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Physics: Post your doubts here!

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Can anyone help me with this?

"A power supply of e.m.f. E and internal resistance R is connected to a 10 kΩ resistor. The current drawn from the power supply is 0.91 mA. A voltmeter of internal resistance 10 kΩ is connected in parallel with the 10 kΩ resistor, and reads 8.3 V. Calculate the e.m.f. and the internal resistance of the power supply."

This question should be answered using Kirchhoff's laws.
 
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Book says :

1.Back Emf seems to imply opposition, something not wanted, yet in a motor it is the induced emf that is responsible for the mechanical work output of the motor.
2.The workdone against the induced emf in a motor is the useful work output of the motor. Workdone against the resistance of the wires is wasted as thermal energy

Can someone explain please?
How is back emf responsible for the ouput of a motor? :S
 
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http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Physics (9702)/9702_s09_qp_4.pdf
help please! 6(b)(i) why does the fact that the force on the magnet is downwards mean that the force on the wire is upwards?
Force on wire is upwards according to newtons third law.
Use Flemings left hand rule. Thumb will point upward, second finger in direction from x to y. So first finger will be from pole p to other pole. Thus pole p is north pole as magnetic field direction is from north to south.
 
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