15 Electromagnetic Induction 5th 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)
When a magnet is moved in and out of a coil, the magnetic field lines of the magnet cut the coil at 90º , which induces an a.c. current.
State three way in which you could increase the current induced?
- More turns in the coil
- Stronger magnet
- Move the magnet faster
Explain why a current is induced in the wire.
- Wire is moved downward (thuMb- MOVEMENT)
- 90º to a magnetic field (First finger- FIELD)
- Therefore a current is induced in the wire (seCond finger- CURRENT)
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.
The coil rotates clockwise.
What would happen if the battery was reversed?
The direction of the current is reversed AND the coil will rotate anticlockwise
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
Explain why a current is induced in the coil
- Magnet is moved into the solenoid (thuMb- MOVEMENT)
- Magnetic field lines around magnet cut the coil at 90º (First finger- FIELD)
- Therefore a current is induced in the coil (seCond finger- CURRENT)
Using the transformer equation below. Describe what would happen to the voltage across a transformer if the number fo turns in the secondary is three times larger than the number of turns in the primary.
If no. of turns triples, the voltage triples.
Voltage in the seconday coil will be three times greater than the voltage in the primary
Step-up transformer
How do we use dot and crosses to represent the direction of a mangetic field or current?
dot means out of the page
cross means into the page
When a magnet is moved in and our of a coil, a current is induced in the coil. What is this phenomenon called?
Electromagnetic induction
Complete the explanation of why the comutator 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
When the magnet is moved into the coil, a current is induced and the needle on the ammeter moves to the left.
Describe what happens when the magnet is pulled out of the coil.
The needle with move to the right
Reversing the direction of the magnet, reverses the induced current
What is a step-down transformer?
no. of primary turns > no. of seconday turns
primary voltage > secondary voltage
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
Why is voltage stepped up using transformers before transmitting long distances?
If current is reduced then the power loss along the transmission lines is reduced.
Less heat loss along the wire
Describe the force on the particle below
No force - as particle is moving parallel to the field (it must move 90º to the field)