1. What are we doing today? Internal Resistance Emf Using a graph to find the internal resistance and emf of a potato cell

2. Internal Resistance Where do we get resistance from? What happens inside a battery? Every cell/battery/power supply has internal resistance due to it’s construction. We use the symbol r to represent internal resistance.

3. Chemical reactions inside the battery Internal Resistance, r Load Resistance, R Total resistance of all the components in the `external’ part of the circuit.

4. What is an emf? Electromotive Force BUT it is NOT actually a force......it is measured in volts. Energy required to take 1 coulomb of charge completely around the circuit including the site of the emf, i.e. the battery itself.

5. Terminal p.d This is the p.d. across the terminals of a cell/battery/power supply when it is supplying a current. It is the work done per unit charge between those two points. Unit – Volt, V The terminal pd, V, is less than the emf, ε, whenever current passes through the source. The difference is the lost pd, v, due to the internal resistance of the source – lost volts

6. emf = Useful volts + Lost volts In energy terms the `lost volts’ is the energy per coulomb dissipated or wasted inside the cell due to the internal resistance. If there is no internal resistance, then the terminal p.d will be the same as the emf.

7. Including internal resistance The resistors are connected in series. So total resistance is:- R + r Current through the cell:-

8. Lost pd So the cell In other words:-

9. Lost pd The terminal pd can be calculated using:- The equation becomes:- The lost pd can be calculated using:-

10. Most power supplies (e.g. car battery) have a low internal resistance – why? Have to deliver a high current. Usually have an internal resistance of less than 1Ω High voltage power supplies – HT (high tension) and EHT (extremely high tension) have very high internal resistances. If they are short circuited then only a small current is allowed to flow – much safer!

11. Potato Cell Experiment Digital multimeters, 2 Leads, 4 mm, 5 Zinc sheet approx 0.5 cm x 2 cm Copper sheet approx 0.5 cm x 2 cm Resistance substitution box Crocodile clips, 2

12. 1) Make your potato cell. Insert the copper and zinc electrodes at either end of the potato. Attach a 4 mm lead to each electrode using a crocodile clip. 2) Set up the circuit as shown. Set the resistance substitution box to 4.7 kΩ. This is the load resistance. Record the current and potential difference values in a suitable table. 3) Change the load resistance and record the values of current and potential difference. Repeat this process to gather data for a range of load resistances. You will have to change the range of your ammeter. Take care not to confuse amps with milliamps or microamps! 4) Plot a graph of V against I. Describe the trend.

13. p.d / V Current / A Start with V = ε – Ir Equation of a straight line Rearrange V = ε – Ir Intercept on the y-axis is ε Gradient is -r

14. Using the following equations: V = ε – Ir ε = V + v ε = I(R + r) V = ε – v 1)A large battery with an internal resistance of 0.8Ω and e.m.f 24V is used to power a dentist’s drill with resistance 4Ω. a) Calculate the current in the circuit when the drill is connected to the power supply. b) Calculate the voltage across the drill while it is being used. 2)A student mistakenly connects a 10Ω ray box to a HT power supply of 500V. The ray box does not light, and the student measures the current flowing only to be 50mA. a) Calculate the internal resistance of the HT power supply. b) Explain why this is a sensible internal resistance for a HT power supply.

15. 3)A 9.0V battery has an internal resistance of 12.0Ω. a) What is the potential difference across its terminals when it is supplying a current of 50.0mA? b) What is the maximum current this battery could supply? 4)A cell in a deaf aid supplies a current of 25.0mA through a resistance of 400Ω. When the wearer turns up the volume, the resistance is changed to 100Ω and the current rises to 60mA. What is the e.m.f and internal resistance of the cell? 5)A battery is connected in series with a variable resistor and an ammeter. When the resistance of the resistor is 10Ω the current is 2.0A. When the resistance is 5Ω the current is 3.8A. Find the e.m.f and the internal resistance of the battery.