Practicals Flashcards
Describe how you would calculate internal resistance.
Set up the circuit with a battery, a variable resistor, an ammeter in series, and a voltmeter across the battery terminals.
Measure the EMF of the battery by setting the circuit to an open circuit (no current flowing).
Adjust the variable resistor to change the current
and record corresponding values of voltage
Plot a graph of V against I
Determine the internal resistance from the graph:
The y-intercept gives the EMF.
The gradient of the straight-line graph :
𝑉 = EMF - Ir is -r, so the internal resistance is the negative gradient.
How would you investigate the characteristics of electrical components?
- Circuit setup: Connect the component in series with an ammeter and a variable DC supply, and a voltmeter in parallel across the component.
- Measurement procedure: Vary the supply in at least 10 equal steps (e.g. 0 V → 10 V), recording I and V at each step.
- Repeat readings: Take measurements while increasing and then decreasing V to check for a hysteresis (lag effect, important for a filament lamp).
Graph plotting: Plot I (y‑axis) against V (x‑axis) to obtain the I–V characteristic curve.
Ohmic vs non‑ohmic: A straight line through origin indicates ohmic behaviour (constant R); a curved “S‑shape” indicates non‑ohmic (e.g. filament lamp’s resistance increases with temperature).
Uncertainty and evaluation: Quote instrument uncertainties (e.g. ±0.01 V, ±0.005 A), discuss systematic errors (meter calibration, contact resistance), and suggest improvements (use digital meters, limit heating).
How would you determine the resistivity of a wire?
Method setup
Measure length of uniform wire using a meter rule (at least 5 different lengths, e.g. 20, 40, 60, 80, 100 cm).
Set up circuit with the wire in series with a variable DC supply, ammeter (in series) and voltmeter (in parallel across portion of wire).
Data collection
For each length, adjust the supply to a fixed current (e.g. 0.50 A) and record the voltage V across that length.
Calculate resistance R=V/I for each length.
Graph and gradient
Plot R (y‑axis) vs L (x‑axis). The gradient 𝑚 = Δ𝑅/Δ𝐿
Calculation of resistivity
Use 𝜌 = 𝑚𝐴 where 𝐴 is the cross‑sectional area of the wire (𝐴 = 𝜋𝑑^2/4 and d is measured with a micrometer (take several readings for accuracy).
Uncertainty and evaluation
Quote uncertainties in L, 𝑉, 𝐼, d.
Discuss systematic errors (e.g. wire not uniform, contact resistance) and improvements (e.g. more lengths, digital meters, zero the micrometer)
