Physics topic 7 Flashcards
(12 cards)
Explain why magnets produce magnetic fields
1) all magnets have two poles, a north and south, meaning they produce a magnetic field where other magnets or magnetic materials experience non-contact force.
2) you can show a magnetic field by drawing magnetic field lines, which always go from north to south, and showing which way the force will act on a north pole.
3) the closer together lines, the stronger the magnetic field, the further away from the magnet you get the weaker the field is.
4) the magnetic field is strongest at the poles of a magnet. Meaning, the magnetic forces are always strongest at the poles.
5) the force between a magnet, and a magnetic material is always attractive, no matter the pole.
6) if two poles of a magnet to put near to each other, they will exert a force on each other. This force can be attractive or repulsive: two poles that are the same will repel each other, two unlike poles will attract each other.
How does a compass show the directions of a magnetic field?
- A bar magnet making it means North Pole will attract south pole of any other magnet nearby, so will point in the direction of the magnetic field.
- You can move the compass around a magnet and trace its position on paper to build up a picture of what the magnetic field looks like.
- When they’re not near a magnet, compasses always point north, this is because the Earth generates its own magnetic field, which shows the inside of the Earth must be magnetic.
What does it mean that magnets can be permanent or induced?
Permanent magnets produce their own magnetic field, whereas induced magnets are magnetic materials that turn into a magnet when they’re put into a magnetic field. The force between permanent and induced is always attractive. When you take away the magnetic field, induced magnet quickly lose their magnetism and stop producing a magnetic field.
How does a moving charge create a magnetic field?
When a current flows a magnetic field is created around the wire. The field is made up of concentric circles perpendicular to the wire. You can see this by placing a compass nearby that is carrying the compass. As you move the compass, it will trace the direction of the magnetic field, changing the direction of the current changes, the direction of the magnetic field. Use the right hand thumb rule to work out which way it goes. The strength of the magnetic field produced changes with the current and distance from the wire, the larger the current or the closer you are to the wire, the stronger the field will be.
What is the right-hand thumb rule?
Using a right-hand point your thumb in the direction of the current income your fingers the direction of your fingers is the direction of the field.
What is a solenoid?
You can increase the strength of the magnetic field the wire produces by wrapping the wire into a coil called a solenoid. This is because the field line up around each loop of wire. This results in lots of field lines, pointing in the same direction to very close together, meaning the field is stronger. You can increase the field strength even more by putting a block of iron in the centre of the coil. This iron core becomes an induced magnet where ever the current is flowing certain wheat with an iron core is called an electromagnet.
Key details about what a solenoid is…
The magnetic field inside a solenoid is very strong and uniform. Outside the coil, the magnetic field is just like the one around a bar magnet. Meaning that the end of a solenoid act like the North Pole and South Pole of a bar magnet. You can work out which end of the solenoids is the north pole, and which is the South Pole, using the right-hand rule.
What happens when a current carrying wire is put between magnetic poles?
The magnetic field around the wire interacts with the magnetic field It has been placed in. This causes the magnet and the conductor to exert a force on each other. This is called the motor effect and can cause the wire to move.
What is the motor effect?
To experience the full force of magnetism the wire has to be at 90° to the magnetic field. The force always acts at right angles to the magnetic field of the magnets and the direction of the current in the wire. A good way of showing the direction of the force is to apply a current to a set of rails inside a horseshoe magnet. A bar is placed on the rails, which completes the circuit, generating a force which will roll the bar. The magnitude of the force increases with the strength of the magnetic field. The force also increases with the amount of current passing through the conductor.
What is the equation of finding the size of a force?
You can remember it as FBI,
Force (newtons) = magnetic flux density (Tesla) x current (amps) x length (metres)
What is Flemings left hand rule?
Point your thumb upwards, your first finger forwards and your second finger at 90° to your first finger.
thuMb (Motion)
First Finger (field)
seCond finger (Current)
Why does a current carrying coil of wire rotate in a magnetic field?
Explain
1) forces act on the two side arms of a coil of wire that is carrying a current.
2) because the coil is on a spindle, and the forces act one up and one down, it rotates.
3) the split ring, commutator is a clever way of swapping the contacts every half term to keep the motor rotating in the same direction.
4) the direction of the motor can be reversed, either by swapping the polarity of the DC supply or swapping the magnetic poles.
5) the speed of the motor can be increased by increasing the current adding more times to the coil or increasing the magnetic flux density.
6) you can use Flemings left hand rule to work out, which way the coil turn.
