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jack12589 > Magnetic Fields + EM Induction > Flashcards

Magnetic Fields + EM Induction Flashcards

1
Q

What is a magnetic field?

A

it is a force field surrounding a magnet or current-carrying wire

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2
Q

What is the motor effect?

A

a current-carrying wire placed at a non-zero angle to the lines of force of an external magnetic field experiences a force due to the field.

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3
Q

What hand do you use for the motor effect

A

left hand

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4
Q

When is the force on a current-carrying wire greatest?

A

when the wire is at right angles to the magnetic field

it is zero when the wire is parallel to the magnetic field.

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5
Q

What is the force on the wire proportional to?

A

the current, the length of the wire

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6
Q

What is magnetic flux density, B?

A

the force per unit length per unit current on a current-carrying conductor at right angles to the magnetic field lines.

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7
Q

What is magnetic flux density measured in?

A

Tesla (T)

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8
Q

What is the simple electric motor?

A

a coil of insulated wire which spins between poles of a U-shaped magnet

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9
Q

What is the role of a split ring commutator?

A

It allows the coil to continue to rotate in one direction by reversing the direction of current.

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10
Q

Why does a current-carrying wire in a magnetic field experience a force?

A

the electrons moving along the wire are pushed to one side by the force of the field.

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11
Q

What is the hall probe used for?

A

to measure magnetic flux density

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12
Q

Why is no work done by a magnetic field on a particle?

A

because it acts at right angles to the velocity of the particle.

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13
Q

Why can a cyclotron not alter the speed of a particle when it is in the field?

A

Because the force acts perpendicular to the velocity of the particle.

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14
Q

What is electromagnetic induction?

A

the induction of an electromotive force (i.e. voltage) across an electrical conductor in a changing magnetic field.

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15
Q

How can the induced emf be increased?

A

moving the wire faster
using a stronger magnet
making the wire into a coil, and pushing the magnet in or out of the coil

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16
Q

What happens to the induced emf when the relative motion between the magnet and coil?

A

emf is zero

17
Q

Name two methods of generating an emf

A

using an electric motor in reverse (as a generator) (falling weight)
using a cycle dynamo

18
Q

what hand do you use for generator (dynamo) effect?

A

right hand

19
Q

What is Lenz’s law?

A

the direction of the induced emf (and hence current) is always such as to oppose the change that caused it.

20
Q

What is the unit of magnetic flux?

A

Weber (Wb)

21
Q

What is the unit of magnetic flux linkage?

A

Weberturns

22
Q

What is Faraday’s Law?

A

the induced emf in a circuit is equal to the rate of change of flux linkage through the circuit.

23
Q

What is back emf?

A

an emf induced in the spinning coil of an electric motor because the flux linkage through the coil changes.

24
Q

Why is the current high at low motor speed?

A

the induced emf is small

25
Q

What is alternating current?

A

a current that repeatedly reverses its direction.

26
Q

What is the root mean square value of an alternating current?

A

the value of direct current that would give the same heating effect as the alternating current in the same resistor

27
Q

What is a transformer?

A

a device that changes an alternating pd to a different peak value. must be alternating current.

28
Q

How does a transformer work?

A

it consists of a primary coil and a secondary coil wrapped around the same iron core. when the primary coil is connected to a source of alternating pd, an alternating magnetic field is produced in the core. This field passes through the secondary coil which induces an emf.

29
Q

What does a step up transformer have

A

more turns on secondary coil than primary coil

30
Q

What does a step down transformer have?

A

less turns on secondary coil than primary coil

31
Q

What are eddy currents in transformers?

A

they are induced currents in the core of the transformer itself

32
Q

Why is transmission of electrical power over long distances more efficient at higher voltages

A

this is because current needed to deliver a certain amount of power is reduced if the voltage is increased

jack12589 (13 decks)

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