Electromagnetism Flashcards
What is electromagnetism?
It’s the link between electricity and magnetism
What do electric currents produce?
Magnetic fields
What is real electric current?
The flow of electrons
What is conventional current?
This is the apparent flow of positive charge
What does a circle with a dot in the middle mean?
Conventional current coming out of the page (towards you)
What does a circle with a cross in the middle mean?
Conventional current going into the page (away from you)
What does the direction of the field lines depend on in a wire?
Depends on the direction of current
How does magnetic field lines look in a circular coil?
Field lines follow the direction of current (right hand grip rule), field lines are straight at the centre at the loop
How does magnetic field lines look like in a solenoid?
Similar to a bar magnet on the outside of the solenoid and they go from north to south, in the inside of the solenoid they are parallel from north to south
What are the 3 ways to increase the strength of an electromagnet?
Increase the current that flows through the solenoid
Increase the number of coils (length of wire remains the same)
Add a soft iron core to create an induced magnet
What is the motor effect?
A current carrying wire will experience a force in the presence of a magnetic field
How to explain the direction of the motor effect in an exam questions?
Magnetic field caused by current in conductor interacts with the permanent magnetic field causing a force which accelerates the wire to the left/ right
Explain what each finger means in flemmings left hand rule:
First finger - Magnetic Field
SeCond finger - Conventional current
Thumb - Thrust/ (direction) force
What is required to make an electric motor rotate 360 degrees?
The current must be changed every half turn which is done by using split-ring commutator
What equipment is involved in an electric motor?
Two magnets, square shaped coil, split-ring commutator, power source
How does a split-ring commutator work?
Swaps the negative and positive connections every half turn so the direction of current swaps every half turn so the forces on the coil are always in the same direction.
What are the 3 ways to increase the speed of rotations in an electric motor?
Increase the current
Add more turns to the coil
Increase the magnetic field strength (use stronger magnets
What is electromagnetic induction/ the generator effect?
Move a coil shaped wire through a magnetic field and it’ll induce a potential difference in the wire
What happens when the wire stops moving through a magnetic field?
The potential difference disappears because it’s the change in the magnetic field that the wire experiences as it moves through it which creates the potential difference
How does the direction of the potential difference swap?
Every time we change the direction of the wire moving through the magnetic field
How can we generate a current using the generator effect?
It requires a complete circuit
How to change the size of the induced potential difference?
Change the strength of the magnetic field –> stronger magnets –> stronger magnetic field –> larger potential difference
Move wire/ magnets more quickly, the faster they move –> faster the magnetic field will change –> bigger potential difference
Shape the wire into a coil, more turns –> bigger potential difference
What is a generator?
It generates electricity from rotational motion (e.g. rotating a coil of wire)
What are two common types of generators?
Alternators and dynamos
How does alternators work?
Coil of wire rotates relative to magnets –> induces magnetic field in coil –> induced current and voltage in coil
Slip rings and brushes means contacts doesn’t swap every half turn
This means it produces an alternating potential difference and current
What happens when the coil rotates faster in an alternator?
The peaks of the oscillations get larger and more frequent (higher amplitude and frequency)
How does dynamos work?
Coil of wire rotates relative to magnets –> induces a magnetic field in the coil –> induced current and voltage in coil
Split ring commutator means contact swaps every half turn
A direct potential difference and current would be produced (as current is always flowing in the same direction)
What happens when the coil rotates faster in a dynamos?
The peaks of the oscillations get larger and more frequent (higher amplitude and frequency)
How do loud speakers work (simplified explanation)?
Loud speakers take electric signals and uses the motor effect to convert them into vibrations generating sound waves
How do microphones work (simplified explanation)?
Microphones detect vibrations by sound waves and uses the generator effect to convert them into electrical signals
Explain the structure of a loud speaker:
Coil of wire is wrapped around one pole of a permanent magnet connected to the cone
The coil of wire is permanently attached to the cone but coil and cone can slide back and forth along magnet
How does a loud speaker work?
Alternating current flows through wire and creates a magnetic field in the coil
The two magnetic fields (of the coil and permanent magnet) will interact and causes a force on the coil which makes it rapidly move back and forth (creating vibrations)
Vibrations are so fast that the cone vibrations cause pressure variations in the air - sound waves
Explain the structure of a microphone:
Similar to loud speakers - but there’s no cone and instead a diaphragm
It’s made to detect sound instead of creating it
How does a microphone work?
Sound waves hit diaphragm –> diaphragm and coil of wire moves
As wire is moving within the magnetic field of the permanent magnet will generate a current (generator effect)
Frequency + amplitude of sound wave determines how much the diaphragm vibrates
How can the frequency and amplitude of the current of a microphone be determined?
Amplitude and frequency of sound wave determines how much the diaphragm vibrates and therefore determines the frequency and amplitude of the current
What is the national grid?
A large network of transformers and wires which spreads across the country to distribute electricity
Where does most of our electricity come from and how does it work?
Power stations - generates a lot of heat which is converted into thermal energy to electrical energy
What time of day is the demand at its peak- national grid?
Afternoon/ evening
How does power stations cope with demand- national grid?
They must have spare capacity –> often run below their maximum power output
Explain why the high current is used to increase the amount of power transmitted - national grid:
High currents means a lot of heat would be produced due to resistance so there would be a high loss of energy so the voltage must be very high instead according to P=IV
Explain the general process of transferring energy:
Electrify is generated at power station –> voltage is increased to approximately 400,000V using a step-up transformer –> wires transmits voltage between pylons –> voltage is reduced to 230V using a step-down transformer
Why must voltage be reduced to 230V when transferring energy to households?
As it’s dangerous and will blow household appliances
How does a transformer work?
Alternating potential difference is applied to the primary coil –> current flows –> generates a magnetic field around the primary coil
Current + magnetic field will alternate
Induced an alternating magnetic field in the iron core
Iron core induces a potential difference in the secondary coil –> causes the current to flow (in a complete circuit)
Why can’t the current be transferred straight across in a transformer?
The wires are insulated in plastic –> can’t pas electricity to the iron core
What is step-up transformer?
Increases voltage and decreases current (depending on amount of turns in primary secondary coil)
What is a step-down transformer?
Decreases voltage and increases current (depending on amount of turns in primary secondary coil)
How to calculate voltage and number of turns in a transformer?
Vp/ Vs = Np/ Ns
Primary voltage/ secondary voltage = number of primary turns/ number of secondary turns
How to calculate voltage and current in a transformer?
VpIp = VpVs
Primary voltage x Primary current = Secondary voltage x Secondary current
What is solenoid?
A loop of wire carrying a current
How does a relay work?
It uses a small current to energise a solenoid to close a switch and turn on a large current
