Find a potential difference \(\varphi_{A}-\varphi_{B}\) between the plates of a capacitor \(C\) in the circuit shown in Fig. 3.47 if the sources have emf's \(\mathscr{E}_ {1}=4.0 \mathrm{~V}\) and \(\mathscr{E}_ {2}=1.0 \mathrm{~V}\) and the resistances are equal to \(R_{1}=10 \Omega, R_{2}=20 \Omega,\) and \(R_{3}=30\) \Omega. The internal resistances of the sources are negligible.