RP4: V-I Characteristics Flashcards
What is the aim of this experiment?
- to investigate the I-V characteristics of a variety of circuit elements, using circuit diagrams to construct appropriate circuits
How would you carry out this practical?
- Construct circuit with variable resistor, battery, resistor (and voltmeter in parallel) and ammeter in series
- Set the variable power supply or variable resistor the the lowest setting for potential difference
- Record the current and voltage over the resistor
- Increase the current from the power supply by 2V and repeat your readings
- Change the resistor to a filament lamp and repeat experiment
* Change filament to a diode and protective resistor (to restrict high currents flowing through the diode) ensuring the diode is the correct direction allowing flow of current though. Change ammeter to a milliammeter since the current measure will be smaller than for the other components - Plot a graph of current against p.d for each component
What are the expected results?
- Components can be described as ohmic or non-ohmic
- Ohmic conductors follows Ohm’s Law (resistance is constant) and so the graph of an ohmic conductor will be a straight line through the origin. The resistor should be ohmic
- Non-ohmic conductors have a resistance that varies with current. The I-V graph of a non-ohmic conductor won’t be linear. The filament lamp and diode are non-ohmic conductors
List some safety precautions
- Wires and components may become hot after a current passes through - allow to cool before handling
- Disconnect the power supply when not taking readings to prevent overheating
- Be aware that too high a voltage my damage components and cause excessive. Switch your circuit off if you think the voltage is too high
What component is required to alter potential difference if you’re not using a variable power pack?
- A rheostat, or variable resistor
What is an ‘ohmic conductor’? State the condition required
- A conductor for which current and potential difference are directly proportional
- Resistance remains constant as current changes
- Temp must be constant
What specific type of ammeter may be required in this experiment and why?
- Milliammeter since the currents and current changes involved may be quite low
What must be present in the circuit when the diode is being tested? Where should it be connected?
- Protective resist to prevent current level getting too high
- Connected in series with the diode
For which component does the polarity of power supply matter and why?
- Diode - diodes only allow current to flow through in one direction
What must be kept constant to get reliable results - how can yo achieve this?
- Temp should remain constant so that the resistance of the components isn’t affect
- You should disconnect the supply when not taking reading to avoid unnecessary heating
How do you determine the resistance of a component from an I-V graph?
- The resistance at a given point is the inverse of the gradient of the line drawn from that point to the origin
- Work out the gradient and the reciprocal is the resistance
What would you expect the I-V graph if a resistor to look like?
- Straight through origin
- From negative to positive
- Diagonal straight line which is directionally proportional
What would you expect the I-V graph of a diode to look like
- Flat at first in the negative quarter, then slowly increases (flat in negative then increases)
- Like a slope?
What would you expect the I-V graph of a filament lamp to look like?
- S shaped
- Through the origin
- Flatish in the negative negative quarter, then increase, then flattens out again
What happens to the resistance of a filament lamp as the temp increases? Why?
- Resistance increases
- Ions in metal have more energy, so vibrate more, causing ore collisions with electrons as they flow thought he metal creating greater resistance to current flow
