The default gateway is the route use if none of the other routes match. So by that definition, there can be only one at a time. 

Except not really, because your routing rules might be more complicated. Modern versions of Linux, for example, support using multiple completely independent routing tables, and making packet-by-packet determination as to which table to consult. So for example, perhaps you have one network configuration used for normal traffic, but a completely different routing table for your web browser. Or perhaps you have another routing table that's only used for packets with specific TOS bits set. But each would have its own default gateway which would reflect the default route for that specific routing table.

Normally, the routing table is as simple as, "local traffic to the local interface, everything else to the router." But if you're using a more complicated network setup, then the tools exist to cope with whatever network scenario you can dream up.

Can a PC have more than one Default Gateway?
Consider an ideal capacitor which has a length of \$\ell_1\$ between its plates. The capacitor terminals are open; they are not connected to any finite valued impedance. Its capacity is \$C_1\$ and it has an initial voltage of \$V_1\$.

What happens to the capacitor voltage if we make the gap between the plates \$\ell_2=2\ell_1\$ without changing the amount of charge on the plates?



My thoughts on this:

Increasing the gap will decrease the capacitance.

$$ C_2 = \dfrac{C_1}{2} $$

Since the amount of charge is unchanged, the new capacitor voltage will be

$$ V_2 = \dfrac{Q}{C_2} = \dfrac{Q}{\dfrac{C_1}{2}} = 2\dfrac{Q}{C_1} = 2V_1. $$

Is this true? Can we change the capacitor voltage just by moving its plates? For example, suppose that I'm wearing plastic shoes and I have some amount of charge on my body. This will naturally cause a static voltage, since my body and the ground act as capacitor plates. Now, if I climb a perfect insulator building (e.g.; a dry tree), will the static voltage on my body increase?
