05 Electromagnetism & Motor Effect - 4th Form Flashcards
- In a speaker, there is a current carrying coil
- 90º to a magnetic field
- Due to LHR there is a force on the coil
- The coil is attached to the cone - therefore the cone moves in one direction
If the current in a speaker is a.c, describe the movement of the coil and cone and how this creates a sound wave.
- The alternating current means the current in the coil changes direction continuously.
- This means the force on the coil/cone changes direction continuously - cone moves in and out (vibrates)
- This creates a sound wave (compression wave) in front of the cone at the same frequency as the alernating current.
Describe the left hand rule (LHR)
First Finger - Field
Second Finger - Current
Thumb - Movement
Draw the magnetic field around a solenoid.
Use the LHR to explain why the wire moves to the left.
- Current carrying wire (seCond finger- CURRENT)
- 90º to magnetic field (First finger field)
- Using LHR this creates a force to the left- wire moves left (thuMb movement)
How do you use the right hand grip rule to work out which end of a solenoid is the north end?
Fingers- wrap your fingers in the direction of the current
Thumb- will point to the North end of the solenoid
How do we use dot and crosses to represent the direction of a magnetic field or current?
dot means out of the page
cross means into the page
Complete the explanation of why the commutator allows the motor to rotate
- There is a current carrying coil
- 90º to a magnetic field
- Using LHR- left side of coil experiences an upward force.
- Using LHR- current is travelling the opposite direction on the right side of coil- it experiences a downward force
- Coil rotates clockwise
- When coil is at 90º, the current in the coil reverses due to the commutator and brushes.
- Current continues to travel IN on the left and OUT on the right
8 Using LHR- left side of coil still experiences an upward force and the right hand side still experiences a downward force
- Coil continues to rotate clockwise
How would you demonstrate the direction of the magnetic field around a single wire?
Place compasses around the wire
The compass will point in the direction of the magnetic field
Describe the force on the particle below
No force - as particle is moving parallel to the field (it must move 90º to the field)
What can be done to increase the magnetic field around a solenoid?
1- more turns
2- increase the current
3- add an iron core
Draw the shape of a magnetic field around a coil.
How do you determine the direction of a magnetic field around a wire?
Use the right hand grip rule
Thumb- direction of current
Fingers- wrap in direction of magnetic field (clock wise or anti-clockwise)
Electromagets are used in electric bells.
- When the button is pushed, a current flows in the electromagnet -This creates a magnetic field around the electromagnet
- The electromagnet attracts the soft iron armature which moves to the right
- This causes the hammer to hit the bell.
What happens next?
The moment the iron armature moves to the right this creates a _break in the circuit- t_he circuit is no longer complete.
The current drops to zero and the magnetic field around the electromagnet disappears.
The iron armature is no longer attracted and springs back to the left
The armature completes the circuit again and the whole cycle repeats alowing the bell to repeatedly sound.
Why is iron a better core to use in an electromagnet than steel?
Iron is magnetically soft- the domains in iron line up easily - it can magnetise and demagentise easily
Steel is magnetically hard- the domains do not line up easily- it cannot magnetise easily and it can be used to make permananet magnets becaouse it also does not easily demagnetise
What happens to the magnetic field around a wire if more wires are added side by side- How can you tell from the image that the magnetic field is stronger?
More wire- stronger field
The closer the magnetic field lines the stronger the field.
iron filing will line up more easily and more closely together in a stronger magnetic field.
The current flows into the left brush making contact with the left of the split ring commutator, this means the current always travels into the page on the left side of the coil. This is 90º to the field and due to LHR the force on the left side of the coil is upward.
Why is the force on the right side of the coil downward?
The direction of the current is reversed or out of the page on the right side of the coil.
Therefore the direction of the force is in the opposite direction OR downward
The coil rotates clockwise
What happens to the magnetic field around a solenoid if you add an iron core (magnetically soft)
Stronger magnetic field
If a vertical wire has the current upward in the wire, work out the direction of the magnetic field using your Right Hand Grip Rule
Thumb- up and in direction of current
Finger wrap in direction of magnetic field.
Describe the shape of the magnetic field around a wire
Circular field shape
field lines getting futher apart- field getting weaker as you move away from the wire
How would you construct a simple electromagnet?
1) wrap a wire around an iron nail
2) connect a cell across the wire
- No force at all
Note: Particle is moving parallel to the field- no force.
