Topic 13 - Electromagnetic induction Flashcards
What is electromagnetic induction?
The induction of a potential difference (and current if there’s a complete circuit) in a wire which is experiencing a change in magnetic field
Induces is a fancy word for….
Creates
How would you produce an electric current by the relative movement of a magnet and a conductor? (ON A SMALL SCALE IN THE LAB)
- You could rotate/move either a magnet in a coil of wire or a conductor in a magnetic field
- If you move or rotate the magnet in the opposite direction, then the p.d/current will be reversed too
- If you keep the magnet (or coil) moving backwards and forwards, or keep it rotating in the same direction, you produce an alternating current
When a current is induced in a wire, that current always produces its own….
magnetic field
The magnetic field created by an induced current always acts against ……
the change that made it
What do transformers do?
They change the size of the pd of an alternating current
How do transformers work?
- They all have two coils of wire, the primary and secondary coil, joined with an iron core
- When an alternating pd is applied across the primary coil, it produces an alternating magnetic field
- The iron in the core is a magnetic material that is easily (de)magnetised, as the coil is producing an alternating magnetic field, the magnetisation in the core also alternates
- This changing magnetic field, induces a pd in the secondary coil
What do step up transformers do?
- They step the pd up (increase it)
- They have more turns on the secondary coil than the primary coil
What do step down transformers do?
They step the pd down (decrease it). They have more turns on the primary coil than the secondary
How efficient are transformers?
Nearly 100% efficient
What formula shows that the input power is equal to the output power in transformers?
Vp * Ip = Vs * Is
V = pd I = current p = primary coil S= secondary coil
How do dynamos generate direct current?
- Generators apply a force to rotate a coil in a magnetic field, like a motor (or a magnet in a coil)
- As the coil (or magnet) spins, a current is induced in the coil. This current changes direction every half turn
- Dynamos are dc generators. They have a split ring commutator
4, This swaps the connection every half turn to keep the current flowing in the same direction
What increases the current induced in a dynamo or alternator?
- Having more turns of wire in the coil
- The magnetic flux density is increased
- If the speed of rotation is increased
What is the only main difference between dynamos and alternators?
- Instead of a split ring commutator, a.c. generators have slip rings and brushes so the contacts dont swap every half turn
- This means an alternator produces an alternating p.d and therefore an a.c. if the coil is part of a complete circuit
How do microphones generate current from sound waves?
- Mics use electromagnetic induction to generate an electrical signal
- Sound waves hits a flexible diaphragm, attached to a coil of wire. The coil of wire surrounds one pole of a permanent magnet and is surrounded by the other pole
- This means as the diaphragm moves, a current is generated in the coil
- The movement of the coil depends on the properties of the sound wave (louder sounds makes the diaphragm move further
- Therefore, mics convert pressure variations of a sound wave into variations in current in an electric circuit
How do loudspeakers work?
- In loudspeakers, the diaphragm is replaced with a paper cone
- The coil is wrapped around one pole of a permanent magnet, so the a.c. signal causes a force on the coil
- When the current is reversed, the force acts in the opposite direction
- These movements makes the cone vibrate, which makes the air around the cone vibrate and creates the variations in pressure that causes a sound wave
Describe how a power station uses a turbine to generate electricity
- Burning fuel is used to heat water, and convert it to steam, which turns a turbine
- The turbine is connected to a powerful (electro)magnet inside a generator - a huge cylinder wound with coils of copper wire
3, As turbine spins, the magnet spins with it, inducing a large pd and alternating current in the coils - Coils are joined together in parallel, to produce a single output from the generator
What is the only type of power generation that doesnt use a turbine and generator?
Solar
What is the national grid?
A network of wires that connects UK power stations to consumers. This is where electricity is taken to once it has been generated
The national grid has to transfer loads of …….., which means it transfers electricity at a high….. (power = energy transferred * time)
- Energy each second
2. Power
To transmit the huge amounts of power needed, what would you need?
Power = current /pd
Either a high pd or a high current
Whats the disadvantage with having a high current?
It makes the wires heat up so lots of energy is wasted to thermal energy stores
What formula can you use to find out the power lost due to resistive heating?
Electrical power = current (squared) * resistance
How could the national grid reduce these losses
- By using more efficient, high voltage, low resistance cables
- by using transformers, which are almost 100% efficient
Why is using high voltage an advantage?
For a given power, as you increase the pd across a coil, you decrease the current through it (Vp * Ip = Vs * Is)
What transformer boosts up the pd really high (400,000 V) and keeps the current low in power stations?
Step up transformers
What transformer brings the pd back to safe usable levels at the consumers end?
Step down transformers
What is the transformer equation?
Vp/ Vs = Np/ Ns * (this equation also works if Vs and Ns are on top)
Vp = input pd Vs = output pd Np= number of turns on primary Ns = number of coils on secondary
