exam questions Flashcards
State what happens to the resistance of the filament lamp as the current increases.
increases
Explain why a filament lamp is a non−ohmic
conductor up to its working power.
current is not directly proportional to voltage
resistance is not constant
State what is meant by electromotive force.
work done by a voltage source per unit charge
Describe how you would make a direct measurement of the emf ɛ of a cell, stating the type of meter you would use.
- voltmeter across terminals with nothing else connected to battery
- voltmeter must have high resistance to give zero current
Describe and explain what happens to the resistance of the thermistor as its temperature increases.
- decrease in resistance
- more charge carriers released at high temperature
Explain what is meant by the root mean square voltage.
the square root of the mean of the squares of all the values of the voltage in one cycle
State and explain why a filament lamp is most likely to fail as it is switched on.
- resistance of lamps will be lower when first switched on
- hence initial current will be larger
- sudden rapid change in temperature
State and explain what happens to the current when another lamp is added to the circuit in series.
→ current decreases
→ as voltage supply is shared
State and explain what happens to the current when another lamp is added to the circuit in parallel.
→ current through the lamps stays the same
→ as both lamps are connected directly to voltage supply
Explain what is meant by critical temperature.
the temperature below which a material
becomes a superconductor (has zero resistance)
Describe how you would use an oscilloscope to check the accuracy of the rms
output voltage and the frequency of the supply.
1) connect to oscilloscope input with time base initially switched off
2) y gain adjusted to get as long a line as possible,
3) length of line = peak to peak voltage - find rms voltage
4) switch on time base + adjust
5) find period + frequency
6) compare vales with stated values
