# DC versus AC current in a coil

IronaSona
Homework Statement:
.
Relevant Equations:
XL =2p x f x L.
Hello, am kind of new to all the physics stuff , and kind a terrible at maths but am trying my best to be good at it one day .So i was told to answer this question

When a direct current of 2A is passed through a coil, the potential difference across the coil is 20V. When an alternating current of 2A at a frequency of 40Hz is passed through the coil, the potential difference across the coil is 140V.

a) Find the current in the coil if it is connected to a supply of 230V, 50Hz.

but i don't even know where i need to start ? do i need to find the coil inductances first and then do:
2 = a constant (2 x 3.1416 = 6.28)
6.28*hz*henries= ... Ohms

240V/...Ohms= ...Amps?

is that right ?

and for (b) i got no idea how to do that

b) What is the phase shift of the current in the coil with respect to the supply voltage?

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Staff Emeritus
Homework Helper
Rather than guessing and plugging numbers into formulas, you need to see the big picture and formulate a strategy. So first, what is the significance of what the sentence below is telling you?
When a direct current of 2A is passed through a coil, the potential difference across the coil is 20V.

IronaSona
Rather than guessing and plugging numbers into formulas, you need to see the big picture and formulate a strategy. So first, what is the significance of what the sentence below is telling you?
voltage across the coil is 20V?

Staff Emeritus
Homework Helper
That's what the sentence literally says, but what does it mean? Why does the DC current produce a voltage drop across the coil?

IronaSona
That's what the sentence literally says, but what does it mean? Why does the DC current produce a voltage drop across the coil?
because there's a resistance

• DaveE
Staff Emeritus