Electric and Magnetic Fields Flashcards
SP12a Magnets and magnetic fields
Describe the differences between a permanent magnet and an induced magnet.
Permanent Magnets;
- This means the material is always magnetic
- A magnet can attract magnetic materials, these include the metals; iron, steel, nickel and cobalt.
- The space around a magnet where it can attract these materials is called it’s magnetic field.
Induced Magnets;
- When a piece of magnetic material enters the a magnetic field, it becomes a magnet itself. This is called an induced magnet.
- It stops being magnetic when taking out of the field again.
SP12a Magnets and magnetic fields
Describe the shape and direction of a magnetic field of a;
A) Bar magnet
B) Uniform field.
A) The field lines are leaving the north-seeking pole and are curving towards the south seeking pole, forming an oval-esq shape.
- As the magnetic field is strongest where there are most lines, this would be at the north & south seeking poles
B) The field lines leave the north-seeking pole and travel in a straight line to the other magnet’s south-seeking pole.
- Due to all field lines being equally spread out, there magnetic field strength is equall throughout the uniform field.
SP12a Magnets and magnetic fields
Describe how a plotting compass can be used to to show the shape and direction of a magnetic field.
Bar Magnet:
1. Place the plotting compass near the magnet on a piece of paper.
2. Mark the direction the compass needle points.
3. Move the plotting compass to many different positions in the magnetic field, marking the needle direction each time
4. Join the points to show the field lines
5. Key Note - The needle of a plotting compass points to the south pole of the magnet.
Earth:
1. The earth has a similar shape to the magnetic field of a bar magnet.
2. As the compass needle will always point towards the Earth’s north pole, that can be used to identify it’s magnetic field.
SP12a Magnets and magnetic fields
Explain how the behaivour of a magnetic compass is proof that the earth’s core is magnetic.
- A magnet suspended on a string will tilt relative to the horizontal by different amounts in different places, compass needles are weighted at one end to keep them level.
- This behaivour of compasses is evidence that the earth has a magnetic field, which is similar to that of a bar magnet.
SP12b Electromagnetism
Describe the effect of a current on a wire.
1. A current flowing through a wire causes a magnetic field.
- You can find the direction of the magnetic field using a compass.
2. The direction of the magnetic field depends on the direction of the current.
- The strength of the magnetic field depends on the strength of the current & the distance from the wire. ( Stronger when closer, weaker when further )
SP12b Electromagnetism
Describe what is ment by an Electromagnet, the magnetic field inside of one & how it can be made stronger.
- An electromagnet is a coil of wire with a current flowing through it, a solenoid ( wire made into a coil ) is an example of this.
- The fields from individual coils add up to make a very strong field inside of the solenoid.
- However, outside the solenoid the fields from one side of the coil tends to cancel out the fields from the other side of the coil, resulting in a weaker field outside the solenoid.
- The magnetic field can be made stronger by putting an iron core inside of an electromagnet. ( The iron becomes an induced magnet )
SP12c Magnetic forces
Describe & Explain the interaction between a current carrying conductor and a magnet.
- A wire carrying current experiences a force when it is placed inbetween 2 magnets.
- The current in the wire creates a magnetic field around the wire which interacts with the magnetic field between the magnets.
- This will produce a magnetic force on the wire that is greatest when the wire is at right angles to the magnetic field produced by the magnets, & 0 when the wire is in the same direction as the magnetic field.
- There is an equal and opposite force on the magnets.
Motor effect - The idea that a current carrying wire in the presence of a magnetic field will experience a force.
SP12a Magnets and magnetic fields
Explain how Electric motors work.
1. The force on a conductor in a magnetic field is used to cause rotation in electric motors.
2. Due to the current flowing through the wire switching directions every half-turn, the direction of the force also switches every half-turn.
- This will prevent the coil from actually turning 360degrees
3. A Split-Ring commutator is used to prevent this, it swaps the positive and negative connections every half-turn, this swaps the direction of the current every half turn which means the force on the coil stays acting in a clock-wise manner.
Sp13a Electromagnetic induction
Explain how an electric current is produced with a magnet & a conductor on a small scale in the lab
- Note: A changing magnetic field can induce a p.d in a wire, causing current to flow.
- A p.d can also be induced if a wire is moved in a magnetic field.
- The wire is often made into a coil so there is more wire in the changing magnetic field.
- Size of induced p.d depends on; number of turns in coil, strength of magnetic field & how fast the magnetic field changes or moves past the coil
- Reversing the direction of change ( + -> - or - -> + ) reverses the direction of the induced p.d.
- If the p.d causes a current to flow, the magnetic field of the current opposes the original change.
SP13a Electromagnetic induction
Describe how a generator works.
- A generator consists of a coil of wire that is rotated inside a magnetic field.
- As the coil turns, a voltage is induced in the wire, The ends of the coils are connected to slip rings.
- Electrical contact with an external circuit is made using carbon brushes, which press on the slip rings.
- A generator like this produces an alternating current, this is often called an alternator.
SP13a Electromagnetic induction
Explain how microphones work in producing a current.
Microphones:
1. They convert the pressure variations in sound waves into variations in current in electrical circuits.
2. This happens as;
- 1. Sound waves cause variations in air pressure.
- 2. The pressure variations in the air causes a diaphragm to vibrate.
- 3. The diaphragm moves a coil of wire backwards and forwards.
- 4. We know a wire moving in a magnetic field will induce a p.d, this will then induce a varying current.
- 5. It is varying as the speed of the sound waves will vary as well.
Note: Varying is NOT the same as alternating.
SP13a Electromagnetic induction
Explain how Loudspeakers work in producing sound waves.
1. Loudspeakers convert variations in an electrical current into sounds waves ( the opposite of a microphone )
- Loudspeakers have similar components to a microphone.
2. The varying current flows through a coil that is in a magnetic field, this causes a force on the coil, which moves back, which moves backwards and forwards as the current varies.
3. The coil is connected to a diaphragm, and the movements of the diaphragm produce sound waves.
SP13a Electromagnetic induction
A) Explain how transformers work.
1. First, an alternating current is passed through the primary coil ( induced via alternating p.d ), this creates an alternating mangetic field that induces the iron core.
- Iron core is used because it can be magnetised easily
2. The magnetic field around the the iron core then induces a p.d in the secondary wire, this will then cause current to flow if the transformer is connected in a circuit.
SP13b The national grid
Explain why transformers are used
1. In the national grid, power stations may transfery energy in transmission lines at high voltages as this means there will be a lower current.
- A lower current is good as it means there is less electrons to collide with the ionic lattice structure of a resistor, meaning their will be less energy lost to heating. ( Improving effiency )
- This is the purpose of a step-up transformer.
2. Step-down transformers are used when this electrical energy is going to houses or factories, this is because to high a voltage is dangerous for people.
SP13c Transformers and energy
Explain how the following formulas;
- Power ( W ) = Energy transferred ( J ) / Time taken ( s )
- Electrical power ( W ) = Current ( A ) x Voltage ( V )
- Electrical power ( W ) = Current^2 ( A ) x Resistance ( Ω )
Show the advantages of high voltage cables.
( When given the voltage, power & resistance )
1. First work out the current by rearranging P = I x V to I = P / V
2. Then using the formula P = I^2 x R, we can work out the amount of power being transferred to the surroundings per second.
3. Then we can work out Energy transferred per hour by doing I = P x 3600s ( 1 hour )
