THE RELATIONSHIP BETWEEN CURRENT AND VOLTAGE Aims In this experiment you will investigate the behaviour of three differe

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THE RELATIONSHIP BETWEEN CURRENT AND VOLTAGE Aims In this experiment you will investigate the behaviour of three different kinds of conductor when a voltage is applied across each. a) An ohmic conductor, for which the resistance is independent of current magnitude and direction; b) A lamp filament, for which the resistance increases with increasing current (i.e. with increasing temperature); c) A rectifier, for which the resistance depends on current magnitude and direction. Theory The current, 1 (measured in amperes (A)), flowing through a conductor depends on the potential difference, V (measured in volts (V)), across its ends. In special circumstances I is proportional to V, and we say that the conductor is ohmic. If this is so the V ratio R= , known as the resistance of the conductor (measured in ohms (22)), is constant. 1 More generally the relationship between V and I is non-linear, or non-ohmic, and the resistance is not constant. Method You are provided with a D.C. power supply, a rheostat, an ammeter, a voltmeter and a reversing switch. Before connecting up the complete circuit for this experiment, it is worthwhile to investigate briefly that part of the circuit known as the potential divider. 1) Connect the following circuit. + D.C. 2) Set the toggle switch on the D.C. power supply to 6 V. 3) Slide the movable contact (b) on the rheostat from c to a, and notice how the voltmeter reading changes from 0 V to nearly 6 V. The p.d. Vnc has been divided into p.d.s Vab and Vhe. It is Voc which provides the variable voltage to the conducting device being tested. 4) Now connect the complete circuit for this experiment as follows.

E1,2 D.C. Reversing switch X V Y 5) Connect the resistor mounted on perspex between X and Y. 6) Use the ammeter and the 0-6 V voltmeter, making sure that their polarities are correct. 7) Note that the meters are unidirectional, and so they must be connected before the two-way switch, which allows current to flow either from X to Y or from Y to X through whatever is connected between X and Y. 8) Set the rheostat so that b is about halfway between a and c, and make the following observations. Reversing Voltmeter reading Ammeter reading switch (A) Open Closed The reason for the voltage drop on closed circuit is that current is now being drawn from the source through R, and so Vab increases. This means that Vbc = Vec - Veb decreases. (V.c is approximately constant.) 9) The ammeter can be placed between the potential divider and the voltmeter, or between the voltmeter and the reversing switch. Which position is more suitable depends on the value of the resistance being measured relative to those of the meters. The position is of importance in part (c), and for that part of the experiment it must be connected as shown above.

(A) A CONDUCTOR AT CONSTANT TEMPERATURE Tabulate below values of V and I for both directions through the resistor. V(V) I(A) VV) (reversed) I (A) (reversed) Results for part (A) Plot a graph of V against I, showing both positive and negative values for V and I. What do you deduce from the graph? From the graph calculate the resistance of the resistor.

(B) A LAMP FILAMENT Replace the resistor used in part (A) with the lamp provided. Replace the voltmeter with a 0-300 mV meter, and before switching on make sure that the potential divider is set to apply zero p.d. across the lamp. Starting with small voltages take readings of V and I. When the millivoltmeter reaches its maximum replace it with the 0-6 V meter and continue taking readings until the lamp glows brightly. Take at least five readings in each range. N.B. Do not switch off the current between readings (or else the filament will cool down). You need not reverse the current in this part. Tabulate your results below. The table includes a column for the resistance of the filament calculated for each value of 1. V (V) I(A) R= 1 r= (12) Results for part (B) Plot separate graphs of V against I and R against I. Compare the graphs of V against I plotted in parts (A) and (B), and account for the differences. -Explain how the resistance of the cold filament can be found from the graph of R against I. Resistance of cold filament =

(A) A CONDUCTOR AT CONSTANT TEMPERATURE The following data was recorded with the resistor connected between X and Y: V (V) I (MA) 1.5 2.5 A 3.0 5.0 D.C. 4.5 7.5 Reversing switch х V 6.0 10.0 7.5 12.5 9.0 15.0 Results for part (A) . Plot a graph of V against I. What do you deduce from the graph? 1 From the graph calculate the resistance of the resistor

ue 011. . a) 11'D 0.1 Voltage (V) Current (A) 0.2 0.52 0.4 1.02 0.6 0.8 881 3.40 1.0

(B) A LAMP FILAMENT The resistor used in part (A) is now replaced with the lamp filament provided The current must not be switched off between readings (or else the filament will cool down) Results are tabulated below. The table includes a column for the resistance of the lamp filament calculated for each value of 1. a Voltage (V) Current (A) 10 U Jux 10 14 A D.C. Reversing switch X Results for part (B)