What is Electromagnetic Induction
- When a conducting rod moves relative to a magnetic field, the electrons in the rod will experience a force
- The electrons will build up on one side of the rod, causing an emf to be induced in the rod
- This is an example of Electromagnetic Induction
- The same happens if you move a bar magnet relative to a coil of wire
- If the coil then forms a complete circuit, a current is also induced
What is Faraday’s Law
The magnitude of induced emf is equal to the rate of change of flux linkage
What is Lenz’s Law
The direction of induced current is such as to oppose the motion causing it
What happens when you move a straight conductor through a magnetic field
- The electrons experience a force pushing them to one end of the conductor
- This creates an emf across the conductor
- The rod obeys Faraday’s law and it is changing flux as it moves through the field therefore an emf is induced
Describe the Equation that links to Faraday’s Law
ε = N x ϕ / t
Electro Magnetic Force (V) = Number of Colis x Magnetic Flux (Wb) / Time (s)
How do you find the magnitude of emf induced by a straight conductor when you know it’s length
- Combing the Velocityand Area Equation and Faraday’s Law
- s = v x t
- From this we can find the area if we consider the distance travelled as width
- A = l x v x t
- We can use this to find the magnetic flux cut by the conductor
- Φ = BA = B x l v x t
- Finally, substitute this into the equation for Faraday’s law ε = NΦ / t = N x B x l x v x t / t = NBlv
- ε = NBlv
What is the equation for the emf induced in a coil rotating uniformly in a magnetic field
ε = B x A x N x ω x sin(ωT)
Emf = Magnetic Flux Density x Area x Number of Colis x Angular Speed x Sin(Angular Speed x Time)
Describe how you would use an oscilloscope
- Oscillioscopes display alternating waves
- The x axis (The time base) shows how long it takes the wave to move 1 division
- The y axis shows how much pd (potential difference) is needed to move the wave up 1 divison
Describe what is meant by Peak voltage, Peak-to-peak voltage ,Root mean square (rms) voltage and Time Period
- Peak voltage (V0 ) - distance from the equilibrium to the highest (or lowest) point.
- Peak-to-peak voltage - distance from the minimum point to the maximum point
- Root mean square (rms) voltage - the average of all the squares of the possible voltages, this value gives you the average value of voltage output by the supply (in either direction)
- Time period (T) - distance from one point on a curve to the point where the curve repeats
What are the equations for the Root Mean Square Current and Voltage
- Irms = I0 / √2
- Root Mean Sqaure Current = Peak Current / √2
- Vrms = V0 / √2
- Root Mean Sqaure Voltage = Peak Voltage / √2
What is the UK’s main voltage
230V
What is the Peak and Peak-to-Peak voltage of UK mains
Peak = 330 V
Peak to Peak = 650 V
What is the function of a transformer
Transformers can be used with alternating currents to change the size of their voltage
How do transformers work
- A primary coil is wrapped around an iron core with an alternating p.d
- This creates an alternating magnetic field
- This magnetic field induces an emf in the secondary coil also wrapped around the core
- This creates a current in the secondary coil
What kind of current is produced by a transformer and why
- An alternating current
- An emf is induced by a changing magnetic field, hence the emf induced is alternating producing an alternating current
Why are transformers used
By changing the number of coils, the transformer can be used to increase the voltage and reduce current when transporting power with minimal energy lost
What is the transformer equation
Ns / Np = Vs / Vp
Number of Coils in Seconday / Number of Coils in Primary = Voltage in Secondary / Voltage in Primary
What are the two types of transformers
- Step-up transformer - increases the input voltage by having more turns on the secondary coil than the primary
- Step-down transformer - decreases the input voltage by having less turns on the secondary coil.
What is the transformer efficiency equation
Efficiency = Is x Vs / Ip x Vp
Efficiency = Current in Secondary x Voltage in Secondary / Current in Primary x Voltage in Primary
What is an eddy current
- Eddy currents are induced in the iron core by the alternating magnetic field in the primary coil
- Due to Lenz’s law they oppose the field that produced them, reducing the field’s flux density
- They generate heat causing energy to be lost
How can eddy currents be reduced
- Using a laminated iron core, meaning that the core is made using layers of iron between layers of an insulator, because the eddy currents cannot pass through the insulator their amplitude is reduced
- Eddy currents can also be reduced by using a core made out of a high resistivity metal.
How can you make transformers more efficient
- Use a laminated iron core to reduce eddy currents
- Use a thick wire which has low resistance to reduce energy lost due to resistance in the coils
- Use a magnetically soft iron core to allow easy magnetisation and demagnetisation
