Electrical Circuits PPQs Flashcards
You are to describe the method to determine accurately the resistivity of one of the metal wires
Your description should include:
- the circuit diagram you would use
- the quantities you would measure
- the graph you would plot
- how you would determine the resistivity
[9]
Circuit diagram:
- resistor and power supply
- ammeter in series and voltmeter in parallel with resistor
Quantities measured:
- current and potential difference or resistance
- length of wire
- diameter/thickness of wire
Graph:
- graph of RA against l
Resistivity:
- gradient of graph
- a = pi r^2
- correct processing to find p consistent with graph
A student has a sample of wire made of an unknown metal.
In order to identify the metal, she determines its resistivity.
She measures the diameter of the wire using a micrometer screw gauge
Explain why the micrometer screw gauge is an appropriate instrument for this measurement [2]
Resolution is 0.01/0.001 mm
Therefore percentage uncertainty is 2/0.2% which is small
A student has a sample of wire made of an unknown metal.
In order to identify the metal, she determines its resistivity.
She measures the diameter of the wire using a micrometer screw gauge
Describe two techniques she should use to make this measurement as accurate as possible [2]
- measure diameter at varying lengths and find mean
- check for zero error OR avoid squishing wire
A student is asked to determine the resistance of a wire
Discuss the advantages and disadvantages of each method of measuring resistance:
Using a digital multimeter, using an analogue ammeter with an analogue voltmeter
- multimeter is easier because it is only one reading
- multimeter is cheaper because it is only one piece of equipment
- multimeter scale is variable whereas analogue is fixed
- analogue requires both series + parallel when setting up
- analogue: heating might change current in wire
- analogue: uncertainties are greater because more readings taken
A student investigated how the current through a diode varies with the p.d. Across it.
Diagram
———-||-||
——A——B———
Bulb— diode—- voltmeter
Voltmeter can slide between A and B
Although the components were not faulty, when the slider was moved from A to B and the voltmeter changed from 0V to 3V, the lamp did not light.
Explain this observation [3]
- the voltmeter has a large/infinite resistance
- so p.d. Across diode is too small
- hence the diode doesn’t conduct so there is no current in the lamp
OR - the voltmeter has a large/infinite resistance
- so there is negligible current through voltmeter
- hence as the bulb is in series with the voltmeter, the current will be too small for the lamp to light.
A student investigated how the current through a diode varies with the p.d. Across it.
Diagram
———-||-||
——A——B———
Bulb— diode—- voltmeter
Voltmeter can slide between A and B
Although the components were not faulty, when the slider was moved from A to B and the voltmeter changed from 0V to 3V, the lamp did not light.
State how the circuit should be modifies so the lamp will light [1]
Place voltmeter in parallel with diode
Or remove voltmeter
Explain the variation of resistance with potential for the filament bulb in terms of particle behaviour [6]
- as V increases, acceleration/energy of electrons increases
- greater energy transfer in collisions with lattice/ions
- increasing the temperature of metal/filament/ions
- amplitude of vibrations of lattice/ions increases
- collision rate between lattice/ions and electrons increases
- due to the increase in resistance, current doesn’t increase in proportion to potential difference
A student carries out a series of measurements to determine how the resistance of a wire varies with its length.
One precaution taken by the student was to keep the current small.
Explain why this precaution was necessary [2]
- temperature increases with increasing current
- resistance would’ve increases with temp
A student carries out a series of measurements to determine how the resistance of a wire varies with its length.
Explain a precaution which should be taken by the student to ensure the accuracy of the results in the table
[2]
- avoid parallax error
- by reading perpendicular to ruler.
A conductor has a resistance R. A constant potential difference is applied across the conductor. The drift velocity of the charge carriers in the conductor is v.
What is the relationship between R and v?
v (fish symbol) 1/R
State what is meant by drift velocity when applied to a metal conductor [1]
Mean velocity of charge carriers
Two conductors of the same material and length carry the same current. Current X has twice the cross-sectional area of conductor Y
By referring to an appropriate equation, compare the drift velocities [2]
v for Y is twice v for X
I=nqvA . n and q are constants
Two conductors of the same material and length carry the same current. Current X has twice the cross-sectional area of conductor Y
Explain the difference in resistance of X and Y in terms of the difference in drift velocity [3]
Resistance in Y is higher bc v is higher
Electrons gain more KE between collisions
More frequent collisions
More energy lost per collision
Greater pd required for a given current
When tidying a prep room, a teacher discovers a tray of resistance wires that have lost their labels. SHe decides to ask her students to carry out experiments to determine the material that each wire is made of by measuring the resistivity of the wires
Explain why the teacher asks the students to measure the resistivity and not the resistance of the wires [2]
Resistivity - constant, different materials have a fixed resistivity
Resistance is a property of wire
You are to describe a method to determine accurately the resistivity of a metal wire
Your description should include
Circuit diagram
Quantities u wld measure
Graph u wld plot
How u wld determine resistivity
[6]
Wire and power supply
Ammeter in series with voltmeter in parallel w resistor
Current and pd
Length of wire
Diameter of wire
Graph: RA against L
Gradient is resistivity
A = pi r squared
