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Flashcards in Electricity Deck (26)
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State what is meant by a superconductor (2)

Zero resistance
At or below critical temperature


What are the required conditions for a material to become superconducting? (2)

Resistivity decreases with temperature
Becomes superconducting when you reach the critical temperature


Conducting putty is rolled into a cylinder. It is reshaped into a cylinder with half the radius and four times the length, determine the change to the resistance (2)

Halving diameter will increase resistance by 4
Increasing length by 4, increases resistance by 4
Resistance will be 16 times greater


How would a voltmeter, ammeter and standard lab equipment be used to determine a value for the resistivity of the putty? (length and radius known) (6)

Measure length with a ruler
Measure diameter with micrometer
Use diameter to calculate cross-sectional area
Measure voltage with voltmeter in parallel
Measure current with an ammeter in series
Calculate resistance using V against I graph (gradient)
Flat metal electrodes at each end to improve connection


Describe a procedure that could obtain an I-V curve for a semiconductor diode (6)

Connect circuit up
Measure current and voltage
Vary resistance
Obtain a range of results
Reverse connections to power supply and repeat
Plot a graph of V against I
Disconnect between readings


Explain the shape of the I against V graph of a filament lamp (4)

At low V, I increases proportionally
As V increases, temperature of filament increases
Resistance increases with temperature due to atoms/ions vibrating and taking up more space.
Rate of increase of I with V decreases
Same form of the curve in negative quadrant


In a circuit of two resistors in series, the second is replaced by a thermistor. State and explain what happens to the reading on the voltmeter on the resistor as the thermistor's temperature increases (3)

Reading will increase
Resistance of thermistor decreases
Current in circuit increases so pd across resistor increases


State and explain why it is an advantage for a rechargeable battery to have a low internal resistance (2)

Internal resistance limits current
Hence can provide higher current


State what is meant by the emf of a battery (1)

Energy changed to electrical energy per unit charge passing through


Explain what is meant by the internal resistance of a battery (1)

The resistance of the materials within the battery


State and explain the effect of attempting to use a battery with a high internal resistance to start a car (2)

Car will probably not start
Battery will not be able to provide enough current


A cell is connected to a variable resistor. Why does the terminal pd (across the cell) decrease as current increases? (2)

Energy loss in internal resistance
Pd across internal resistance increases with current


Why does a voltmeter across a battery have a reading lower than the emf of a battery in a circuit? (3)

Battery has internal resistance
Current passes through this resistance
Voltage lost, which reduces value of emf


How is emf and internal resistance found from the V against I graph for a cell? (2)

Emf is the y-intercept
Gradient is negative internal resistance


State Ohm's Law (1)

The current through a conductor between two points is directly proportional to the potential difference across the two points,provided the temperature remains constant


Explain why resistivity of wire changes with temperature (3)

Increasing temperature, increases resistivity
Lattice ions vibrate with greater amplitude
Rate of movement of charge carriers (electrons) reduced


Define resistance (1)

Ratio of voltage across component to current through component


Explain in terms of electron motion, why the I-V characteristic for the filament lamp is a curve (4)

Increase in voltage causes temp increases
This causes an increase in the movement of the atoms
Increase in rate of collisions with electrons
So resistance increases as shown by V/I changing


Define current

Rate of flow of charge


Define potential difference

Work done per unit charge


What is the resistance of an ideal ammeter?



What is the resistance of an ideal voltmeter



Describe and explain how the resistance of a NTC thermistor varies with temperature

As temperature increases, resistance decreases.
The increase in temperature causes atoms to vibrate and take up more space. This also causes more electrons to be released for conduction. the extra electrons released over compensates for the atoms vibrating more and taking up more space.


What are the rules for current, potential difference and resistance for series circuits

Current - same at all points
p.d. from battery shared amongst components
Resistance increases as more components are added.


What are the rules for current, potential difference and resistance for parallel circuits

Current - splits up but total current must stay the same
p.d. from battery same across each branch
Resistance decreases as electrons have more branches/paths to take


What is the relationship between resistance and light intensity for an LDR

As light intensity increases resistance decreases.