P7 - Magnetism Flashcards
Where are magnetic forces strongest?
What happens when we bring two magnets together?
What happens when two like poles meet?
What happens when two Different poles meet?
Magnetic forces are strongest at the poles.
When we bring two magnets close to each other they exert a force on eachother
Two like poles with repel each other
two unlike poles attract each other
These are examples of non-contact forces.
What does a permanent magnet do?
What does an induced magnet do?
Permanent magnet produces its own magnetic field. When we bring two permanent magnets together they can either repel or attract towards eachother depending on the direction of the poles.
An induced magnet is an object that becomes a magnet when placed in magnetic field. Induced magnetism always causes a force of attraction. If we take away the permanent magnet then all the induced magnet lose most or all of their magnetism quickly.
What are the four magnetic materials?
- Iron
- Steel
- Cobalt
- Nickel
What is a magnetic field?
What is the strength of magnet dependent on?
A magnetic field is a region around a magnet where a force acts on another magnet or a magnetic material.
Strength of the magnetic field depends on the distance from the magnet.
Where are magnetic fields strongest?
Where the fields lines are closest is the part where the magnetic field is strongest.
What happens when a current flows through a conducting wire?
What is the strength of the magnetic field depend on in an electromagnet?
What happens if we change the direction of the current?
When a current flows through a conducting wire a magnetic field is produced around the wire.
We can prove this using a compass. When the current of the wire turns on the direction of the compass should change.
The strength of the magnetic field depends on the size of the current. The larger the current is the stronger the field. It is also strongest closes to the wire.
If we change the direction of the current we also change the direction of the field. A compass placed near this wire would deflect in the opposite direction to before.
What is the right hand grip rule?
Hold out your right hand with your thumbs tucked in and your thumb pointing upwards:
the thumb is equal to the direction of current
the fingers are equal to the magnetic field direction
What are the three ways to increase the strength of a magnetic field in a solenoid?
Another way to increase the strength of a magnetic field is to coil the wire aka a solenoid. When we turn on the current there is a strong and uniform magnetic field inside the solenoid.
- Increase size of current
- Increase number of turns on coils because each loop or turn of the coil sets up its own magnetic field.
- place of piece of iron in the solenoid
A solenoid containing an iron core is called a electro magnet. You can change the strength of the electromagnet and whether it is on or off.
What is the motor effect?
How do you calculate the size of the force?
A wire carrying a current creates a magnetic field. This can interact with another magnetic field, causing a force that pushes the wire at right angles. This is called the motor effect.
F = BIL
A coil of wire carrying a current in a magnetic field tends to rotate. This is the basis of an electric motor.
What is Flemings Left Hand Rule?
Direction of current is always from positive to negative.
How is sound created by loudspeakers and headphones?
Speakers use the motor effect to convert variations in current in electrical circuits to the pressure variations in sound waves.
1) a current in the coil creates a magnetic field
2) the magnetic field interacts with the permanent magnet generating a force, which pushes the cone outwards
3) the current is made to flow in the opposite direction
4) the direction of the magnetic field reverses
5) the force on the cone now pulls it back in
6) repeatedly alternating the current direction makes the cone vibrate in and out
7) the cone vibrations cause pressure variations in the air - which are sound waves
What is the generator effect?
How can you increase the induced potential difference and induced current?
Direction of current switches when direction of movement also switches.
If an electrical conductor moves relative to a magnetic field or if there is a change in the magnetic field around a conductor, a potential difference is induced across the ends of the conductor. If the conductor is part of a complete circuit, a current is induced in the conductor. This is called the generator effect.
The generator effect is the induction of a potential difference (leading to a current) when a wire experiences a change in magnetic field. In a closed circuit a current is induced
For example, if a magnet is moved into a coil of wire, the induced magnetic field tends to repel the magnet back out of the coil. This effect occurs whether a magnet is moved into a coil, or a coil is moved around a magnet.
An induced potential difference or induced current will increase if:
- the speed of movement is increased
- the magnetic field strength is increased
- the number of TURNS on the coil is increased
What is an Alternator?
What is a Dynamo
An alternator is a coil of wire rotating in a magnetic field. Produces a alternating current.
A Dynamo produces a direct current and contains a split-ring commutator.
How does a microphone use the generator effect to induce a changing current from the pressure variations of sound waves?
1) pressure variations in sound waves cause the flexible diaphragm to vibrate
2) the vibrations of the diaphragm cause vibrations in the coil
3) the coil moves relative to a permanent magnet, so a potential difference is induced in the coil
4) the coil is part of a complete circuit, so the induced potential difference causes a current to flow around the circuit
5) the changing size and direction of the induced current matches the vibrations of the coil
6) the electrical signals generated match the pressure variations in the sound waves
What are transformers?
What is the basic transformer made from?
Transformers
A transformer is a device which can change the potential difference of an alternating current.
