20. there are two electrical devices, one is electrical kettle of 2kw-200v and
next one is electrical heater of 1kw-240 v. which device has highest
resistance?

Electrical kettle:  

P = 2 KW = 2000 W

U = 200 V

⇒  I = P/U = 2000 / 200 = 10 A

We apply the Law of Ohm:

R = U / I = 200V / 10A = 20 Ω

Electrical heater:

P = 1 KW = 1000 W

U = 240 V

⇒  I = P/U = 1000 / 240 = 4,16 A

We apply the Law of Ohm:

R = U / I = 240V / 4,16A = 57,69 Ω

The device with the highest resistance is the electric heater.

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3. dark matter

empty space could also be a simple vacuum