P15: Electromagnetism (Y11 - Spring 2)

Question 1

🟠 How do you find the direction of the field around a straight wire?

Answer 1

To find the direction of the field around a straight wire, use the RIGHT HAND GRIP rule

Question 2

🟠 What is a Solenoid

Answer 2

A wire coiled into a spiral is called a solenoid.

Question 3

🟠 Describe the magnetic field inside the solenoid

Answer 3

The magnetic field is much stronger than if the wire was straight. The field lines are parallel to the axis of the solenoid, and thet are all in the same direction (i.e, unform. The magnetic field inside a solenoid is stong and uniform.

Question 4

🟠 Describe the magnetic field outside the solenoid

Answer 4

The magnetic field lines bend around from one end of the solenoid to the other end of the solenod. The magnetic field outside is like the field of a bar magnet, except that each field line is a complete loop because it passes through the inside of the solenoid.

Question 5

🟠 What 3 things can you do to make the field around a solenoid stronger

Answer 5

• Increase the current
• Add more turns to the coil
• Add an iron core (making an electromagnet)

Question 6

🟠 What is an Electromagnet

Answer 6

An alectromagmet is a solenoid in which the insulated wire is wrapped around an iron bar (the core). When a current is passed along a wire, magnetic field is created around the wire. Because of this, the magnetic field of the wire magnetises the iron bar. When the current is switched off, the iron bar loses most of its magnetism.

Question 7

🟠 Fill in the gaps:

Iron, temporary, current, magnetic, switch, induced, paper clips, attract

When the __________ is pressed, an electric __________ will flow in the circuit. When this happens, the wire has a __________ field around it, which means that it can pick up, or __________ magnetic materials, such as __________, which are made from __________. An electromagnet is an example of __________ magnetism because the magnetic field is no longer present when the current is turned off. The iron material is picked up because the opposite pole is __________ in the end nearest the electromagnet’s poles.

Answer 7

When the switch is pressed, an electric current will flow in the circuit. When this happens, the wire has a magnetic field around it, which means that it can pick up, or attract magnetic materials, such as paper clips, which are made from iron. An electromagnet is an example of temporary magnetism because the magnetic field is no longer present when the current is turned off. The iron material is picked up because the opposite pole is induced in the end nearest the electromagnet’s poles.

Question 8

🟠 How would you record your results table of the investigation:

Investigate how the number of coils affects the number of paper clips attracted to an electromagnet – keep the current the same in this experiment.

Answer 8

• The number of cells would be on the left of the table, as it’s the independant variable (x-axis)
• The number of apper clips attracted would be on the right, as its the dependant variable (y-axis)

Question 9

🟠 How would you record your results table of the investigation:

Investigate how the size of the current affects the number of paper clips attracted to an electromagnet – keep the number of coils the same in this experiment

Answer 9

• The current would be on the left of the table, as it’s the independant variable (x-axis)
• The number of apper clips attracted would be on the right, as its the dependant variable (y-axis)

Question 10

🟠 Fill in the conclusion:

If the current is increased, then there will be an __________ in the number of paper clips picked up because the __________ field around the electromagnet will become __________.

Answer 10

If the current is increased, then there will be an increase in the number of paper clips picked up because the magnetic field around the electromagnet will become stronger.

Question 11

🟠 Give one way on how the experiment of investigating how the ‘size of the current affects the number of paper clips attracted to an electromagnet’ can be improved

Answer 11

Could be improved by repeating the experiment to increase reliablility and create a mean

Question 12

🟠 Why are electromagnets useful?

Answer 12

Electromagnets are useful as they can be turned on and off when required as they are temporary magnets (meaning they are not permament magnets)

Question 13

🟠 How does the scrap yard crane work?

Answer 13

Scrap vehicles are lifted in a scrap yard using powerful electromagnets attached to cranes. The steel frame of a vehicle sticks to the electromagnet which current passes through the coil of the electromagnet. When the current is switched off, the vehicle frame falls off the electromagnet.

Question 14

🟠 How does an electric bell works? and why doesn’t a bell keep ringing?

