· Explain the use of step-up and step-down transformers in the large-scale generation and transmission of electrical energy
· http://youtu.be/LZKhGGBcYFI 
· http://youtu.be/LZKhGGBcYFI
6.18 answers
Sunday, 5 February 2012
6.20
· recall and use the relationship (for 100% efficiency): 
input power = output power
Vp Ip = Vs Is
6.19 and 6.20 Plenary answers
a. Vs/Vp=ns/np
Vs/2=80/20
Vs=8V
b. Vs/Vp=ns/np
Vs/10=20/100
Vs=2V
c. Vs/Vp=ns/np
Vs/240=20/400
Vs=12V
d. Vs/Vp=Ns/Np
Vs/4=2000/50
Vs=160V
2. Vs/Vp=ns/np
3/24=ns/480
ns=60turns
a. Iron (Why? Iron is a magnetically soft material - it can be magnetised and lose its magnetism easily. This is necessary in a transformer as the magnetic field needs to change repeatedly)
b. electrical energy in the primary coil => magnetic energy in the core =>electrical energy in the secondary coil
c. dc current produces a steady magnetic field in the core. To induce a current in the secondary coils there must be a changing magnetic field in the core.
a. Step down (because the voltage decreases)
b. Vs/Vp=ns/np
12/240=ns/1000
ns=50turns
c. P = V x I
24 = 12 x I
I = 2A
d. Power in secondary = 24W
e. Power in primary = 24W
f. P = V x I
24 = 240 x I
I = 0.1A
g. The current would be greater/double
Why?
Efficiency = Useful Pout/Total Pinx100
50 = 24/Total Pin x 100
Total Pin = 48W
P = V x I
48 = 240 x I
I = 0.2A
6.19
· recall and use the relationship between input (primary) and output (secondary) voltages and the turns ratio for a transformer:
input (primary voltage = primary turns
output (secondary) voltage secondary turns
Vp/Vs = np/ns
6.17
· Recall the structure of a transformer, and understand that a transformer changes the size of an alternating voltage by having different numbers of turns on the input and output sides
6.17 Practical - model answers
· If you…
· Turn the powerpack to dc
No current is induced in the Secondary Coil. To induce current you need a changing magnetic field and this is not produced by applying dc to the Primary Coil
· Turn the powerpack to ac
ac is induced in the Secondary Coil. To induce current you need a changing magnetic field and this is produced by applying ac to the Primary Coil
3. Increase the number of turns in the Secondary Coil
The size of the induced voltage in the secondary coils increases
4. Increase the voltage on the Primary Coil
The size of the induced voltage in the secondary coils increases
5. Decrease the number of turns in the Secondary Coil
The size of the induced voltage in the secondary coils decreases
6. Decrease the voltage on the Primary Coil
The size of the induced voltage in the secondary coils decreases
6.16
· Describe the generation of electricity by the rotation of a magnet within a coil of wire and of a coil of wire within a magnetic field; also describe the factors which affect the size of the induced voltage
· Magnet rotating near coil
· Coil rotating near magnet
· ac generator
6.16 Practical - model answers
· Connect a hand turned generator to a light bulb. Observe the light bulb when you...
· Rotate the generator slowly
The induced voltage decreases
· Rotate the generator quickly
The induced voltage increases
3. Increase the strength of the magnet
The induced voltage increases
4. Increase the number of turns in the coil
The induced voltage increases
6.16 generator simulation
Website:
http://www.walter-fendt.de/ph14e/generator_e.htm
6.16 Plenary
· What are the 3 ways that you can increase the size of the current induced in a generator?
Answers
· Increase the strength of the magnets
· Increase the speed of the relative motion
· Use a coil with more turns of wire
6.15
6.15 starter
· What’s the motor effect?
· "If there’s a magnetic field perpendicular to a current in a wire, the wire moves in a direction perpendicular to the field and the current" (FLHR)
· So what about if we move a wire in a magnetic field? What happens in the wire?
· When we move a wire in a magnetic field, a current is induced in the wire
6.15
· 6.15 recall that a voltage is induced in a conductor or a coil when it moves through a magnetic field or when a magnetic field changes through it; also recall the factors which affect the size of the induced voltage
6.15 Practical - model answers
· If you...
· Push the North pole of the magnet into the coil
A negative current flow shown by a negative deflection on the ammeter
· Keep the magnet stationary within the coil
No current
3. Pull the North pole of the magnet out of the coil
A positive current
4. Push the South pole of the magnet into the coil
A positive current
5. Push the North pole of the magnet slowly into the coil
A smaller negative current
6. Push the North pole of the magnet quickly into the coil
A larger negative current
7. Change the coil for one with more turns of wire and push the North pole of the magnet into the coil
A larger negative current
8. Push the North pole of a neodymium (strong) magnet into the coil
A larger negative current
9. Move the magnet in and out of the coil repeatedly. What sort of current is this?
An alternating current
6.15 Plenary answers
16 January 2012
· Explain carefully how you can induce a current in a wire (3 marks)
· State 3 ways you can increase the size of this induced current (3 marks)
Answers
· The wire must be perpendicular to a magnetic field
· The wire and magnetic field must move relative to each other – the wire must “cut” through the magnetic field lines/lines of magnetic flux as it moves
· A current is induced in the wire. The induced current is perpendicular to both the field and the motion
· Increase the strength of the magnets
· Increase the speed of the relative motion
· Use a coil of wire instead of a single piece of wire
Sunday, 22 January 2012
6.14
6.146.14 recall that the force on a current-carrying conductor in a magnetic field increases with the strength of the field and with the current
Exam style question
· What changes can you make to a simple dc motor to increase the force it can exert? (3 marks)Model Answer
· Increase the strength of the magnetic field· Increase the current in the coil· Increase the number of turns on the coil
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