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• Work out the voltage of a torch by adding up the voltages of all its batteries: if it has 4 batteries each of which is 1.5V, the total voltage is at 6V, which is not enough to do any harm.
• A laptop power supply (picture) is connected to the mains (240V), so that part is dangerous, but the part that supplies the laptop is only 18V, so that's safe to do repairs on: you'll get at most a bit of a tingle if you touch it.
• A mobile phone charger is connected to the mains, so that part (240V) is dangerous, but the part that plugs into the phone is probably 5V (check the label on the part that plugs into the mains), so very safe.
• USB sockets provide a maximum of 5V, so they're very safe.