Ronaldo Degazon Saturday 03/11/12 Physics Lab #16 Electricity

Aim: To investigate the IV relationship for a filament lamp.

Apparatus: Power source, voltmeter, Rheostat, filament bulb, switch, connecting wires

Diagram:

Procedure:

1. The apparatus was set up as shown in the diagram.

2. Using the rheostat the voltage was adjusted to a maximum of 2.6V.

3. The current was recorded.

4. The voltage was decreased by 0.2V and the corresponding current was recorded.

5. Step 4 was repeated 9 more times, each time recording the current.

6. A graph of current vs voltage was drawn.

Results:

Voltage (V/V) Current (I/A)

2.6 0.50

2.4 0.46

2.2 0.44

2.0 0.42 1.8 0.40

1.6 0.38 1.4 0.36

1.2 0.34

1.0 0.32 0.8 0.28

Discussion:

Ohm’s law states that; at a constant temperature, the potential difference across the ends of a

conductor is directly proportional to the current through it. It is represented by the equation

V=IR.

Voltmeters must be connected in parallel since they measure the potential difference across a

circuit device. This means that you have to connect the ends of the voltmeter immediately

before and immediately after a circuit component. This then, by definition, means that the

voltmeter is placed in parallel with a device in the circuit. If you connect the voltmeter in series,

it will not be able to be connected to either side of a device; it will either be completely before

or completely after the device. Also it is important to note that voltmeters have high

resistances so that they would not affect the current flowing and eventually the voltage in the

circuit.

Ammeters however must be connected in series since you simply need to calculate the current

flowing through a certain point in the circuit. If an ammeter is connected in parallel, it is not

doing the job of measuring the current flow in the circuit. Instead it is generating a new current

loop and will show an inaccurate reading. Ammeters have very low resistances so that they

would not affect the true value of current flowing through the point which it is measuring.

An ohmic device is one which obeys Ohm’s law. In other words, its current and voltage are

proportional. Non-ohmic devices do not obey Ohm’s law. This means that the current and

voltage do not vary proportionally.

The gradient of the graph represents the inverse of the resistance of the circuit. If the slope of

an IV graph for a device is straight, then the device obeys Ohm’s law. If it is not straight, then

the device can be considered non-ohmic.

A filament lamp obeys Ohm’s law only within certain limits of voltage and current. However,

the resistance of a filament lamp increases as the temperature of its filament increases, thus

causing the current to change. The filament lamp in this experiment obeyed Ohm’s law for the

majority of the experiment.

Sources of Error:

1. The voltage may been too high.

2. The components of the circuit may not have been connected correctly.

3. There may have been loose connections in the circuit.

4. There may have been parallax errors when reading the instruments.

Precautions:

1. It was ensured that the voltage was below 3 volts.

2. It was ensured that the components of the circuit were connected correctly.

3. It was made sure that the there were no loose connections.

4. It was ensured that the voltmeter was connected parallel to the load being tested.

5. The instruments were read in way such as to reduce parallax error as much as possible.