Principles of Electromagnetic Induction
- Electromagnetic induction is a phenomenon which occurs when an e.m.f is induced when a conductor moves through a magnetic field
- When the conductor cuts through the magnetic field lines:
- This causes a change in magnetic flux
- Which causes work to be done
- This work is then transformed into electrical energy
- Therefore, if attached to a complete circuit, a current will be induced
- This is known as electromagnetic induction and is defined as:
The process in which an e.m.f is induced in a closed circuit due to changes in magnetic flux
- This can occur either when:
- A conductor cuts through a magnetic field
- The direction of a magnetic field through a coil changes
- Electromagnetic induction is used in:
- Electrical generators which convert mechanical energy to electrical energy
- Transformers which are used in electrical power transmission
- This phenomenon can easily be demonstrated with a magnet and a coil, or a wire and two magnets
Experiment 1: Moving a magnet through a coil
- When a coil is connected to a sensitive voltmeter, a bar magnet can be moved in and out of the coil to induce an e.m.f
The expected results are:
- When the bar magnet is not moving, the voltmeter shows a zero reading
- When the bar magnet is held still inside, or outside, the coil, the rate of change of flux is zero, so, there is no e.m.f induced
- When the bar magnet begins to move inside the coil, there is a reading on the voltmeter
- As the bar magnet moves, its magnetic field lines ‘cut through’ the coil, generating a change in magnetic flux
- This induces an e.m.f within the coil, shown momentarily by the reading on the voltmeter
- When the bar magnet is taken back out of the coil, an e.m.f is induced in the opposite direction
- As the magnet changes direction, the direction of the current changes
- The voltmeter will momentarily show a reading with the opposite sign
- Increasing the speed of the magnet induces an e.m.f with a higher magnitude
- As the speed of the magnet increases, the rate of change of flux increases
- The direction of the electric current, and e.m.f, induced in the conductor is such that it opposes the change that produces it
- Factors that will increase the induced e.m.f are:
- Moving the magnet faster through the coil
- Adding more turns to the coil
- Increasing the strength of the bar magnet
Experiment 2: Moving a wire through a magnetic field
- When a long wire is connected to a voltmeter and moved between two magnets, an e.m.f is induced
- Note: there is no current flowing through the wire to start with
The expected results are:
- When the wire is not moving, the voltmeter shows a zero reading
- When the wire is held still inside, or outside, the magnets, the rate of change of flux is zero, so, there is no e.m.f induced
- As the wire is moved through between the magnets, an e.m.f is induced within the wire, shown momentarily by the reading on the voltmeter
- As the wire moves, it ‘cuts through’ the magnetic field lines of the magnetic, generating a change in magnetic flux
- When the wire is taken back out of the magnet, an e.m.f is induced in the opposite direction
- As the wire changes direction, the direction of the current changes
- The voltmeter will momentarily show a reading with the opposite sign
- As before, the direction of the electric current, and e.m.f, induced in the conductor is such that it opposes the change that produces it
- Factors that will increase the induced e.m.f are:
- Increasing the length of the wire
- Moving the wire between the magnets faster
- Increasing the strength of the magnets
Applications of EM Induction
- Electromagnetic (EM) induction is commonly used in power generation and power transmission, most commonly:
- Electrical generators
- Electric motors
- Transformers
Generators
- Generators are used inside power stations
- It is a machine which converts mechanical energy to electrical energy for transmission over power lines and for electrical power required for transport
- Sources of mechanical energy can be from steam turbines, water turbines, or wind turbines
- An electrical generator consists of a rotor
- This is an electromagnet made by coiling wires around two or more poles of a ferrous metal, ie. iron core
- These are rotated between the poles of a magnet
- As the coil rotates, the sides of the coil cut through the magnetic field lines, and current flows through the coil as an e.m.f is induced
- This flows out from both sides of the coil through slip rings and produces an alternating current output
Motors
- Electric motors work in a similar way to generators, except they convert electrical energy to mechanical energy
- Motors also consist of a rotor between the poles of a magnet
- The two poles of the permanent magnet remain the same, but the poles of the electromagnet reverse when the direction of electric current reverses
- For example, the end of the electromagnet, ie. its north pole, will be attracted to the south pole of the permanent magnet
- This pulls the electromagnet towards the permanent magnet
- When the electric current changes direction, the same end of the electromagnet becomes its south pole
- The electromagnet is now repelled by the south pole of the permanent magnet, and moves away from it
Transformers
- Transformers are used to:
- Increase the voltage of electricity generated in a power station so less energy is lost as heat when the current is transmitted down power lines
- Decrease the voltage before it reaches buildings to be used in electrical appliances
- A transformer is made up of two coils, the primary and secondary coil, each with a different number of coils wrapped around an iron core
- The number of coils on each side determines whether the voltage will be stepped-up (increased) or stepped-down (decreased)
- When A.C is applied through the first coil (called the primary coil), a varying magnetic flux is generated inside the iron core
- The varying magnetic flux passes through the iron core to the secondary coil which induces an e.m.f in the second coil
- From Faraday’s Law, the induced e.m.f in the secondary coil can be increased by the number of turns