Section 4 -Electricity Exam Questions Flashcards
Electric Current Direct Current Circuits
State what is meant by a superconductor (2)
Zero resistance
At 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
A copper cylinder cable contains several filaments. State and explain what happens to the cable’s resistance when the filaments are made superconducting (5)
Resistance decreases to zero Copper still has resistance But this is in parallel with filaments So total resistance is zero Current goes through filaments
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
Measure current
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
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
Describe the procedure of obtaining accurate and reliable measurements of the resistance of a thermistor at different temperatures (6)
Thermistor is connected in a suitable circuit with voltmeter
Thermistor is heated in a beaker of water and a thermometer measures the temperature at small regular intervals
Resistance is found at various temperatures by diving voltage by current
Water must be stirred.
Temperature in range 20-90
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 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
A conducting putty of 20Ω is set up in series with a cell, ammeter and 10Ω resistor. The voltmeter is across the cell. Explain the problems for showing the putty obeys Ohm’s law (4)
Voltmeter position should not be across the cell
Voltmeter should be connected across the putty
10Ω resistor is not suitable to control the current
Because its resistance is only half of the putty
With reference to two uses for superconductors, explain the advantage of their use compared with copper conductors (3)
Electromagnet
Produces very high magnetic field strength
Transmission
Less energy wasted
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 reduced
Define resistance (1)
Ratio of voltage across component to current through component
State and explain, in terms of electron flow, how the resistance of the filament lamp changes as the current in the lamp increases (3)
Resistance increases
Temperature rise increases lattic vibration
More frequent collisions
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
