# PR 5 - PowerPoint PPT Presentation

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PR 5. Comments. Record of L, I, v, r. Record of measurements and calculations Headings with units with at least 6 sets of readings for l , I , V, R Acceptable values and correct d.p for l , I , V l in cm, 1 d.p. I in A, 2 d.p. V in V, 2 d.p.

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PR 5

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## PR 5

### Record of L, I, v, r

• Record of measurements and calculations

• Headings with units with at least 6 sets of readings for l,I, V, R

• Acceptable values and correct d.p for l,I, V

• l in cm, 1 d.p.

• I in A, 2 d.p.

• V in V, 2 d.p.

• correctly calculated and correct s.f. for R

• Wide range (at least 70 cm) and fairly well spread values of length l

### Y-intercept

• Theoretically, should be zero. Anywhere near zero is fine!

• Note that y-intercept is a value, not the coordinates of a point!

• “Y-intercept” need not have units. However, you must include units if they ask what is the potential difference when R=0.

• Do NOT include 0.0 cm as a reading for l !

### conclusion

• Potential difference across the resistance wire is directly proportional to its resistance.

• OR

• (When graph does not pass through the origin, or not close to it due to experimental error)

• Potential difference across the resistor increases linearly (or proportionally) as the resistance of the resistance wire increases.

### precautions

Note that precautions should always come with a “so as to”

Avoid using parallax error and zero error!!

Acceptable

unacceptable

Minimise parallax error by taking reading when the needle coincides with its reflection in the mirror.

Adjust needle of voltmeter and ammeter to eliminate zero error.

• Ensure that wire clippings are securely/tightly attached to the circuit components so that the readings will not fluctuate.

• Press the jockey down firmly so as to minimise contact resistance which can reduce current flow.

Current can flow smoothly is very vague…

Use only if there are no better answers

### precautions

Note that precautions should always come with a “so as to”

Avoid using parallax error and zero error!!

Acceptable

unacceptable

To ensure a more “accurate reading” is too general! Try to be specific about what quantity is affected. At least say “accurate reading of current”

Ensure wires are not faulty.

Wait for needles to stabilise before taking reading.

• Open the switch when not taking readings so as to prevent the wire from heating up which can cause its resistance to change.

• Place the jockey perpendicular to the length of the wire so that l measured will be an accurate representation of the length of resistance wire considered.

### Source of error

Highly recommended

A good source of error should explain clearly which reading will be affected and as far as possible, how it will be (increase or decrease)

• When using the jockey to press down on the resistance wire, the contact force and angle might differ for each reading. This results in differences in the resistance at the contact point, causing the voltmeter reading to have random errors.

### Source of error

Acceptable

unacceptable

Parallax error (Putting it here is worse than putting it under precaution!!)

Just stating the problem without explaining how it affects the experiment.

• Readings of l and V cannot be simultaneously taken – our hands may shift when reading off the voltmeter, thus resulting in an inaccurate reading of l.

• Resistance in the other connecting wires in the circuit will cause the current in the circuit to be lower than expected.

### Source of error

Acceptable

unacceptable

Not pressing jockey down perpendicularly, affecting readings on voltmeter and ammeter (The perpendicular part is for a good reading of l more than I and V.)

• Resistance of the resistance wire may not be uniform (due to non-uniform cross-sectional area of the wire). Therefore, potential difference across the wire may be inconsistent.

• Resistance wire may heat up during the course of the experiment, causing its resistance to change.