A basic transformer is made from two coils of wire, a primary coil from the alternating current (ac) input and a secondary coil leading to the ac output. The coils are not electrically connected. Instead, they are wound around an iron core. Iron is easily magnetised and can carry **magnetic fields** from the primary coil to the secondary coil.
What is the equation to calculate power, primary voltage and secondary voltage?
Example Question?
Vs × Is = V p × I p
Where Vs × Is is the power output (secondary coil) and Vp × Ip is the power input (primary coil).
What is the national grid?
What does national grid do to decrease the energy transfers to the environment?
What is an Electromagnet useful for?
Examples of Electromagnetic Devices
- Electromagnets are used in several devices, for example, a scrapyard crane or an electric bell
- Scrapyard cranes:
- When the electromagnet is switched on it will attract magnetic materials
- When the electromagnet is switched off it will drop the magnetic materials
- Electric bell:
Animation showing an electric bell in operation
- When the button K is pressed:
- A current passes through the electromagnet E creating a magnetic field
- This attracted the iron armature A, causing the hammer to strike the bell B
- The movement of the armature breaks the circuit at T
- This stops the current, destroying the magnetic field and so the armature returns to its previous position
- This re-establishes the circuit, and the whole process starts again
What is the right hand grip rule?
- The right-hand thumb rule can be used to work out the direction of the magnetic field
- Reversing the direction in which the current flows through the wire will reverse the direction of the magnetic field
Use Fleming’s left-hand rule to show that if the current-carrying wire is placed into the magnetic field between the poles of the magnet, as shown below, there will be a downwards force acting on the wire.
Step 1: Determine the direction of the magnetic field
- Start by pointing your First Finger in the direction of the (magnetic) Field.
Step 2: Determine the direction of the current
- Now rotate your hand around the first finger so that the seCond finger points in the direction of the Current
Step 3: Determine the direction of the force
- The THumb will now be pointing in the direction of the THrust (the force)
- Therefore, this will be the direction in which the wire will move
- The THumb will now be pointing in the direction of the THrust (the force)
How does a DC motor work?
The motor effect can be used to create a simple d.c electric motor
- When the current is flowing in the coil at 90o to the direction of the magnetic field:
- The current creates a magnetic field around the coil
- The magnetic field produced around the coil interacts with the field produced by the magnets
- This results in a force being exerted on the coil
- The direction of the force can be determined using Fleming’s left-hand rule
- As current will flow in opposite directions on each side of the coil, the force produced from the magnetic field will push one side of the coil up and the other side of the coil down
- This will cause the coil to rotate, and it will continue to rotate until it is in the vertical position
- When the coil is in the vertical position there will be a force acting upwards and a force acting downwards
The split ring commutator swaps the contacts of the coil
* This **reverses the direction in which the current is flowing** * **Reversing the direction of the current will also reverse the direction in which the force**s are acting * As a result, the **coil will continue** to **rotate** * The **commutator reverses the direction of the current in the coil every** **half turn** * This will keep the c**oil rotating continuously as long as the current is flowing**
Factors Affecting the D.C Motor
- The speed at which the coil rotates can be increased by:
- Increasing the current
- Use a stronger magnet
- The direction of rotation of coil in the d.c motor can be changed by:
- Reversing the direction of the current
- Reversing the direction of the magnetic field by reversing the poles of the magnet
- The force supplied by the motor can be increased by:
- Increasing the current in the coil
- Increasing the strength of the magnetic field
- Adding more turns to the coil
Alternating Potential Difference trace on an oscilloscope?
- An alternator consists of a rotating magnet next to (or inside) a coil
- When the magnet is in position 1 the magnetic field lines of the magnet do not cut the coil
- This means that there is no potential difference induced in the coil
- When the magnet is in position 2 the magnetic field lines of the magnet are at 90o to the coil
- This means that there will be maximum potential difference induced in the coil
- When the magnet is in position 3 the magnetic field lines of the magnet do not cut the coil
- This means that there is no potential difference induced in the coil
- When the magnet is in position 4 the magnetic field lines of the magnet are at 90o to the coil
- This means that there will be maximum potential difference induced in the coil
- As the poles of the magnet are reversed compared to position 2 the induced potential difference will also be in the opposite direction compared to position 2
- This means that the trace is negative
How does the Pd differ on a Dynamo?
Direct Potential Difference Trace on an Oscilloscope
- A dynamo consists of a coil, which is rotated by a mechanical process, inside a magnetic field
- When the coil is in position 1 the magnetic field lines of the magnet do not cut the coil
- This means that there is no potential difference induced in the coil
- When the coil is in position 2 the magnetic field lines of the magnet are at 90o to the coil
- This means that there will be maximum potential difference induced in the coil
- When the coil is in position 3 the magnetic field lines of the magnet do not cut the coil
- This means that there is no potential difference induced in the coil
- When the coil is in position 4 the magnetic field lines of the magnet are at 90o to the coil
- This means that there will be maximum potential difference induced in the coil
- The trace will remain positive because there has been no change to the direction of the magnetic field
