## February 13, 2012

This animation is great for demonstrating the physics of electromagnets

### 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)
$Current=24/240$
$Current=0.1\, Amps$
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

### 6.14 Increasing the Force

Recall that the force on a current-carrying conductor in a magnetic field increases with the strength of the field and with the current.

In the motor effect if you increase either the current in the conductor/wire or the strength of the magnetic field the force of the movement will also increase

Exam style question:
What changes can you make to a simple dc motor to increase the force it can exert? [3]
• Increase the strength of the magnetic field [1]
• Increase the current in the coil [1]
• Increase the number of turns on the coil [1]

### 6.13 Fleming’s Left Hand Rule

Use the left hand rule to predict the direction of the resulting force when a wire carries a current perpendicular to a magnetic field

We can predict the direction of the force from the motor effect using Fleming’s left hand rule.
• Use your First finger (index finger) to point in the direction of the Field
• Your seCond finger in the direction of the Current (at 90° to the first finger)
• Your thumb is now pointing in the direction of the force/motion.
• Remember to always use your LEFT HAND

### 6.12 Motor effect

Recall that a force is exerted on a current-carrying wire in a magnetic field, and how this effect is applied in simple dc electric motors and loudspeakers

Motor effect: The fact that a wire carrying a current perpendicular to a magnetic field will experience and force (mutually perpendicular to both the field and the current)
What this means is that when a wire carrying a current is passed through a magnetic field a force is applied on the wire such that it exits the magnetic field and no longer “cuts” the field. This is because the electromagnetic field surrounding the wire is repelled by the field in the permanent magnet

### 6.11 Charged Particles

Appreciate that there is a force on a charged particle when it moves in a magnetic field as long as its motion is not parallel to the field

When charged particles enter a magnetic field the charged particles get deflected. Charged particles from the sun enter our magnetic field and are deflected to produce what we call the aurora borealis (northern lights)

### 6.10 Sketch Magnetic fields

Sketch and recognise magnetic field patterns for a straight wire, a flat circular coil and a solenoid when each is carrying a current

Straight Wire
If you have a straight wire with a current flowing in a known direction, pointing your right-hand thumb in the direction of the current you can find the direction of the magnetic field by looking at the direction your hands curl around the wire.
For example, as in the diagram below, if the current is flowing upwards pointing your thumb upwards reveals that the magnetic field is flowing in a anti-clockwise motion.

Flat circular coil
Using the Right Hand Grip Rule (RHGR) we can predict that a coil of wire will produce a magnetic field similar to that of a small bar magnet.

Solenoid
A cylindrical coil of wire is called a solenoid. Passing a current through a solenoid produces a flux roughly the same shape as a bar magnet. Because of its shape is also produces a uniform magnetic field inside the solenoid.
You can predict the poles of a solenoid by using the graphic shown above. If you are looking at the solenoid from one end and the current is flowing anti-clockwise that end is the North Pole, this means that looking from the other direction the current would flow clockwise producing the South Pole.

### 6.8 and 6.9 Electromagnets

6.8 Recall that an electric current in a conductor produces a magnetic field round it.
6.9 Describe the construction of electromagnets

Passing an electric current through a conductor (e.g. a copper wire) produces and magnetic field known as an electromagnet. The advantage of having an electromagnet rather than a permanent magnet is that an electromagnet can be turn off simply by stopping the current in the conductor. You can also increase the strength of a electromagnet by increasing the current in the circuit or by increasing the number of coils around the iron core.

To construct a simple electromagnet coil insulated copper wire around a soft iron core and apply a current through. The video below demonstrates this: