Faraday's Lab

This animation is great for demonstrating the physics of electromagnets

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6.20 100% Efficiency

Recall and use the relationship (for 100% efficiency):

We know that energy cannot be created nor destroyed so in the case of 100% efficiency the input power must always equal the output power of a transformer
Remember: Power = Current x Voltage

Note: In reality Transformers are roughly 99% efficient

6.19 Turns

Recall and use the relationship between input (primary) and output (secondary) voltages and the turn ratio for a transformer:

The ratio between voltage and turns is shown above, this can be used to predict the output voltage of a transformer. It is worth noting that that because this is a ratio you could place the secondary turns and voltage on top as long as both are on the top or bottom (i.e. Vp and Ns on top would be incorrect)

Example question:

a) This is a step-up transformer because the voltage is being decreased

b)

        

     

        

        

c)

     
 
 
d) 24 Watts of Power is delivered to the lamp [assuming 100% efficiency]
e) 24 Watts [see 6.20]
f)
    

 
 
g) If the transformer is only 50% efficient, half of the energy would be wasted. In order to retain 24Watts of power the primary current must be doubled so that the primary power is 48Watts.

6.18 Transformers in Power Stations

Explain the use of step-up and step-down transformers in the large-scale generation and transmission of electrical energy


After electricity is generated in a power plant it is transformed into very high voltage so that it can be transported across the country through power lines with little energy loss. It is then transformed down to the voltage used in household sockets.

A transformer that increases voltage is called a step-up transform, and a transformer the decreases voltage is a step-down transformer.

6.17 Transformer

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

A transformer consists of a circular iron core with input and output coils wrapped around opposite sides. In order for it to work there needs to be a changing magnetic field; this is why DC power cannot be transformed


http://micro.magnet.fsu.edu/electromag/java/transformer/index.html
The Java applet at this URL shows a simple transformer

6.16 Generator

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 recall the factors which affect the size of the induced voltage

In a generator a magnet is rotating near a coil of wire. This rotatory motion induces a current in the wire generating electricity. On the other hand you could also rotate the wire inside a magnetic field

It is important to note that because of the rotations Alternating Current (AC) is produced

6.15 Electromagnetic induction

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

We know that if there’s a magnetic field perpendicular to a current in a wire, the wire moves perpendicular the field and the current.
But what happens if we move a wire in a magnetic field…? We get a current induced in the wire

We can show this by moving wire connected to an ammeter through a magnetic field. Move the wire in one direction we get a positive reading; and in the other, a negative. But when there is no movement there is no current

We can use this to identify that:

• Using a magnetic field and movement we can create a current
• Using a current and a magnetic field we create movement
• \ We can see that any combination of a current, magnetic field and motion create the other one.

Model question:

Explain carefully how you can induce a current in a wire [3]

• The wire must be perpendicular to the magnet
• The wire and magnetic field must move relative to each other
• The wire must cut through the magnetic field lines as it moves
• The induced current perpendicular to both the field lines and the motion