Magnetism and electromagnetism pt2

Question 1

How can the potential difference be induced?

Answer 1

• an electric conductor moving relative to a magnetic field

- a change in the magnetic field

Question 2

A potential difference is induced across _____________

Answer 2

the ends of the conductor

Question 3

What is the generator effect?

Answer 3

if the conductor is part of a complete circuit, a current is induced in the conductor

Question 4

What does an induced current generate?

Answer 4

a magnetic field that opposes the original change* which produces it.

• the movement of the conductor, or the change in the magnetic field

Question 5

How do you increase the size of induced potential difference?

Answer 5

• increase the speed of the movement
• size of the magnetic field
• shape wire into a coil: greater number of coils

Question 6

What reverses the direction of any induced current?

Answer 6

reversing the direction of the movement or magnetic field

Question 7

What happens in an alternator?

Answer 7

• a rotating coil is used in a magnetic field
• this allows for the kinetic energy of a rotating coil to be converted to the electrical energy of an induced electric current
• As the magnet rotates, the direction of the field through which the coil passes alternates: it cuts through the field lines
• this induces an alternating potential and an alternating current

Question 8

How can the induced p.d and current in an alternator be increased?

Answer 8

• rotating the coil more quickly
• increasing the magnetic field strength
• increasing the number of turns on the coil
• increasing the area of the coil

Question 9

For alternator, the speed of rotation is _________

Answer 9

the frequency of the alternating current

Question 10

How is current able to pass out of the coil in alternators?

Answer 10

the coil is connected to 2 commutators

Question 11

When is maximum and minimum p.d created in alternators and dynamos?

• why is this

Answer 11

A p.d s induced when the wire passes a magnetic field

Maximum p.d is achieved when the alternator is horizontal because at this point the wire is sweeping directly through the magnetic field lines at the fastest possible rate

Minimum p.d is created when the coil is vertical = 0 p.d = because the coil is moving parallel to the field line, so it’s not cutting through magnetic field lines

Question 12

why are there negative values for p.d in alternators?

Answer 12

As the coil continues moving around we get a maximum p.d again but in the opposite direction = because the 2 sides of the coil are now moving in a different direction than before

Question 13

P.d values in alternators change from + to -.

What does this mean?

Answer 13

that it produces alternating current

Question 14

Explain why alternators produce alternating CURRENT

not p.d!!!

Answer 14

When the given side of the coil is moving upwards than the magnetic field, the current will flow through it in one direction.

when the same side move down, the current will flow through it in the opposite direction

Question 15

How do you plot a magnetic field around a magnet?

Answer 15

1) place the plotting compass near the magnet on a piece of paper
2) mark the direction the compass needle points
3) move the plotting compass to many different positions in the magnetic field, marking the needle direction each time
4) join the points to show the field lines

Question 16

How is a dynamo different from an alternator?

What does it do?

Answer 16

it has a split- ring commutator instead of two separate commutator

it produces direct current

Question 17

What happens in a dynamo?

Answer 17

one side of the coil is connected to one side of the split-ring commutator.

this side moves upwards whiles the opposite side of the coil moves downwards

Question 18

WHy doesnt the p.d and current reverse in a dynamo?

Answer 18

when the coil moves around the side of the coil are connected to the same side of split ring commutator

Question 19

What does a basic transformer consist of?

why is iron used?

Answer 19

a primary and secondary coil wound on an iron core

iron is used because it is easily magnetised

Question 20

Describe step-up transformer?

Answer 20

The secondary coil has more coils that the primary coil

this increases p.d it has more turns on the secondary coil than the primary coil

Question 21

Describe step-down transformer?

Answer 21

The primary coil has more coils that the secondary coil

this reduces the p.d

Question 22

What would happen if transformers were 100% efficient

Answer 22

the electrical power output ould equal the electrical power input

Question 23

How does current flow through a transformer?

Answer 23

an alternating current flows through the primary coil, (which is a solenoid) and an alternating magnetic field is produced around it)

the alternating magnetic field in the iron core induces an alternating potential difference in the secondary coil

In a circuit, the magnetic field is transmitted along with the iron core and passes through the secondary coil and an alternating current flows through

Question 24

Why do transformers only work with the alternating current?

Answer 24

because we need a changing magnetic field to induce a potential difference

Question 25

How do microphones work?

Answer 25

1) Sound waves hit a flexible diaphragm that is attached to a coil of wire, wrapped around a magnet

2) This causes the coil of wire to move in the magnetic field, which generates current
* the diaphragm oscillates due to the changes in air pressure related to the sound wave

3) The microphone diagram and coil vibrate at the same frequency as the incoming sound. The bigger the amplitude of the sound, the bigger the amplitude of the vibration
4) this induces a potential difference and current in the coil with the same frequency of the incoming and wave and an amplitude dependent on the amplitude of the incoming wave

Question 26

What do microphones use the generator effect to do?

Answer 26

to convert the pressure variations in sound waves into variations in current in electrical circuits

Question 27

Draw the construction of a DC motor

Describe how it is operated

Answer 27

urrent in the right hand part of coil causes a downward force and the left hand part of the coil causes an upward force
the coil rotates clockwise

When the coil is vertical, it moves parallel to the magnetic field, producing no force. However, the momentum of the motor carries it over a little + split ring commutator changes the current direction every half turn

Once the conducting brushes reconnect with the commutator after a half turn current flows in opposite direction through wire in the coil.

This means that the motor effect forces continue to cause clockwise rotation of the coil.