SP12 and 13 Flashcards
What happens to unlike magnetic poles?
- They attract
What happens to like magnetic poles?
- They repel
What is a permanent magnet?
- A magnet that is always magnetic and cannot be turned on and off
What is an induced magnet?
- A magnet that can be turned off and on and is only magnetic when placed inside a magnetic field
What are some uses of magnetic materials?
- Compasses
- Motors
- Knife holders
- Loudspeakers
Which way do arrows point in a magnetic field when drawing lines?
- Field lines go in from the South pole and out from the North pole
How do field lines show strength of a magnetic field?
- When field lines are denser (more close together), the field is stronger
How can plotting compasses be used to show shape of magnetic field?
- The tiny bar magnet points in the direction of the magnetic field, which can show the direction of the field at different points around a magnet
How do compasses show that the Earth’s core is magnetic?
- The tilt by different amounts, relative to the horizontal. This shows how the magnetic field of the Earth causes a shift in the compass position
What is the shape of the magnetic field produced when a current is passed through a wire?
- Magnetic field made of multiple circles around each other, closest together nearest to the wire
- The ‘Right hand grip’ rule can be used to how the direction of the field compared to the current flow
- As distance from the wire increases, the strength of the magnetic field decreases
What is a solenoid and describe the magnetic field in it?
- A solenoid is a series of coils of wire, one after the other
- The magnetic field is strong and uniform within a solenoid, because they add together
- The magnetic field is weaker on the outside as they cancel each other
What are four ways of increasing the strength of an electromagnet?
- Increase the current flowing through the solenoid
- Increase the number of turns (or coil) in the solenoid
- Decrease the length of the solenoid, whilst keeping number of turns the same
- Add a soft iron core in the middle of the solenoid
What happens when a current carrying conductor is placed near a magnet?
- It experiences a force, whilst an equal and opposite force acts on the magnet (this is known as the Motor Effect)
How does a magnetic force occur?
- It is due to interactions between magnetic fields, where it will either cause attraction or repulsion
What is Fleming’s left-hand rule?
- A method of determining the force on a conductor carrying a current
- Can be done by holding your left hand, with the first three fingers at right angles from each other
- The direction of the thumb represents Force, the index finger represents the Magnetic field and the middle finger represents the Current
How do you calculate the strength of the force?
F = B x I x L
- F= force and is measured in Newtons (N)
- B=magnetic flux density and is measured in Teslas (T)
- I= current and is measured in Amperes (A)
- L=length and is measured in metres (M)
How does an electric motor spin?
- There is a magnetic field caused by two magnets on either side of the motor
- There is a current flowing through the motor, which flows in opposite directions at either end
- Due to Fleming’s left-hand rule, each side of the motor will experience an opposite force, causing it to spin clockwise
What is the role of a split-ring commutator?
- Every 180 degrees that the motor turns, the currents will now be facing in opposite directions, resulting in the forces acting in opposite directions and the motor turning anticlockwise
- To overcome this, a split-ring ring commutator swaps the positive and negative connections every half-turn, allowing the currents to swap so that the motor can turn a full 360 degrees as the forces will always act in the same direction
What are 3 ways of increasing the speed of rotations in a motor?
- Increase the current
- Increase the number of coils
- Increase magnetic flux density by increasing magnet strength
What is electromagnetic induction?
- The effect seen as the movement of a wire or magnet in a field will induce a voltage
- Each time direction changes, the direction of the potential difference changes
What factors affect the size of the induced potential difference- and therefore the current?
- Change strength of magnetic field by using stronger magnets
- Move wire/magnets quicker, to faster change the field
- Shape wire into a coil, as more turns mean an increased potential difference
What factors affect the direction of the potential difference?
- Direction by which magnet-wire is moved into/out of the field
- Direction of poles in the magnet
How does a generator produce an alternating current?
- Consists of a coil of wire rotated inside a magnetic field
- As the coil rotates, a potential difference is induced
- The coils are connected to slip rings, which come into contact with carbon brushes, which press on them and allow a current to flow
- These types of generators are called alternators
How does a generator produce a direct current?
- It uses a split-ring commutator, which switches the connections every half-turn and allows a direct current to flow
- This is known as a dynamo
How does a microphone work?
- A person singing causes sound waves and air pressure that hit the diaphragm and causes vibrations
- This causes the coil of wire within the microphone to move backwards and forwards
- This results in electromagnetic induction, causing a current and allowing the signal to be amplified
How does a loudspeaker work?
- Similar to microphone, yet it starts out with current and ends with air pressure
- An alternating current and magnetic field within the speaker causes a force to be achieved (Fleming’s left-hand rule)
- The AC means the force moves backwards and forwards, causing a paper cone to do so too, causing vibrations and amplifying the noise
What is the role of a transformer?
To increase (step-up) or decrease (step-down) the size of an alternating voltage
How does a transformer work?
- An alternating potential difference is applied across the primary coil, causing a current to flow and generate a magnetic field around the coil
- This induces an alternating magnetic field in the iron core
- This induces a potential difference in the secondary coil
- If this is part of a complete circuit, it causes current to flow
How does a transformer step-up or step-down the voltage?
- By increasing or decreasing the number of turns in the secondary coil, it increases or decreases the potential difference, respectively
Why and where are transformers used?
- An increase in voltage means that the wire gets hitter, and so more energy is lost through heating
- Transformers step down the voltage before entering a transmission line, to reduce energy lost as electricity travels over a long distance
- They then step up the voltage at the end, so more electricity can travel to homes and power buildings
