a