P15 - Magnetism And Electromagnetism✔️

Question 1

What are happens when magnets come into contact with each other?

Answer 1

They exert a force on each other - two like poles repel each other - two unlike poles attract each other the attraction and repulsion are examples of non-contact forces

Question 2

What is a permanent magnet?

Answer 2

Produces its own magnetic field

Question 3

What is an induced magent?

Answer 3

A material that becomes a magnet when it is placed in a magnetic field - when removed from the magnetic field an induced magnet loses most/all of its magnetism quickly

Question 4

What does the strength of a magnetic field depend on?

Answer 4

The distance from the magnet - the field is strongest at the poles of the magnet

Question 5

How does a compass work?explain with refrence to magnets?

Answer 5

Compass contains a tiny bar magnet which uses the earths magnetic field and the compass will point in the direction of this field

Question 6

What happens when a current flows through a conducting wire?what does the strength of the field depend on?

Answer 6

A magnetic field is produced around the wire - the strength of the magnetic field depends on the current through the wire and the distance from the wire

Question 7

How can you increase the strength of a magnetic field?explain how this works?

Answer 7

By wrapping the wire into a coil called a solenoid - this happens because the field lines around each loop of wire line up with each other which results in lots of field lines pointing in the same direction and very close to each other

Question 8

Effect of the field lines being closer together?example of this being used?

Answer 8

The stronger the field is - seen when wrapping a wire into a coil (solenoid) which increases the strength of the magnetic field

Question 9

Features of a coiled wire (solenoid)?

Answer 9

Increased magnetic field compared to a straight wire - with the magnetic field inside the solenoid being strong and uniform (same strength and direction at every point) - outside the coil magnetic field is like a bar magnet

Question 10

How can you increase the strength of a solenoid?

Answer 10

Putting a block of iron in the centre of the coil - which becomes an induced magnet whenever current is flowing

Question 11

What is an electromaganet?

Answer 11

A solenoid with an iron core (a magnet whose magnetic field can be turned on and off with an electric current)

Question 12

Define the motor effect?

Answer 12

When a wirer carrying a current (or any conductor carrying a current) is placed in a magnetic field the magnet producing the field, and the conductor, exert a force on each other and can cause the wire to move

Question 13

Example of two uses of electromagnets?

Answer 13

• within circuits as switches
• in cranes e.g. in scrap yards to pick up things made from magnetic materials like iron and steel

Question 14

What are the three things a force acting on a conductor in a magnetic field depend on?

Answer 14

• the magentic flux density - how many field (flux) lines there are in a region
• size of the current through the conductor
• length of the conductor in the magnetic fied

Question 15

What is magnetic flux density?

Answer 15

How many field (flux) lines there are in a region which shows the strength of the magnetic field

Question 16

What does each finger in flemings left-hand rule symbolise?

Answer 16

• Thumb - motion
• first finger - field
• second finger - current

Question 17

Why is a split-ring commutator used in a dynamo motors and basic dc motors?

Answer 17

Swaps the contacts (positive and negative connections) every half turn to keep the motor rotating in the same direction

Question 18

Explain how speakers and headphones use the motor effect - Step 1 to 3?

Answer 18

1) ac current is sent through a coil of wire attached to the base of a paper cone

2) coil surrounds a permanent magnet and is surrounded by the other pole so the current causes a force on the coil - causes the cone to move

3) when current reverses force acts in the opposite direction which causes the cone to move in opposite direction aswell

Question 19

Explain how speakers and headphones use the motor effect - Step 3 to 5?

Answer 19

4)varitaions in the current makes the cone vibrate - which makes the air around the cone vibrate and creates variations in pressure, causing a sound wave

5)frequency of sound wave is the same as the frequency of the ac - so changing the ac you can alter the sound wave produced

Question 20

How does the cone moving in speakers cause sound waves?

Answer 20

The variations in the current makes the cone vibrate which makes the air around it also vibrate and creates variations in pressure which cause sound waves

Question 21

Define the generator effect?

Answer 21

The induction of a potential difference (and current if its a complete circuit) in a wire which is moving relative to a magnetic field or expirencing a change in magnetic field

Question 22

What are three factors which increase the induced potnetial difference and current?

