electromagnetism help! A2

an electromagnet can support a load of 42N with a 25 turn coilof 3.2A

what other combinations could support a load of 42N?

a) 3.2A, 25 Turns

b) 3.2A , 30 turns

c)6.4A , 10 turns

d) 2.0A , 50 turns

the answer is a) b) and d) but I am not sure why? anyone care to explain? :smile:

thanks

Reply 1

Stonebridge

13

Original post by Mr Tall

an electromagnet can support a load of 42N with a 25 turn coilof 3.2A

what other combinations could support a load of 42N?

a) 3.2A, 25 Turns

b) 3.2A , 30 turns

c)6.4A , 10 turns

d) 2.0A , 50 turns

the answer is a) b) and d) but I am not sure why? anyone care to explain? :smile:

thanks

It's an odd question that tells you that 25 turns and 3.2A supports the load, then in one of the answers gives the same values!

Anyway
I expect you are supposed to know that the field strength at the end of a solenoid is proportional to NI (turns x current)
3.2 x 25 = 80

So any pair of values that is equal to or greater than 80 should be ok.
You can check for yourself why the answer is as given.

Reply 2

uberteknik

21

Original post by Mr Tall

an electromagnet can support a load of 42N with a 25 turn coilof 3.2A

what other combinations could support a load of 42N?

a) 3.2A, 25 Turns

b) 3.2A , 30 turns

c)6.4A , 10 turns

d) 2.0A , 50 turns

the answer is a) b) and d) but I am not sure why? anyone care to explain? :smile:

thanks

Bit of a trick question methinks.

In the absence of any other information, all you can say is that:

F = kNI where k is some constant representing the lumped values of permeability, coil diameter etc. etc.

So by inspection

42N = kNI

where NI = 3.2 x 25 = 80 Amp turns.

Assuming k remains constant, then any combination which offers >= 80 Amp turns value will support 42N

Hence a, b and d will do this.

Reply 3

Mr Tall

OP

9

Original post by Stonebridge

It's an odd question that tells you that 25 turns and 3.2A supports the load, then in one of the answers gives the same values!

Anyway
I expect you are supposed to know that the field strength at the end of a solenoid is proportional to NI (turns x current)
3.2 x 25 = 80

So any pair of values that is equal to or greater than 80 should be ok.
You can check for yourself why the answer is as given.

sorry stonebridge, my mistake.

Nope, I have never been taught that the field strength at the end of a solenoid is proportional to NI (turns x current)..

I just know that the EMF is proportional to the N delta I

Please explain :s-smilie:

thanks

Reply 4

Stonebridge

13

Original post by Mr Tall

sorry stonebridge, my mistake.

Nope, I have never been taught that the field strength at the end of a solenoid is proportional to NI (turns x current)..

I just know that the EMF is proportional to the N delta I

Please explain :s-smilie:

thanks

The formula for the magnetic field strength at the end of a solenoid

http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/solenoid.html

To do this question you have to make some sort of assumption about what the strength of the field is, and hence the size of the force.
More turns means more force.
Bigger current means more force.

electromagnetism help! A2

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