P15 - Magnetic Fields

Question 1

What are all of the magnetic metals?

Answer 1

• Iron
• Cobalt
• Nickel

Question 2

What is a magnetic field.

Answer 2

The region around the magnet is called magnetic field. Any other magnetic material placed in this space experiences a force caused by the first magnet.

Question 3

What is the shape and direction of the lines of force in the magnetic field around a bar magnet.

Answer 3

• Field Lines point from North to South
• Strength decreases with distance from the magnet
• Direction always points to south pole and away from north pole, at any point

Question 4

What are the rules to drawing a magnetic field diagram?

Answer 4

• The lines should never cross in the diagram.
• The lines should be going North to South ( represented by arrows on the lines).
• Closer lines means a stronger force.
• At least 3 lines need to be drawn on each side with at least one going from both ends of the magnet.

Question 5

Describe how to plot a magnetic field.

Answer 5

Using a pencil, put a dot on the paper where the needle is pointing. Place the plotting compass so that the needle points to the dot you have made. Draw a new dot at the other end of the needle. Keep going until you either go off the page or you loop back around to the other side of the magnet. Repeat this so that you have at least 3 lines need to be drawn on each side with at least one going from both ends of the magnet.

Question 6

What is a compass and how does it work?

Answer 6

A magnetic compass contains a small bar magnet. The Earth has a magnetic field. The compass needle points in the direction of the Earth’s magnetic field.

Question 7

What is induced magnetism?

Answer 7

Induced magnetism is magnetism created in an
unmagnetised magnetic material when the material is
placed in magnetic field.

Question 8

What is the right hand grip rule?

Answer 8

The right hand grip rule is where the direction of you thumb points to the direction of the current of a straight wire and your fingers point to the direction of the magnetic field.

Question 9

For questions using the right hand grip rule, how do you know when the wire is coming out the page or going in the page?

Answer 9

• coming out of the page is represented by a circle with a dot at the centre.
• Going in the page is represented by a circle with an x or cross going through it.

Question 10

What is a solenoid?

Answer 10

A wire coiled into a spiral is called a SOLENOID.

Question 11

Describe the shape of the field around a solenoid.

Answer 11

Inside the solenoid:
The magnetic held is much stronger than if the wire was straight. The field lines are parallel to the axis of the solenold, and they are all in the same direction (i.e. uniform) The magnetic field inside a solenoid is strong and uniform.

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

Question 12

How can you make the field around a solenoid stronger?

Answer 12

• Increase the current of the solenoid.
• Add more turns to the coil.
• Add an iron core (which makes it an electromagnet)

Question 13

What is an electromagnet?

Answer 13

An electromagnet is a solenoid in which the isolated wire is wrapped around an iron bar.When a current is passed along a wire, a magnetic field is created around the wire.Because of this, the magnetic fields of wire magnetised the core (iron)

Question 14

How do we make an electromagnet?

Answer 14

An electromagnet is a solenoid in which the insulated wire is wrapped around an iron bar(the core). When a current is passed along a wire, a 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 15

Why are electromagnets useful?

Answer 15

When the current is switched off, the iron bar loses most of its magnetism.

Question 16

How does a scrap yard crane work?

Answer 16

The steel frame of a vehicle sticks to the electromagnet when current passes through the coil of the electromagnet. When the current is switched off, the vehicle frame falls off the electromagnet.

Question 17

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

Answer 17

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

Question 18

What is the difference between a circuit breaker and a relay?

Answer 18

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.
The relay is used to switch an electrical machine on or 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 19

What is Fleming’s left-hand rule? And describe it.

Answer 19

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

First finger = Field
SeCond finger = Current
ThuMb= Movement

Question 20

What is the motor effect?

Answer 20

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 21

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

Answer 21

The size of the force can be increased by :
Increasing the current .
Using a stronger magnet.

Question 22

In which position would the force be the strongest on a wire in the motor effect?

Answer 22

Greatest when wire is perpendicular to the magnetic field.

Question 23

In which position would the force be the weakest on a wire in the motor effect?

Answer 23

Zero when the wire is parallel to the magnetic field lines.

Question 24

When using Fleming’s left hand rule, what needs to be remembered about the current?

Answer 24

That conventional current is used, going from positive to negative.

Question 25

How can the force on a conductor ( motor effect) be changed?

Answer 25

- Using a stronger magnet, so a stronger magnetic field increases the size of the force. - Increasing the current increases the size of the force. - Length of wire in the field

Question 26

How do you calculate the force exerted on the conductor in the motor effect?

Answer 26

By using the equation: Force(N) = magnetic flux density(T) × current(A) × length(m)

Question 27

Describe the electric motor.