Answer 14

When an electric bell is connected to a battery, the iron armature (arm) is pulled onto the electromagnet. This opens the make-and-break switch, and the electromagnet is switched off. Because of this, the armature (arm) springs back and the make-and-break switch closes again, so the whole cycle repeats itself.

Question 15

🟠 What is a circuit breaker and how does it work

Answer 15

A circuit breaker is a switch in series with an electromagnet. The switch is held closed by a spring. When the current is too large, the switch is pulled open by the electromagnet and it stays open until it is reset manually.

Question 16

🟠 What are Relay Switches used for?

Answer 16

The relay is used to switch an electrical machine on ir off. A small current through the coil of the electromagnet magnetises the iron core, which then pulls the armature onto the electromagnet. This closes the switch gap and switches the machine on. In this way, a small current (in the coil) is used to switch on a machine with a much bigger current.

Question 17

🟠 Explain how turning the ignition makes a current flow in the starter motor

Answer 17

When the ignition key is turned, a small current flows through the coil of the electromagnet magnetises the iron core, which then pulls the iron arm/armature onto the electromagenet. This closes the switch gap and switches the machine on ( the contacts are pushed together). In this way, a small current (in the coil) is used to switch on a machine with a much bigger current (the motor).

Question 18

🟠 Why is the spring needed in a lock

Answer 18

To re-lock it

Question 19

🟠 The connections to the coil in a door lock were accidentaly reversed. Would the lock still work and why

Answer 19

The iron bar would still be attracted / the coil would still be magnetised, so it would still work due to the fact the iron bar is still attracted and the coil is till magnetised.

Question 20

🟠 Explain what is meant by induced magnetism

Answer 20

When a piece of unmagnetised magnetic material becomes magnetic when in contact with a magnetic field.

Question 21

🟠 What is the Motor Effect (+How does it happen)

Answer 21

When a conductor carrying a current is placed in a magnetic field the magnet producing the field and the conductor exert a force on each other.

This is called the motor effect.

Question 22

🟠 What factors impact the size of the force produced on the current carrying wire?

Answer 22

The size of the force can be increased by:

• Increasing the current
• Using a stronger magnet
• (+ The Length of the wire in the field)

Question 23

🟠 What position is for a wire between a North pole and a south pole is the strongest in the Motor Effect

Answer 23

The position is strongest when when the wire is perpendictular to the magnetic field.

(Zero is when the wire is parallel to the magnetic field lines)

Question 24

🟠 What is Flemming’s Left-Hand Rule

Answer 24

Fleming’s left-hand rule represents the relative orientation of the force, the current in the conductor and the magnetic field.

Question 25

🟠 What does the First Finger, Second Finger, and Thumb represent in Flemming’s Left-Hand Rule

Answer 25

First finger = Field

Second finger = Current

Thumb= Movement

Question 26

🟠 Identify what direction the conductor will move when a current flows through it when:

• The current is flowing to the right
• The North pole is above the wire and the Siuth pole is below the wire

Answer 26

The conductor will move In (directly fowards) from a 2d burd eye’s view

(See P15.4 for more details)

Question 27

🟠 What key variables need to be considered to calculate the size of the force produced on a current carrying wire in a magnetic?

Answer 27

• Magnetic field strength
• Size of current
• Length of wire in the field

Question 28

🟠 Equation for Force involving Magnetic Flux Density, Current, Length

Answer 28

F = B x I x L

Force (N) = Magnetic Flux Density (T) × Current (A) × Length (M)

(T = Tesla - Strength of the magnetic field)

Question 29

🟠 Worked Example:

If the magnetic flux density is 0.05 T, the length of the wire in the magnetic field is 75 mm and the size of the current flowing is 2 A.
Calculate the force on the wire

Answer 29

F = 0.05 T x 2 A x 0.075 m F = 0.0075 N

Question 30

🟠 Example Question:

A horizontal wire 5 cm long and mass 4 g is placed at right angles to a magnetic file of flux density 0.5 T. Calculate the current that must be passed through the wire so that it is self-supporting

Answer 30

0.004 x 9.81 = 0.3924N

(F = BIL, I = F/BL)

0.3924N / 0.5 x 0.5 = 1.57A

Question 31

🟠 How does an Electric Motir Woek

Answer 31

When an electric current passes through a coil in a magnetic field, the magnetic force produces a turning effect which turns a DC motor

Rectangular coil of insulated wire (armature coil) that is forced to rotate . The coil is connected to the power supply by two metal or graphite brushes. The brushes press onto a metal commutator fixed to the coil.