Answer 22

• the strength of the magnetic field
• faster we move the wire
• the greater number of turns on the coil

Question 23

How can we create the generator effect? What will not work?

Answer 23

• inducing a potential difference by moving a wire up through a magnetic field or keep the wire still but move the magnet field (wire has to pass through the magnetic field)
• moving the wire along the magnetic field will not induce a potential difference or current

Question 24

How can you reverse the direction of the potential difference and current with the generator effect?give two ways?

Answer 24

By moving the magnet in the opposite direction or if the polarity of the magnet is reversed the the potential difference /current will be reversed too

Question 25

How can you create an alternating current by turning magnets or turning a coil inside a magnet- step 1? explain in depth?

Answer 25

as you turn a magnet, the magnetic field through the coil changes which induces a potential difference which can make a current flow in the wire

Question 26

How can you create an alternating current by turning magnets or turning a coil inside a magnet- step 2? Explain in depth?

Answer 26

when you’ve turned the magnet through half a turn, the direction of the magnetic field through the coil reverses which reverses the potential difference - so current flows in the opposite direction arround the coil of wire

Question 27

How can you create an alternating current by turning magnets or turning a coil inside a magnet- step 3? explain in depth?

Answer 27

If you keep turning the magnet in the same direction - the potential difference will keep reversing every half turn and you will get an alternating current

Question 28

Why does an induced current oppose the change that made it?

Answer 28

A second magnetic field is creates around the wire because when current flows through a wire, a magnetic field is created around the wire - this magnetic field (created by an induced current) acts against any changes that made it

Question 29

Why do alternators have slip rings?

Answer 29

So the contacts don’t swap every half turn

Question 30

Explain simply how alternators generate ac?

Answer 30

1)generators rotate a coil in a magnetic field

2)as the coil (or magnet) spins, a current is induced in the coil - the current chages direction every half turn

3)they have slip rings and brushes instead of a split ring commutator so they produce alternating potential difference

Question 31

Explain how a motor works?

Answer 31

Coil is in a magnetic field on a spindle and the current in the left-hand side of the coil creates a downward force and the current in the right creates an upward force - it uses a split ring commutator to swap the contacts every half turn to keep the motor rotating in the same direction

Question 32

Explain how Dynamos generate d.c?

Answer 32

1)generators rotate a coil in a magnetic field

2)as the coil (or magnet) spins, a current is induced in the coil - the current chages direction every half turn

3)they have split-ring commutators to swap the connection every half turn to keep the current flowing in the same direction

Question 33

What does ac look like on an oscilloscope?

Answer 33

A line that goes up and down crossing the horiontal axis

Question 34

What does dc look like on an oscilloscope?

Answer 34

Isn’t straight but stays above the axis (pd is always positive)

Question 35

How do microphones work - Step 1?

Answer 35

1) soundwaves hit a flexible diaphragm that is attached to a coil of wire (wrapped around a magnet)

2) causes coil to move in the magnetic field and generate a current

Question 36

How do microphones work - Step 2?

Answer 36

3) movment of coil (and generated current) depend on the properties of the sound wave

4)pressure variations of sound waves are converted into variations in current in an electric cicruit

Question 37

What do transfomers do?

Answer 37

Change the size of the potential difference of an alternating current

Question 38

Structure of transformers?

Answer 38

They have two coils of wire, the primary and the secondary joined with an iron core (iron is easily magnetised)

Question 39

what happens when an alternating potential difference is applied across the primary coil?

Answer 39

The iron core magnatises and demagnetises quickly - this changing magnetic field induces an alternating pd in the secondary coil (if second coil is part of a complete circuit it causes current to be induced)

Question 40

What do step-up transformers do?which coil has more turns?

Answer 40

Step the potential difference up (increase it) - have more turns on the secondary coil than the primary coil

Question 41

What do step-down transformers do?what coil has more turns?

Answer 41

Steps the potential difference down (decreases it) - they have more turns on the primary coil than the secondary

Question 42

What materials are used to make magnets?

Answer 42

Any iron or steel objects can be magnetised (or demagnetised if its already magnetised) - only a few other materials like cobalt and nickel can be magnetised and demagnetised