7 - magnetism and electromagnetism

Question 1

which direction do magnetic field lines go

Answer 1

north to south

Question 2

how do compasses show the dircetion of a magentic field

Answer 2

• A compass contains a small bar magnet on a pivot so that it can rotate.
• The compass needle points in the direction of the Earth’s magnetic field, or magentic field of the magnet

Question 3

difference between a permanent or induced magnetic

Answer 3

PERMANENT
- produce their own magnetic field

INDUCED
- magnetic material that turns into a magnet when in a magnetic field.

Question 4

whats the thumb rule for direction of magnetic field

Answer 4

RIGHT HAND thumb points in direction of current and fingers curl round to see direction of magnetic field

Question 5

how to increase the stength of a magentic field

Answer 5

• add more coils of wire
• increase the current
• ad an iron core - because it becomes sn induced magnet whenever current flows through

Question 6

example of an electromagnet

Answer 6

• a solenoid with an iron core
• its magantic field can be turned on an off with an electric current

Question 7

uses of electromagnets

Answer 7

• attract and pick up things from scrap yards and then drop them in a dumb
• in circuits as switches

Question 8

whats happens during the motor effect when the force is parrallel VS perpendicular

Answer 8

it will experience a full force when the wire perpendicular
but wont get any force at all if its parrallel

Question 9

what is the motor effect

Answer 9

• when a wire carrying a current creates a magnetic field
• This interacts with another magnetic field
• causing a FORCE that pushes the wire at right angles

Question 10

flemmings left hand rule - 3 fingers

Answer 10

thumb - FORCE
first finger - MAGNETIC FIELD DIRECTION
second finger - CURRENT DIRCETION

Question 11

whats the role of a split-ring commutator in the motor effect

Answer 11

allows the current to change direction through the coil every half turn so that the coil rotates in the same direction.

Question 12

whats the definition of the generator effect

Answer 12

• when PD is created in a wire due to a change in the magnetic field
• if the wire is a full circuit then a current will flow through the wire

Question 13

alternators (AC)

Answer 13

• generator rotates coil in mag field
• when coil spins current is induced in coil making current change direction every half turn
• have slips and brushes (same as split ring commuter) so they spin 360
• produce AC

Question 14

dynamos (DC)

Answer 14

• same as alternators
• but instead of slips and brushes they use SPIT RING COMMUTATORS
• produces DC

Question 15

whats an oscilloscope and what happens when you increase the frequency of revolutions

Answer 15

• show how PD generated in a coil, changes
• when you increase the frequency of revolutions you increase the overal pd but also creates more peaks

Question 16

what does a oscilloscope graph look like for AC and DC

Answer 16

AC
- up and down with large curves over the x - axis

DC
- smaller curves up and down ABOVE the x - axis

Question 17

step up transformers VS step down tranformers

Answer 17

STEP UP
- increase the PD
- have more turns in secondary coil

STEP DOWN
- decrease the pd
- more turns in primary coil

Question 18

link loud speaks and microphones

Answer 18

• loudspeakers - take electrical signals and use MOTOR effect to convert them to vibrations generating sound waves
• microphones - detect vibrations from sound waves and use GENERATOR effect to convert them to electrical signals

Question 19

how do loud speakers work

Answer 19

1. ac flows through wire creates mag field
2. mag field of coil interacts w mag field of permanent mag
3. interacting meg fields exert a force (motor effect) - move back and forth
4. causes cone to change shape - vibrations
5. vibrations in air - sound waves

Question 20

how do microphones work

Answer 20

1. sound waves hit diaphram
2. diaphram and coil of wire move bc of this
3. wire moves in mag field of permanent mag - current produced (generator effect)
4. freq and amplitude of sound waves determines the fre and amp of current

Question 21

how to increase the size of the induced pd in the GENERATOR EFFECT

Answer 21

• change strength of mag field (stronger field - larger pd)
• more wire faster (larger pd)
• shape wire into coil (more turns larger pd)