Answer 27

When an electric current passes through a coil in a magnetic field, the magnetic force produces a turning effect which turns a DC motor. A rectangular coil of insulated wire (armature coil) is forced to rotate . The coil is connected to the power supply by two metal or graphite brushes. The brushes press onto a split ring commutator fixed to the coil, which reverses the direction of the current whenever the rings swap sides, allowing for the motor to always keep spinning in the same direction.

Question 28

How do you change the speed of the electric motor?

Answer 28

Increase the current.

Question 29

How do you change the direction of the motor?

Answer 29

Reverse the direction of the current.

Question 30

How can a potential difference be induced in a wire in a magnetic field?

Answer 30

By changing the magnetic field - either by varying the strength of the magnets or by moving the wire relative to the magnets.

Question 31

If a wire was parallel to a magnetic field, what would the induced potential difference be?

Answer 31

0V , since the magnetic field doesn't change.

Question 32

How could the size of the induced potential difference be increased of a wire between a 2 magnets ( generator effect)?

Answer 32

- Moving the wire/magnets faster. - Using a stronger magnet. - Replacing the wire with a coil of wire.

Question 33

When a permanent magnet is moved through a coil, what happens?

Answer 33

A potential difference and current is induced.

Question 34

Where is the north and south pole of a solenoid?

Answer 34

Looking at each end, the south pole is the end where the current is clockwise, and the north pole is the end at which the current is anticlockwise.

Question 35

How does a dynamo generator work?

Answer 35

A dynamo is a direct current generator. - As the coil spins (conductor) it sees a changing magnetic field inducing a potential difference across it. - As the coil spins it reconnects the coil the opposite way around in the circuit every half turn, due to the split-ring commutator. - The change in direction occurs every time the coil is perpendicular with the magnetic field. - The direction of induced potential does not change.

Question 36

How does an AC generator work?

Answer 36

- A simple a.c. generator consists of a coil of wire rotating in a magnetic field. - As one side of the coil moves up through the magnetic field, a potential difference is induced in one direction. - As the rotation continues and that side of the coil moves down, the induced potential difference reverses direction. - This means that the alternator produces a current that is constantly changing. This is alternating current or a.c.

Question 37

Describe the properties of an alternator.

Answer 37

- Uses a two ring commutator to allow current to change direction. - Produces alternating current (AC) that reverses direction periodically. -Used in power stations, household electricity, and large-scale applications. - The output voltage is sinusoidal (wave -like pattern)

Question 38

Describe the properties of a dynamo.

Answer 38

- Uses a split-ring commutator to maintain the direction of current. - Produces direct current (DC) that flows in one direction only. - Commonly used in battery charging, small motors, and electronic devices.

Question 39

What are the similarities between dynamos and alternators?

Answer 39

- Both work on the principke of electromagnetic induction. - Both use a coil rotating in a magnetic field. - Both induce current by the relative motion of the coil and magnetic field.

Question 40

How do speakers work?

Answer 40

- The moving coil loudspeaker creates sound waves when an alternating p.d is applied to its coil. - The coil is in a magnetic field, the current in the coil cause a force on the coil due to the motor effect. - The alternating current causes the force to constantly change direction, making the coil and the diaphragm to vibrate. - The vibration causes sound waves to be produced of the same frequency as the alternating p.d source.

Question 41

How do microphones work?

Answer 41

- The moving coil in microphone generates an alternating p.d. as the sound waves make the diaphragm + coil vibrate. - The coil is attached to a diaphragm which is mounted between magnetic poles. - The pressure variations of incoming sound waves cause the diaphragm to vibrate along with the coil. - The coil moves backwards and forwards relative to the magnetic field. - An alternating p.d is induced in the coil. -The induced alternating p.d is the same frequency as the incoming sound wave.

Question 42

How do transformers work?

Answer 42

A transformer has two coils of insulated wire, both wound around the same iron core. Iron is used for the core because iron is easily magnetised and demagnetised. The coils are called the primary coil and the secondary coil. The primary coil is connected to an a.c. supply. When the alternating current passes through the primary coil, an alternating potential difference is induced in the secondary coil. This happens because: • alternating current passing 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 43

Do transformers work with both DC and AC electrical supply?

Answer 43

Transformers only work with alternating current. With a direct current, there is no changing magnetic field, so the secondary potential difference is zero.

Question 44

What is the equation linking potential difference and turns for a transformer?

Answer 44

V_primary / V_secondary = no. turns on primary coil/no. turns on secondary coil

Question 45

For efficient transformers what is the equation?

Answer 45

V_secondary x I_secondary = V^primary x I^primary

Question 46

In the national grid how are energy loses reduced in transformers?

Answer 46

- Ensuring the coils of wire are as thick as possible to reduce resistance. - Making them from layers of iron separated by an insulator to ensure that induced current do not build up in the core( eddy current) which would cause inefficiency due to heating.