A Switch, Resistor, And Capacitor Walk Into A Bar

Electricity and Magnetism Level 2

If the capacitor is initially uncharged, then how much current (in $\text{mA}$ ) flows through the circuit just after the switch is closed?

The answer is 1500.

1 solution

Pranshu Gaba
Mar 10, 2016

Just after the switch is closed, the capacitor has no charge so it behaves as plain wire, i.e. potential difference across it is 0. We can apply ohm's law across the resistor.

$V = IR$

The potential difference across the resistor is $6 \ \text{V}$ . The resistance of the resistor is $4 \ \Omega$ . Therefore the current flowing through resistor, and hence through the circuit is $I = \frac{V}{R} = \frac{6}{4} = 1.5$ A.

To convert Ampere to milliAmpere, we must multiply by 1000. Thus the current flowing through the circuit just after the switch is closed is $\boxed{1500 \text{ mA} }$ $_\square$

Bt...capacitor has a tendency to block dc...isn't it??

vaibhav AGGARWAL - 5 years, 3 months ago

@vaibhav AGGARWAL : Capacitor blocks DC(acts as an open circuit), only when its fully charged. But until then, in circuit above it will keep charging. So when switch closed, at that very instant capacitor is closed circuit

Ajit Deshpande - 5 years, 3 months ago

But capacitor is a set of plates with a gap....how can DC work at all?? I mean how could current transfer at all!!!

Rajat Pathak - 5 years, 3 months ago

@Rajat Pathak – Shouldn't the answer be zero!!??

Rajat Pathak - 5 years, 3 months ago

@Rajat Pathak – Yes, the current should be zero. 1500 m apm is the initial current after that the current will decay to ZERO.

Ossama Ismail - 3 years, 10 months ago

@Rajat Pathak – Non-zero current can pass through capacitors, otherwise there would be no differences between a capacitor and an open switch and there would be no use of capacitors. As you pointed out, there is a gap between the conducting plates, so electrons cannot flow through it. However, charge can move from one plate of the capacitor to the other through the rest of the circuit and this creates a net transfer of charge from one plate to the other. To read more, you may go through the Capacitors wiki.

Pranshu Gaba - 5 years, 3 months ago

@Pranshu Gaba – i got that, you are right...i need to revise my concepts....thanks

Rajat Pathak - 5 years, 2 months ago

A Switch, Resistor, And Capacitor Walk Into A Bar

The answer is 1500.

1 solution

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