Here, the test charge is an electron, i.e.,
q0 = –1.6 × 10 –19 C
VA = 20 V
and VB = –40 V
Work done by external force
(WB – A ) external force = q0 (VA – VB )
= (–1.6 ×10 –19 )[(20) – (–40)]
= –9.6 × 10 –18 J
Work done by electric force
(WB – A ) electric forc e = –(WB – A ) external force
= – (–9.6 × 10 –18 J) = 9.6 × 10 –18 J