Electro-magnetism

• A current carrying wire produces a magnetic field around it, this is called an electromagnet.
• An electric motor converts electrical energy into mechanical energy by using the force on moving charges due to a changing magnetic flux through a coil.
• A magnetic field is a region where a force is exerted on magnetic materials.
• When current flows in any conductor, there's a magnetic field around the conductor. We can use the right-hand rule to figure out the direction of the field.
• A current perpendicular to a uniform magnetic field will induce a force. (Flemings left hand rule).
• The size of the force on a current-carrying conductor at a right angle to an external magnetic field is proportional to the magnetic flux density.
• Magnetic flux density is defined as: The force on one metre of wire carrying a current of one amp at right angles to the magnetic field.
• Magnetic flux density is a vector that is measured in Tesla(T).
• Force=Magnetic flux density x current x length of wire
• Force is greatest when the wire and field are perpendicular. F=BILsinx
• By Fleming's left-hand rule the force on a moving charge in a magnetic field is always perpendicular to its direction of travel. This results in circular motion.
• F=BQV F=mv^2r^-1 solve for r
• Radius of curvature depends on charge, mass, speed and magnetic flux density. (r=mv/BQ)
• velocity selectors are used to separate out particles of a certain velocity from a stream of accelerated charged particles moving at a range of speeds. They do this by applying both a magnetic and an electric field at the same time perpendicular to each other, while a stream of particles is fired perpendicularly to both fields at a device with a narrow gap called a collimator. Particles will deflect if forces are not balanced. Therefore BQv=EQ. To simplify v=E/B.
• Velocity sectors are used in mass spectrometers to ensure that the accelerated particles entering the magnetic field have the same velocity.
• If there is relative motion in a conducting rod and a magnetic field, the electrons in the rod will experience a force, which causes them to accumulate at one end of the rod. This induces and emf across the ends of a rod exactly as a battery would, this is called electromagnetic induction. If the rod is part of a complete circuit, then an induced current will flow through it.
• You can induce an emf in a flat coil or solenoid by: Moving the coil towards or away from the poles of a magnet. Or moving a magnet towards or away from the coil. emf is caused by change in magnetic flux linkage. If part of a complete circuit an induced current will flow through it.
• The direction of the induced current is given by Lenz's law. The direction of the induced current opposes the cause of its production. In this case the induced current flows so as to create a magnetic field that opposes the original changing magnetic field.
• Magnetic flux can be thought of as the amount of magnetic field lines passing through an area. (Magnetic flux density x cross-sectional area). Remember flux is continuous though.
• If a coil has N turns you need to talk about flux linkage which is just magnetic flux x Number of turns.
• The Right-hand rule uses your thumb as the direction of the current and the direction your finger curls towards is the direction of the magnetic field.
• When the magnetic flux isn't perpendicular to the area, you need to use trig to find the component of magnetic flux that is perpendicular to the area. (BAcosx)
• Induced EMF= Change in flux linkage/Time taken
• A change in flux linkage of one weber per second will induce an electromotive force of 1 volt in a loop of wire.
• The size of the EMF is shown by the gradient of a graph of flux linkage against time.
• The area under the graph of EMF against time gives the change in flux linkage.
• The direction of the induced EMF/current is such as to oppose the change that caused it. (Lenz's Law)
• A search coil is a small coil of wire with a known number of turns and a known area.
• Generators or dynamos convert kinetic energy into electrical energy.m They induce an electric current by rotating a coil in a magnetic field. The output voltage and current change direction with every half rotation of the coil producing alternating current.
• Transformers use EM induction to change the size of the voltage for an alternating current. They consist of two coils of wire wrapped around an IRON CORE. An alternating current flowing in the primary coil produces a changing magnetic field in the iron core. The changing magnetic field is passed through the iron core to the secondary coil where it induces an AC voltage of the same frequency as the input voltage. The ratio of the number of turns on each coil along with voltage across the primary coil determines the size of the voltage induced in the secondary coil.
• Step-up transformers increase the voltage by having more turns on the secondary coil than the primary.
• Step-down transformers reduce the voltage by having fewer turns on the secondary coil.
• Real-life transformers aren't 100% efficient. Using a laminated core reduces energy loss.
• Ideal transformers are 100% efficient so power in= power out.
• N1/N2=V1/V2=I2/I1 Be careful for current
• Electricity from power stations is sent around the country in the national grid at the lowest possible current. This is because losses due t resistance in the cables is equal to P=I^2R. SO if you double the current you quadruple power lost.
• Transformers allow us to step up the voltage to around 400,000V for transmission through the national grid, and then reduce it again in substations to 230V for home use.
• Lots of electronic devices like laptops and mobile phones can't function using a standard 230V mains supply. They need a much lower voltage. Therefore the chargers for these devices contain transformers to adjust the voltage - they are contained in the plug or a box in the cable.
• Investigate relationship between number of turns and the voltages across the coil.

  1. Put two C-cores together and wrap wire around each to make the coils. Begin with 5 turns in the primary coil and 10 in the secondary coil (a ratio of 1:2).
  2. Turn on AC supply to primary coil. Use a low voltage - remember transformers increase voltage so make sure you keep it a safe level . Record voltage across each coil.
  3. Keep primary voltage the same so its a fair test. Repeat experiment with different number of turns. Try 1:1 and 2:1. Divide Ns by Np and Vs by Vp. Should find that Ns/Np=Vs/Vp
• Investigating the relationship between number of turns, voltage across and current of the transformer coils:

  1. Put two C-cores together and wrap wire around each to make coils. Add a variable resistor to the primary coil circuit and an ammeter to both circuits.
  2. Turn on the power supply and record the current through and voltage across each coil.
  3. Leaving the number of turns constant, adjust the variable resistor to change the input current. Record the current and voltage for each coil, then repeat this process for a range of input currents
  4. You should find Ns/Np=Vs/Vp=Ip/Is