Question 32

🟠 What is the Problem with the Motor Effect, and how is Solved

Answer 32

The problem with the motor effect is that when the wire loop flips over, the forces are reversed. This would not create continuous motion.

This problem is solved by a split ring communicator. The brushes (made of graphite) from the power supply contact the commutator, meaning that the direction of the current can be reversed whenever the rings swap sides. This keeps the motor always spinning in the same direction.

Question 33

🟠 Why does the coil spin when a current is passed through it in an Electric Motor

Answer 33

• A force acts on each side of the coil due to the motor effect
• The force on one side is in the opposite direction to the force on the other side.

Question 34

🟠 Why are Graphite brushed used to press onto the split-ring commutator in an Electric Motor

Answer 34

Graohite is a dorm of carbon that conducts electricity and is very slippery. Graphite causes very little friction when ut is in cintadt with the rotating commutator.

Question 35

🟠 What makes Electric Motors work

Answer 35

A coil of wire carrying a current in a magnetic field tends to rotate. This is the basis of an electric motor.

Question 36

🟠 What is the Generator Effect

Answer 36

The induction of a potential difference (and a current if there’s a complete circuit) in a wire which is moving relative to a magnetic field, or experiencing a change in magnetic field.

Question 37

🟠 When does the Generator Effect create a Potential Difference in a Conductor?

Answer 37

The generator effect creates a potential difference in a conductor, and a current if the conductor is part of a complete circuit.

Question 38

🟠 How can you create a Potential Difference in a Conductor

Answer 38

You can do this by moving a magnet in a coil of wire OR moving a conductor (wire) in a magnetic field (“cutting” magnetic field lines)

Shifting the magnet from side to side creates a little “blip” of current if the conductor is part of a complete circuit.

Question 39

🟠 What happens if you move the magnet (or conductor in the opposite direction, and backwards and forwards when your creating potential difference with the Generator effect?

Answer 39

If you move the magnet (or conductor) in the opposite direction, then the potential difference/current will be reversed. Likewise, if the polarity of the magnet is reversed, then the potential difference/current will be reversed too.

If you keep the magnet (or the coil) moving backwards and forwards, you produce a potential difference that keeps swapping direction - an laternating current.

Question 40

🟠 What does the Induced current generate around the coil when the magnet is moving, and how does this make the generator work

Answer 40

The induced current generates a magentic field in and around the coil, but only when the magnet is moving - a magnetic field is created around the wire (this magnetic field is different to the one whose field lines were being cut in the first place).

The magnetic field created by an induced current always acts against the change that made it. This means the induced current always opposes the change that made it.

As a result, work has to be done by the person moving the magnet. The electricity generated is the result of the work done by the person moving the magnet.

Question 41

🟠 What is the Solenoid rule to work out the direction of the induced current whether end A of the coil is like the north pole or the south pole of a bar magnet

Answer 41

The north-pole is the end where current is anti-clockwise

The south-pole is the end where current is clockwise

Question 42

🟠 How can you Increase the strength of the induced current

Answer 42

• Increasing the strength of the magnetic field
• Increasing the speed that the waves or magnets move at
• Adding more turns to the coil

Question 43

🟠 What is an Alternator and What does it do?

Answer 43

A simple alternator is an alternating-current generator. It is made up of a rectangular coil that is forced to spin in a unifrom magnetic field. The coil is connected to a centre-reading meter by metal brushes that press on two slip rings. The slip rings and brushes provide a cintinuous connection between the coil and meter.

When the coil turns steadily in one direction, the meter pointer deflects first one way, then the opposite, and then back again. This carries on as long as the coil keeps turning in the same direction. The current in the circuit repeatedly changes its direction through the meter because the induced potential difference in the coil repeatedly changes its direction. So the induced potential difference and the current alternate because they repeatedly change direction.

Question 44

🟠 When is the size of the induced potential difference greatest in an Alternator

Answer 44

The size of the induced potential differende is greatest when the plane of the coil is parallel to the direction if the magnetic field. At this position, the sides of the coil cross directly through the magnetic field lones. So the induced potential difference is at its peak value.

