Page 127 - Physics Form 5 KSSM_Neat

3. Record the voltmeter reading as e.m.f., Ԑ in Table 3.13. CHAPTER 3
4. Close the switch and record the voltmeter reading as potential difference, V. Electricity
5. Calculate the difference in voltmeter readings before and after the switch is closed to determine
the voltage drop.
6. Repeat steps 2 to 5 using the brand B dry cell.
KEMENTERIAN PENDIDIKAN MALAYSIA
Results:
Table 3.13

Brand A Brand B

Voltmeter reading before switch is closed, Ԑ / V

Voltmeter reading after switch is closed, V / V

Voltage drop, Ir = Ԑ – V

Discussion:
1. Why should new dry cells be used in this activity?
2. Which brand of dry cell experiences greater voltage drop?
3. Why is there a difference in voltage drop between the two dry cells? Explain.

Internal resistance, r of a dry cell is the resistance caused by electrolyte in the dry cell.
The S.I. unit for internal resistance, r is ohm (W). Internal resistance causes:
• loss of energy (heat) in dry cells as work has to be done to move one coulomb charge
against the resistance within the dry cell
• the potential difference across the dry cell terminals to be less than the e.m.f., Ԑ when
current flows in a complete circuit

Ɛ
r
I
I

R

V

Voltage drop, Ir = Ԑ – V
Figure 3.25 Internal resistance of a dry cell

LS 3.3.2 117