Electromagnetic Induction

Question 1

What is Electromagnetic Induction

Answer 1

• 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

Question 2

What is Faraday’s Law

Answer 2

The magnitude of induced emf is equal to the rate of change of flux linkage

Question 3

What is Lenz’s Law

Answer 3

The direction of induced current is such as to oppose the motion causing it

Question 4

What happens when you move a straight conductor through a magnetic field

Answer 4

• 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

Question 5

Describe the Equation that links to Faraday’s Law

Answer 5

ε = N x ϕ / t

Electro Magnetic Force (V) = Number of Colis x Magnetic Flux (Wb) / Time (s)

Question 6

How do you find the magnitude of emf induced by a straight conductor when you know it’s length

Answer 6

• 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

Question 7

What is the equation for the emf induced in a coil rotating uniformly in a magnetic field

Answer 7

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)

Question 8

Describe how you would use an oscilloscope

Answer 8

• 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

Question 9

Describe what is meant by Peak voltage, Peak-to-peak voltage ,Root mean square (rms) voltage and Time Period

Answer 9

• 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

Question 10

What are the equations for the Root Mean Square Current and Voltage

Answer 10

• I_rms = I₀ / √2
• Root Mean Sqaure Current = Peak Current / √2
• _Vrms = V₀ / √2
• Root Mean Sqaure Voltage = Peak Voltage / √2

Question 11

What is the UK’s main voltage

Answer 11

230V

Question 12

What is the Peak and Peak-to-Peak voltage of UK mains

Answer 12

Peak = 330 V

Peak to Peak = 650 V

Question 13

What is the function of a transformer

Answer 13

Transformers can be used with alternating currents to change the size of their voltage

Question 14

How do transformers work

Answer 14

• 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

Question 15

What kind of current is produced by a transformer and why

Answer 15

• An alternating current
• An emf is induced by a changing magnetic field, hence the emf induced is alternating producing an alternating current

Question 16

Why are transformers used

Answer 16

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

Question 17

What is the transformer equation

Answer 17

N_s / N_p = V_s / V_p

Number of Coils in Seconday / Number of Coils in Primary = Voltage in Secondary / Voltage in Primary

Question 18

What are the two types of transformers

Answer 18

• 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.

Question 19

What is the transformer efficiency equation

Answer 19

Efficiency = I_s x V_s / I_p x V_p

Efficiency = Current in Secondary x Voltage in Secondary / Current in Primary x Voltage in Primary

Question 20

What is an eddy current

Answer 20

• 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

Question 21

How can eddy currents be reduced

Answer 21

• 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.

Question 22

How can you make transformers more efficient

Answer 22

• 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