Question 45

🟠 When is the size of the induced potential difference lowest (at 0) in an Alternator

Answer 45

The size of the induced potential difference is zero when the plane of the coil is perpendicular to the magnetic field lines. At this position, the sides of the coil move parallel to the field lines and do not cross through them. So, the induced potential difference is zero.

Question 46

🟠 What happens when ‘The faster the coil rotates’ on an Alternator

Answer 46

The faster the coil rotates:

• The bigger the frequency (i.e, the number of cycles per second) of the alternating current. This is because each full cycle of the alternating potential difference takes the same time as one full rotation of the cell
• The bigger the peak value of the alternating current. This is because the sides of the coil move faster and so they cross through the magnetic field lines at a faster rate.

Question 47

🟠 How can you increase the peak value of an Alternator

Answer 47

The peak value can also be incteased by using a magnet with a stronger magnetic field, and by using a coil with a bigger area with more turns of wire on it.

Question 48

🟠 How can the Alternating Potential Difference (from an Alternator) be displayed

Answer 48

The alternating potential difference can be displayed on an oscilloscope screen. If the generator is rotated faster, the screen display will show more waves on the screen and the waves will be taller.

Question 49

🟠 What is a Direct-Current Dynamo and how does it work?

Answer 49

A dynamo is a direct-current generator. A simple dynamo is the same an alternator, except that the dynamo has a split-ring communicator instead of two separate slip rings. As the coil spins, the split-ring communicator reconnects the coil the ipposite way around the circuit every half-turn. Thsi happens each time the coil is perpendicular to the magnetic field lines. Because of this, the induced potential difference does not reverse its direction as it does in the alternator. The induced potential differnce varies from zero to a maximum value twice each cycle, and never changes polarity.

Question 50

🟠 What do Loudspeakers and headphones use the motor effect for

Answer 50

Loudspeakers and headphones use the motor effect to convert variations in current in electrical circuits to the pressure variations in sound waves.

Question 51

🟠 How does the moving coil microphone work

Answer 51

The moving coil microphone generates an alternating potential difference as sound waves make the coil vibrate. The coil is attached to a small diaphragm and is between the poles of a cylindrical magnet.

The pressure fariations or the sound wave on the diaphragm, make it vibrate so the coil vibrates in the magnetic field. The alternating potential differende induced on the coil has the same frequency as the sound waves.

Question 52

🟠 How does the moving coil microphone work

Answer 52

The moving coil loudspeaker creates sound waves when an alternating potential difference is applied to its coil. The coil is in a magnetic field. The current in the coil causes a force on the coil due to the motor effect. Because the currnet alternates, this force repeatedly reverses direction and makes the coil and the diaphragm vibrate. This creates sound waves of the same frequency as the alternating potential difference.

Question 53

🟠 What is Electromagnetism

Answer 53

When a current flowing in a conductor creates a magnetic field

Question 54

🟠 What is Electromagnetic Induction

Answer 54

When a changing magnetic field induces a potential difference in a conductor.

Question 55

🟠 What is an Alternating Current

Answer 55

A current that routinely changes direction and size

Question 56

🟠 Parts of a Transformer

Answer 56

• Primary Coil
• a.c. input
• Secondary Coil
• a.c. output
• Magnetic field in an Iron Core

Question 57

🟠 What do Step-up Transformers do

Answer 57

• Increases potential difference
• More turns on the secondary coil
• Higher potential difference increases efficiency of electricity transmission

Question 58

🟠 What do Step-down Transformers do

Answer 58

• Decreases potential difference
• Fewer turns on the secondary coil
• Lower potential difference is safer for use in the home

Question 59

🟠 Why does the National Grid use Step-up transformers

Answer 59

Ncrease the efficiency of electricity transfer

Question 60

🟠 Why does the National Grid use step-down transformers?

Answer 60

To make electricity safe for use in homes

Question 61

🟠 Why are high and low potential differences used in the National Grid

Answer 61

High potential differences are used to increase the efficiency of electricoty transfer, a low potential differences are used to make electricity safe to use.

