The current through the heater is 8 A if potential difference is increased to 120 V. 
Explanation:
- According to Ohm's Law, the potential difference (Voltage) is directly proportional to the current flowing through a conductor.
That is, V ∝ I 
⇒ V = IR where R is the constant of proportionality called resistance. 
- In this case, given voltage = 60 V and current = 4 A. To find the current, first find the constant of proportionality, resistance of the heater. 
R = V/I = 60/4 = 15 ohm
-  If the potential difference is increased to 120 V, find the current flowing through it. 
I = V/R = 120/15 = 8 A