"What is the effect of the current account balance on the exchange rate?" - Vincent
To answer this question, let's take the case of a current account surplus. The current account is determined largely by the level of net exports- the other components of the current account (net factor payments and net transfers) are generally relatively small. So a current account surplus implies that exports are larger than imports.
Paying for exports requires domestic currency, and imports foreign currency. Thus an increase in exports will result in increased demand for domestic currency, and a decrease in imports in decreased demand for foreign currency (=supply of domestic currency)- so positive net exports imply upward pressure on the value of the currency, as the demand for domestic currency is increasing faster than the supply. Thus, to answer your question, a current account surplus will result in upward pressure on the currency. The arguments reverse for a current account deficit.
Empirically, there is not alway a clear link between the value of the currency and the current account, and even where there is, we often observe the exact reverse: after a lag, a decrease in the currency results in an increase in the current account, and vice versa. So what explains this link?
The explanation is that exchange rates are determined largely by capital account flows, rather than current account flows, as the former are much larger. Suppose there is a large capital outflow, for example. This will push down the value of the currency. But as the value of the currency decreases, exports become relatively cheaper and imports relatively more expensive. The direct effect of these price effects is to result in a decrease in the current account balance.
To put this another way, the current account balance may be defined as
CA = Price(exports) x Quantity(exports) - Price(imports) x Quantity(imports)
The effect of the currency depreciation on prices will decrease the current account surplus. But that is ignoring the quantity effects. Over time, trade flows adjust to the exchange rate change, and the quantity of relatively cheaper exports will rise, while the quantity of relatively more expensive imports will fall. After a year or more, the quantity effects will tend to be larger than the price effects, so that the current account will start to rise.
We typically refer to the relationship between exchange rates and trade flows as the "J-curve," since a depreciation results in an initially fall in net exports but an eventual rise, much like the letter J.