Question 62

🟠 What are the 4 steps for the operation of the transformer

Answer 62

1. An a.c. input (alternating current) creates a changing (regularly reversing mgnetic field in the primary coil.
2. The magnetic field is transferred into the iron core to make it an induced electromagnet
3. The changing magnetic field flows through the whole core into the secondary coil
4. This changing magnetic field induces a potential difference in the secondary coil.

Question 63

🟠 Write a paragraph to explain how an alternating current in one coil is used to induced a current in a second coil in transformers

Answer 63

• An a.c. input (alternating current) creates a changing (regularly reversing) magnetic field in the primary coil.
• The iron core transfers the magnetic field to make it an electromagnet with a changing magnetic field.
• The changing magnetic field is now present (flows through) in the secondary coil
• This chaning magnetic field induces a potential difference in the secondary coil. If this is connected to an external circuit, a current will flow.

Question 64

🟠 What do Isolating Transformers do?

Answer 64

• No change in potential difference

- Used for safety

Question 65

🟠 What are Step-Up Transformers and what are they used for

Answer 65

Step-up transformers are used to increase the size of an alternating potential difference. Step-up transformers are used to step uo p.d from about 25kW at power stations to a much higher p.d (typically 132,000V) on the National Grid

Question 66

🟠 What are Step-Up Transformers and what are they used for

Answer 66

Step-down transformers are used to decrease the size of an alternating potential difference. Step-down transformers are used to supply electricity from the National Grid to consumers.

Question 67

🟠 How Does a Transformer Work?

Answer 67

A transformer has two coils of insulated wire, both wound around the same iron core. Iron is used for the core because the iron id easily magnetised and demagnetised. The coils are called the primary coil and secondary coil. The primary coil is connected to an a.c. supply. When the alternating current passes through the primary coil, and alternating potential difference in the secondary coil.

Question 68

🟠 Why is an Alternating Potential Difference Induced in the Secondary Coil?

Answer 68

This happens because:

• Alternating current pasing through the primary coil generates an alternating magnetic field in the iron core
• The lines of the alternating magnetic field in the iron core pass through the secondary coil
• The magnetic field in the secondary coil induces an alternating potential difference between the terminals of the secondary coil

Question 69

🟠 What is The Transformer Equation (The Equation can be used to calculate any one of these factors if you know the other factors)

Answer 69

Potential Difference Across Primary Coil, Vp / Potential Difference Across Secondary Coil, Vs = Number of Turns on Primary Coil, np / Number of Turns on Secondary Coil, ns

Question 70

🟠 For A Step-Up Transformer, is the Number of Secondary turns, ns greater or smaller than the number of Primary Coils

Answer 70

For a step-up transformer, the number of secondary turns,ns is greater than the number of primary turns, np. So Vs is greater than Vp.

Question 71

🟠 For A Step-Down Transformer, is the Number of Secondary turns, ns greater or smaller than the number of Primary Coils

Answer 71

For a step-down transformer, the number of secondary turns, ns is less than the number of primary turns, hp. So Vs is less than Vp.

Question 72

🟠 How Efficient are Transformers?

Answer 72

Transformers are almost 100% efficient. When a device is connected to the secondary coil, almost all the electrical power supplied to the transformer is delivered to the device.

Question 73

🟠 How to work out the Power supplied to the Transformer, and the Power delivered by the Transformer?

Answer 73

• Power supplied to the transformer (i.e, input power) = Primary Current, Ip x Primary Potential Difference, Vp
• Power delivered by the transformer (i.e, output power) = Secondary Current, Is x Secondary Potential Difference, Vs

Question 74

🟠 What is the Equation between Primary Potential Difference and Primary Current against Secondary Potential Difference and Secondary Current if 100% efficiency is assumed?

Answer 74

If 100% efficiency is assumed:

Primary Potential Difference, Vp x Primary Current, Ip = Secondary Potential Difference, Vs x Secondary Current, Is

Question 75

🟠 What does a Step-Up Transformer do at a Power Station in the Grid System

Answer 75

The electrical power supplied to any appliance depends on the appliances current and its political difference. To supply a specific amount of power, the current can be lowered can be lowered if the potential difference is raised. This is what a step-up transformer does at a power station in the grid system.