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dc motor's energy conservation
發問:
一個係magnetic field入面既motor,如果有external force 令到佢個coil rotate,咪會有current產生既By Fleming's left hand rule , 係一個magnetic field中既current 都會產生一個opposite to external force 既magnetic force ,balance 個external force,令到個coil moves at a constant speed我既問題就係,如果個coil moves at a constant speed ,即KE gained=1/2mv^2-1/2m^2=0... 顯示更多 一個係magnetic field入面既motor,如果有external force 令到佢個coil rotate,咪會有current產生既 By Fleming's left hand rule , 係一個magnetic field中既current 都會產生一個opposite to external force 既magnetic force ,balance 個external force,令到個coil moves at a constant speed 我既問題就係,如果個coil moves at a constant speed ,即KE gained=1/2mv^2-1/2m^2=0 咁點解仲會有electric energy produce出黎既? 更新: THX!! but there's still a point that I don't understand. I wanna ask that whether the external force(external torque) is larger than the magnetic force?And if the answer is 'yes',why does the coil still move in a constant speed? 更新 2: I am really sorry that I messed up d.c.motor and d.c. generator, but you still understand my question LOL!Thanks again
最佳解答:
When a coil rotates in a magnetic field, there is electromotive force (emf) induced (Faraday's Law of Electromagnetic Induction). A current flows only when the coil is connected to an external circuit. Assume there is no friction on the rotation coil, no force is needed to keep the coil rotating at constant speed if the coil is NOT connected to an external circuit (i.e. the coil is in open circuit). In the absence of an external circuit, there is no current flowing, and hence no magnetic force is produced on the coil to counter-balance its rotational motion. The coil can rotate by its own inertia. Once the coil is connected to an external circuit, current flows through the coil due to induced emf. The current then gives rise to a magnetic force (more exactly, a magnetic couple) that acts in direction opposite to the motion of the coil. The coil tends to slow down. Therefore, to keep the coil continuously rotating at constant speed, an external force (external torque) must be applied to the coil. It is the mechanical work done by this force (or torque) that transforms into electrical energy in the external circuit. Failing to have an external force applied to the coil, the coil will gradually slow down its rotational speed. The electrical energy so appeared in the external circuit comes from the rotational kinetic energy of the coil. 2014-02-05 19:41:07 補充： Your suppl question: Under an ideal condition (no friction, no air or other resistance), the external torque equals to the magnetic torque. The coil thus rotates at constant speed. The work done by the external torque totally transformed to electrical energy. 2014-02-05 19:43:06 補充： (cont'd)... But in reality, the external torque should be a bit larger than the magnetic torque due to presence of friction and air resistance.
dc motor's energy conservation
發問:
一個係magnetic field入面既motor,如果有external force 令到佢個coil rotate,咪會有current產生既By Fleming's left hand rule , 係一個magnetic field中既current 都會產生一個opposite to external force 既magnetic force ,balance 個external force,令到個coil moves at a constant speed我既問題就係,如果個coil moves at a constant speed ,即KE gained=1/2mv^2-1/2m^2=0... 顯示更多 一個係magnetic field入面既motor,如果有external force 令到佢個coil rotate,咪會有current產生既 By Fleming's left hand rule , 係一個magnetic field中既current 都會產生一個opposite to external force 既magnetic force ,balance 個external force,令到個coil moves at a constant speed 我既問題就係,如果個coil moves at a constant speed ,即KE gained=1/2mv^2-1/2m^2=0 咁點解仲會有electric energy produce出黎既? 更新: THX!! but there's still a point that I don't understand. I wanna ask that whether the external force(external torque) is larger than the magnetic force?And if the answer is 'yes',why does the coil still move in a constant speed? 更新 2: I am really sorry that I messed up d.c.motor and d.c. generator, but you still understand my question LOL!Thanks again
最佳解答:
When a coil rotates in a magnetic field, there is electromotive force (emf) induced (Faraday's Law of Electromagnetic Induction). A current flows only when the coil is connected to an external circuit. Assume there is no friction on the rotation coil, no force is needed to keep the coil rotating at constant speed if the coil is NOT connected to an external circuit (i.e. the coil is in open circuit). In the absence of an external circuit, there is no current flowing, and hence no magnetic force is produced on the coil to counter-balance its rotational motion. The coil can rotate by its own inertia. Once the coil is connected to an external circuit, current flows through the coil due to induced emf. The current then gives rise to a magnetic force (more exactly, a magnetic couple) that acts in direction opposite to the motion of the coil. The coil tends to slow down. Therefore, to keep the coil continuously rotating at constant speed, an external force (external torque) must be applied to the coil. It is the mechanical work done by this force (or torque) that transforms into electrical energy in the external circuit. Failing to have an external force applied to the coil, the coil will gradually slow down its rotational speed. The electrical energy so appeared in the external circuit comes from the rotational kinetic energy of the coil. 2014-02-05 19:41:07 補充： Your suppl question: Under an ideal condition (no friction, no air or other resistance), the external torque equals to the magnetic torque. The coil thus rotates at constant speed. The work done by the external torque totally transformed to electrical energy. 2014-02-05 19:43:06 補充： (cont'd)... But in reality, the external torque should be a bit larger than the magnetic torque due to presence of friction and air resistance.
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