Core Practical 3: EMF and internal resistance

Question 1

CP3 Method

Answer 1

1. Connect voltmeter in parallel to battery, Ammeter and variable resistor in series
2. Measure p.d and current across resistor
3. Vary resistance of resistor and record values for V/I.

Question 2

CP3 Analysis

Answer 2

Circuit mathematical model is ϵ-Ir = V
Rearrange to get V = -rl+ϵ
Plot graph of V against I

gradient = -r
y intercept = ϵ

Question 3

Eval

Answer 3

Circuit components modelled as negligible resistance
- Results become less accurate

Question 4

Why is the ammeter connected in series?

Answer 4

It has a near zero resistance so it can measure resistance going through it.

Question 5

Why is voltmeter connected in parallel?

Answer 5

It has very high resistance as it measure p.d between 2 points.

Question 6

Assess use of voltmeter instead of oscilloscope

Answer 6

• Voltmeter draws current; current received by resistor changes
• Voltmeter would indicate 0 for a.c
• Accuracy depends on calibration of voltmeter
• ## Digital voltmeter would give better resolution than measuring trace height on oscilloscope