Use an oscilloscope to measure the frequency and voltage from an signal generator
Using lemons, limes and potatoes to power an LED zinc copper
Making Batteries There is nothing special about batteries – but these have a high internal resistance. R l What do you think the best way to minimise the internal resistance of your battery? Think about resistivity (lemon is a poor conductor)
Batteries What is the main energy transfer in a battery? Electrical energy This is powered by two 1.5 V AA cells in series - what is the supply voltage? What is the emf of the supply? After a while the battery needs to be replaced; why? What determines how quickly it runs down? What determines how much current is drawn from the supply?
EMF is the external work expended per unit of charge to produce an electric potential difference across two open-circuited terminals
This is the same definition as voltage but on an open circuit (no current flow)
Why is this definition important?
Batteries have internal resistance The circuit now has two resistors in The internal resistance of the battery, r is very small. R is much larger The total resistance of this circuit is R total = R +r I = E / (R+r)
Batteries have internal resistance As charge goes around the circuit the sum of emfs must equal the sum of voltage drops leading to EMF = I R + I r The terminal voltage is equal to I R so this can be rearranged to give: V = E – I r and interpreted as terminal voltage = emf – ‘lost volts’
R -small R total = r + R -small The current will now be larger as the total resistance of the circuit is much lower The voltage lost across r V = I (large) r The voltage lost will now be a problem This case is of a small load resistance connected to a battery is seen in a starter motor on a car http://www.youtube.com/watch?v=al6Yz3Nv7dY http://www.youtube.com/watch?v=Ut7yBdIehYY&NR=1
Starter motor on a conventional car The headlamps are connected in parallel across a twelve-volt battery. The starter motor is also in parallel controlled by the ignition switch. Since the starter motor has a very low resistance it demands a very high current (say 60 A). The battery itself has a low internal resistance (say 0.01 Ω). The headlamps themselves draw a much lower current (they have a higher resistance) Lamp R Starter motor Ignition switch 12V What will happen to the lights?
1. A 9.0 V battery has an internal resistance of 12.0 .
(a) What is the potential difference across its terminals when it is supplying a current of 50.0 mA?
What is the maximum current this battery could supply?
Draw a sketch graph to show how the terminal potential difference varies with the current supplied if the internal resistance remains constant. How could the internal resistance be obtained from the graph?
2. A cell in a deaf aid supplies a current of 25.0 mA through a resistance of 400 . When the wearer turns up the volume, the resistance is changed to 100 and the current rises to 60 mA. What is the emf and internal resistance of the cell?
V = E – I r V = IR E = I(R +r) You need to set up a simultaneous equation
Answers 1. (a) pd = E – I r = 9 – (50 x 10 -3 x 12) = 8.4 V (b) Max current = E/r = 9 / 12 = 0.75 A 2. E = I(R +r) E = 25 x 10 -3 (400 + r) and E = 60 x 10 -3 (100 + r) So 25 x 10 -3 (400 + r) = 60 x 10 -3 (100 + r) so r = 114.3 E = 10 + (25 x 10 -3 x 114.3) = 12.86 V I V
Oscilloscope Use the signal generator to create A/C (alternating current) on the Oscilloscope Draw 2 signals of 2 different frequencies Work out the frequency of the Oscilloscope Add the magnitude of the wave height