Electromagnetic Effects (continued)
Visão Geral
Send a current through a wire that sits in a magnetic field and the wire is pushed sideways. Wind that wire into a coil and the push becomes a spin: this is how every electric motor turns. Pair two coils on an iron core and a changing current in one induces a voltage in the other, which is how transformers raise and lower the voltages that power the world.
In this lesson you will describe the force on a current-carrying conductor and how reversing the current or field reverses it, find force directions and deflect charged particles, explain the turning effect on a coil and the d.c. motor with its commutator, and work through transformers from construction to the turns equation, the efficiency equation, and why high-voltage transmission cuts the power lost as \(I^2R\) heating in cables.
Objetivos
- Describe an experiment to show that a force acts on a current-carrying conductor in a magnetic field, including the effect of reversing: (a) the current (b) the direction of the field
- Know that a current-carrying coil in a magnetic field may experience a turning effect and that the turning effect is increased by increasing: (a) the number of turns on the coil (b) the current (c) the strength of the magnetic field
- Describe the construction of a simple transformer with a soft-iron core, as used for voltage transformations
- Use the terms primary, secondary, step-up and step-down
- Recall and use the equation Vₚ / Vₛ = Nₚ / Nₛ where p and s refer to primary and secondary
- Describe the use of transformers in high- voltage transmission of electricity
- State the advantages of high-voltage transmission
- [Supplement] Recall and use the relative directions of force, magnetic field and current
- [Supplement] Determine the direction of the force on beams of charged particles in a magnetic field
- [Supplement] Describe the operation of an electric motor, including the action of a split-ring commutator and brushes
- [Supplement] Explain the principle of operation of a simple iron-cored transformer
- [Supplement] Recall and use the equation for 100% efficiency in a transformer I V = I V p p s s where p and s refer to primary and secondary
- [Supplement] Recall and use the equation P = I2R to explain why power losses in cables are smaller when the voltage is greater
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Avaliação da Lição
Parabéns por concluir a lição em Electromagnetic Effects (continued). Agora que você explorou o conceitos e ideias-chave, é hora de colocar seu conhecimento à prova. Esta seção oferece uma variedade de práticas perguntas destinadas a reforçar sua compreensão e ajudá-lo a avaliar sua compreensão do material.
Irá encontrar uma mistura de tipos de perguntas, incluindo perguntas de escolha múltipla, perguntas de resposta curta e perguntas de redação. Cada pergunta é cuidadosamente elaborada para avaliar diferentes aspetos do seu conhecimento e competências de pensamento crítico.
Use esta secção de avaliação como uma oportunidade para reforçar a tua compreensão do tema e identificar quaisquer áreas onde possas precisar de estudo adicional. Não te deixes desencorajar pelos desafios que encontrares; em vez disso, vê-os como oportunidades de crescimento e melhoria.
- The force on a current-carrying wire in a magnetic field is reversed by A. increasing the current B. reversing the current direction C. using a longer wire D. switching off the current Answer: B
- Fleming's left-hand rule gives the direction of the A. magnetic field B. induced current C. force on the wire D. e.m.f. Answer: C
- A transformer with 200 primary turns and 50 secondary turns is a A. step-up transformer B. step-down transformer C. d.c. transformer D. rectifier Answer: B
- A transformer has Vp = 240 V, Vs = 12 V and Np = 1000 turns. Ns equals A. 20 B. 50 C. 200 D. 2000 Answer: B
- Electricity is transmitted at high voltage mainly to A. increase the current B. reduce the power lost as heat in the cables C. change a.c. to d.c. D. raise the resistance of the cables Answer: B
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Summarize properties of magnets and describe how some nonmagnetic materials can become magnetized
"Physics: Magnetic Fields, Field Lines, and Force" by OpenStax, OpenStax, Rice University, licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Source: https://openstax.org/books/physics/pages/20-1-magnetic-fields-field-lines-and-force. Access for free at https://openstax.org/books/physics/pages/1-introduction
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