electricity

An adaptor is just a transformer that converts high voltage at the input(220V from socket outlet) to low voltage(5V, 12V or 24V) at the output to provide adequate voltage for the equipment or product.

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Wall wart

Back to your question,
CASE1 - if the output of your adaptor is 5V and 500mA, the Maximum output power of this adaptor is 5x0.5=2.5W (P=VxI). That means the adaptor is capable to provide 1W or less. This adaptor is also suitable for your equipment that only requires 5x0.2 = 1W.
CASE2 - the adaptor only capable for 1W, but the equipment requires 2.5W. The adaptor may get overheated, since more current I is passing thought the adaptor (500mA vs 200mA).
For more information about adaptor, please visit this link http://www.answers.com/topic/wall-wart-1
All these answers are from my own understanding, hope it can be helpful for you.

2009-05-06 08:45:12 補充：
"If It Give Out 500mA,,
But Your Use In 200mA,,
You Will Exceed 300mA,,
It May Broken Your Electrical Appliances"
I dont agree with that.

2009-05-06 08:45:21 補充：
The amount of output current is depending on the Load resistance (your electrical compliant). V=I/R, V is 5V, I = 200mA, R=5/0.2=10ohm.

The resistance is always fixed, and V = 5V, the electrical compliant can only draw 200mA. There is no way that it can draw 500mA.

2009-05-06 08:46:43 補充：
Therefore, the electrical appliance will not be broken in all cases.

If It Give Out 500mA,,

But Your Use In 200mA,,

You Will Exceed 300mA,,

It May Broken Your Electrical Appliances

Another One,,

If You Use In 500mA,,

But It Just Have 200mA,,

It Don't Have Enough Energy,,

Tahn Your Electrical Aplliance Will Be Not Work~