Week 12 - Triple 23rd March

This is more of a procedure than an investigation, so you are less likely to be asked about these variables.

Independent variable: Potential difference (voltage) across the component.

Dependent variable: Current through the component.

Control variables: Temperature of components; same meters/wires; voltage step size/time at each step. 

• Ammeter

• Voltmeter

• Variable resistor

Accuracy can be improved by using a narrower crocodile clip so you can measure the length of the wire exactly. Digital ammeters and voltmeters have a higher resolution than analogue ones.

Validity can be improved by: A variable resistor can be used to adjust the potential difference. The components can quickly overheat if left on and this increases resistance. The circuit should be switched off quickly after taking measurements. 

. 

Describe: Resistor gives a straight line (Ohm’s law).  A

Lamp curves (resistance rises when hot).  B

Diode conducts in one direction only above a threshold. C

Explain: Heating raises resistance in the filament; Diodes only allow current to flow in one direction.

• Set up a circuit with power supply

• Add an ammeter in series

• Add a voltmeter in parallel.

• Measure Potential difference (Voltage) and Current for each component

• Use the variable resistor to adjust the potential difference

• Reverse the current and repeat (to give the data for the left side of the graph)

• You could Use V= IR and rearrange to Resistance = Voltage / Current to calculate resistance.

Independent variable (IV): Antimicrobial type or concentration.

Dependent variable (DV): Size of clear zone (diameter/area) around each disc (inhibition zone).

Control variables: Bacterial species/strain, agar type/depth, incubation temperature & time, inoculum volume and spreading method, disc size & soak time, volume of antimicrobial per disc.

Nutrient agar plates, 

bacterial culture (e.g. E. coli), 

Inoculating/spreader (glass spreader), 

Bunsen burner/bench disinfectant for asepsis, 

sterile filter paper discs

clear tape (to secure lids), marker. 

Improve accuracy with using photographs and image analysis to measure exact coverage size

Improve validity by ensuring plates are all stored at the same temperature. The bacterial solution is equally spread out.

Antibiotics/Antiseptics diffuse away from the discs. The further they diffuse, the lower the concentration becomes.

The larger the clear zone

The more bacteria have been killed

Because the antibiotics are effective at a lower concentration.

1. Aseptic set-up; pour/obtain set nutrient agar plates; label.
   
   

2. Spread a standard volume of bacterial suspension to make a uniform lawn.
   
   

3. Using sterile forceps, place soaked discs (different antiseptics/antibiotics or concentrations) evenly spaced.
   
   

4. Tape the lid (not all the way round), invert, and incubate for a set time/temperature.
   
   

5. Measure diameter of zones of inhibition / clear zones with a ruler.

6. Calculate each zone's area using PiR2