I have Toshiba satellite l505 laptop and the ac-adapter is damaged. i found new adapter but it's output is 19v 3.4a and the output for the laptop is 19v 4.7a. is that safe to use the lower voltage?
Understanding Voltage and Current.
An analogy with water in pipes is often helpful in understanding the basics of electricity.
Voltage is like water pressure. It is a force. If the force is too strong your pipes will burst. Too low and your shower may not work at all.
Current is like flow-rate. Litres per second or how fast your bath fills. It depends on how big your supply pipes are. If your water supply has a low flow rate, you won't be able to fill your swimming pool in 10 minutes. If your water supply can fill your pool in 10 minutes, you can still use it to connect a drip-watering system for your pot-plants that only draws a cupfull a day of water.
If the laptop requires 19V you should use an power-supply that provides the same voltage. If the power supply voltage is significantly more, it will damage your device and could start a fire. If the voltage is less, the device will not work, actual damage is less likely.
If the laptop need to draw 4.7A, your should use a power supply that is capable of providing At least that number of Amps. More is ok. Less means the device will probably not operate correctly.
Normally when you use adapter, that has lower power output than the device you supply power to requires, adapter will simply heat up and die. In case of laptops, where battery has some charging logic, it will probably not charge or take very long time to charge, depends on how you use this laptop. I don't know if the adapter will explode in this case though. I will certainly not recommend using lower amperage adapter than Toshiba specifies.