# DC versus AC current in a coil

• IronaSona
In summary: The second sentence saysWhat's the relationship between those two quantities in terms of ##R##, ##L##, and frequency ##f##? Rather than using numbers, just call them ##I## and ##V## for now.The relationship between the current in the coil (#I) and the supply voltage (V) is given by ##I = V/R##.
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?

Last edited by a moderator:
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?
IronaSona said:
When a direct current of 2A is passed through a coil, the potential difference across the coil is 20V.

vela said:
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?

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

vela said:
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
OK, good, so the coil isn't simply an inductor but an inductor ##L## and a resistor ##R##.

The second sentence says
When an alternating current of 2A at a frequency of 40Hz is passed through the coil, the potential difference across the coil is 140V.
What's the relationship between those two quantities in terms of ##R##, ##L##, and frequency ##f##? Rather than using numbers, just call them ##I## and ##V## for now.

## What is the difference between DC and AC current?

DC (direct current) flows in one direction while AC (alternating current) changes direction periodically.

## How does a coil affect DC current?

A coil can block or reduce the flow of DC current due to its inductance, which resists changes in current.

## How does a coil affect AC current?

A coil can cause AC current to oscillate or vibrate due to its inductance, which stores energy in its magnetic field and releases it when the current changes direction.

## Which is better for long-distance transmission, DC or AC current?

AC current is better for long-distance transmission because it can easily be transformed to higher or lower voltages using transformers, while DC current requires expensive and inefficient converters.

## What are some common applications of DC and AC current in coils?

DC current is commonly used in batteries, electronic circuits, and electric motors. AC current is used in power grids, household appliances, and electric generators.

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