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Resistance Lab. Aim - To investigate the effective resistance (total resistance) of three resistors connected in a series circuit.

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Introduction

Physics

Lab Report – Resistors in Series

• Aim - To investigate the effective resistance (total resistance) of three resistors connected in a series circuit.
• Background – A resistor is an electrical component which opposes the flow of current in a circuit. When a resistor is connected in a circuit it produces a voltage across its terminals (measured using a voltmeter) using this voltage and the current (measured using a milliammeter) flowing through the circuit the resistance of the resistor can be calculated. Thus Resistance = Voltage/Current, this formula has basically been derived from Ohm’s Law which states that Voltage = Current * Resistance. In this experiment we have connected three resistors with unknown resistances in series circuit, the voltage of each resistor was measured using a separate voltmeter while another one was used to measure the combined overall voltage of all three resistors, further by using the above formula we can find the individual resistance of each resistor which can be added to give the effective (total) resistance of the three resistors connected in series. This was the first process used to find the effective resistance, the second process involved using the same formula only this time the total voltage across all the three resistors was used along with the current to directly find the effective (total) resistance of the three resistors connected in series. Both the above processes were repeated with a different value of current so that an average of the effective (total) resistance of the three resistors connected in series could be calculated for each process. Thus in this way the effective resistance of three resistors connected in series was calculated and verified using two separate processes.
• Method -
• Variables –

1) Controlled – The entire set of apparatus.

2) Independent – The current used.

3) Dependent – Voltage across each resistor (individual resistance) along with combined voltage of the circuit (effective resistance).

• Apparatus – A power pack, 4 voltmeters, a milliammeter, 3 resistors and connecting wire (with crocodile clips).
• Diagram -
• Method (contd.) -
• Procedure –
1. First set up the circuit shown above. In order to this connect the power pack, the ammeter and the 3 resistors in a series circuit using some connecting wire. Then connect three voltmeters across each of the three resistors and then finally connect the 4th voltmeter (Vs) across all three resistors and all three voltmeters (as shown above).
2. Now set the current in the circuit to 150 milliamps using the power pack and the ammeter.
3. Quickly take the readings for the voltage across each individual resistor (V1, V2 and V3) and the combined voltage across all three resistors (Vs).
4. Now repeat the above step until you get 4 sets of readings for each value of the different voltages.
5. Next repeat the entire above procedure using a different value of current (200 milliamps)
6.  Now for each value of current separately calculate an average value of the voltages on V1, V2, V3 andVs.
7. Usingthe average values of V1, V2, V3 and the value of the current calculatethe values of resistance (R1, R2 and R3) for the three resistors, with both values of current, respectively.
8. Now add the individual values of resistance (R1, R2 and R3) for the three resistors, with both values of current, to get the value of the effective resistance.
9. Usingthe respective value of Vs and the value of the current in both cases, calculatethe value of the effective resistance (Rs) for the entire circuit and cross check with your above answer.
• Results –
• Raw Data Table –

Middle

Trial 1

Trial 2

Trial 3

Trial 4

Trial 1

Trial 2

Trial 3

Trial 4

V1 (volts)

1.2

1.4

1.3

1.2

1.8

1.6

1.7

1.8

V2 (volts)

1

0.9

0.8

0.9

1.2

1.4

1.3

1.2

V3 (volts)

3.7

3.5

3.9

3.6

5

5.1

4.8

5.1

Vs (volts)

6

6.2

5.9

6

8

8.2

8.1

8

• Processed Data Table –

Processed Data Table showing all the values used to calculate the effective resistance of all the three resistors by both processes for both values of current along with the overall average value of effective resistance.

 Current = 0.15 amps Current = 0.2 amps Average value of V1 (volts) (1.2 + 1.4 + 1.3 + 1.2)/4 = 1.275 (1.8 + 1.6 + 1.7 + 1.8)/4 = 1.675

Conclusion

I made sure that the wires would not heat up too much by keeping the circuit switched off as much as possible.I took four trials of each voltage reading for both sets of to get a more accurate average set of results.
1. Safe Test –

In order to conduct the experiment safely, I took the following measures.

• I handled all the apparatus carefully so that no one was hurt and so that no apparatus was damaged, the resistors were handled with special care.
• The circuit was always kept off when not in use.
• The work space around me was clear so that there were no accidents and so that the experimental setup was intact.
• Evaluation –
1. Factors that could have led to experimental inaccuracies–

The factors, which could have affected the experiment negatively thus causing any experimental inaccuracies, are given below.

• A slight error while measuring the volatges could have led to anomalies.
• The current flowing through the circuit could have been slightly more or less than required, thus leading to inaccurate results.
• While keeping the circuit on for too long the wire could have heated up thus affecting the resistances in the circuit.

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