Topic 3 Electric circuits Flashcards
Describe a procedure to determine the internal resistance and E.M.F of an electrical cell. (3)
-Connect a cell, ammeter, variable resistor in series and a voltmeter in parallel to the cell.
-Recording pairs of readings of terminal P.D and current.
-Use variable resistor to obtain 5 other pairs of readings.
Describe how you would use your results from the procedure to set up a circuit with internal resistance. To find a value for the E.M.F and internal resistance of the cell. (3)
-Plot a graph of terminal P.D on y axis and current on x axis.
-Y intercept = E.M.F
-Gradient = -r
A student set up a circuit with a cell, LDR, and resistor in series with a voltmeter parallel to the resistor.
As the student moved a lamp towards the circuit she observed the potential difference (p.d.) changing across the fixed resistor.
With reference to the electrons in the LDR explain this observation. (5)
-Electrons gain energy from the light.
-As the lamp is moved towards the LDR the intensity of light falling on LDR increases.
-This increased the number of conduction electrons.
-Which reduced the resistance of the LDR.
-Decreasing the P.D across the LDR.
What is the expression for lost volts? (1)
Ir
What is the correct expression for terminal P.D of the cell? (1)
V=-Ir +E.M.F
A student carried out an experiment to calibrate a thermistor. She connected the thermistor in series with a resistor and a power supply as shown. Then she placed the thermistor in a
beaker of hot water and used a thermometer to record the temperature θ of the water.
The student recorded θ and corresponding values of the reading V on the voltmeter as the water cooled.
Explain, making reference to charge carriers, why V increased as the water cooled. (3)
-The number of free charge carriers in thermistor decreases.
-Hence the resistance of the thermistor increases.
-And a larger fraction of supply P.D is across thermistor.
A light dependent resistor and a negative temperature coefficient thermistor are connected in series.
What factors have to be changed to obtain highest combined resistance of these two components? (1)
-Conditions which are dark and cold.
With reference to the charge carriers in the thermistor, explain what happens to the P.D across the thermistor as the temperature increases? (3)
-As temperature increases, number of charge carriers increases so its resistance decreases.
-Thermistor resistance as a proportion of total resistance decreases.
-P.D across thermistor decreases.
A potential difference is applied across the metal filament of a light bulb and charge flows.
By referring to the mean drift velocity of the electrons, explain what happens to the current in the metal filament if the potential difference is unchanged and the temperature of the metal
increases. (3)
-Amplitude of lattice vibrations increases.
-Resulting in more frequent successful collisions between electrons and lattice ions.
-This results in a smaller drift velocity and due to I=nqvA, I decreases.
Resistance can also be affected by temperature.
Explain why the resistance of a metal sample increases with an increase of temperature. (3)
-A rise in temperature causes amplitude of vibrating ions to increase.
-Causes number of collisions per second between ions and moving electrons to increase.
-So rate of flow of electrons decrease causing resistance of metal to increase.
A cell, LDR and resistor are connected in series.
What happens to the resistance across the LDR and P.D across the resistor when light intensity is increased? (1)
-Resistance of LDR decreases, P.D across resistor increases.
