16. P15 Magnetism Flashcards
Permanent magnets
Metal with their own magnetic field
Induced magnets
Metals which become magnetic when placed in a magnetic field - lose their magnetism when taken out
3 magnetic metals (can be induced)
Iron Nickel Cobalt
Direction of magnetic field
North to South
magnetic field around a bar magnet
…
Describe how you use the right hand rule (cork screw rule)
Align your right hand thumb with the direction of conventional current along the wire, the fingers align to show direction of the circular magnetic field around the wire.
The strength of a magnetic field around a solenoid can be increased by…?
- Increase the size of current through wire, 2. Increase the number of coils, 3. Use an iron core inside the coils (becomes an electromagnet)
Describe the shape of the magnetic field inside the coils fo a solenoid.
Strong (lines close together), uniform and parallel (running down the middle of the coils)
Describe the shape of the magnetic field around the outside of a solenoid.
Similar to the magnetic field around a bar magnet.
To increase the strength of a magnetic field around a current carrying wire.
Coil the wire into a solenoid
State what the three fingers represent in Flemings Left Hand Rule.
Thumb = motion/force, First finger = magnetic field direction, middle finger = current
Describe what is meant by the motor effect?
When a current carrying wire sits within a permanent magnetic field, the wire experiences a force.
What three factors affect the size of the force exerted on a current carrying wire, within a magnetic field.
1) Size of current, 2) Length of wire, 3) Strength of magnetic field.
Describe how the electric bell works? (Physics Only)
Person closes switch > circuit is complete current flows > electromagnet is turned on > attracts the iron armature hammer hits the gong > breaks the iron contact > circuit is open > no current flows > electromagnet is demagnetised > armature flicks back into place > circuit is closed again > process repeats
Describe how a relay circuit works. (Physics Only)
Person closes switch > current flows in input circuit > electromagnet is turned on > L shaped armature is attracted and so perpendicular end presses switch in output circuit > current flows in output circuit
What is the symbol and units for magnetic field strength?
B - measured in Tesla (T)
What is the symbol and units for current?
I - measured in Amperes
Purpose of a split ring commutator in an electric motor?
It causes the current travelling through the coil, to change direction every half-turn, so motor turns in one direction.
How to increase the torque in a motor?
1) Increase current, 2) Increase number of coils, 3) Increase strength of permanent magnets (magnetic field)
How loudspeakers work (Physics Only)
The current flows through the solenoid produces a magnetic field which its size and direction depend on the current flowing. This coil is placed in the middle of the permanent magnetic field, so the wires experience a force and move depending on the direction and size of the current. This makes the cone vibrate and produce sound.
The generator effect (Physics Only)
If a wire moves with respect to a magnetic field, the changing field cutting the wire induces a current in the wire.
To increase the size of the current induced as it moves through a magnetic field (Physics Only)
1) Move the wire/coil faster, 2)Stronger magnetic field, 3) coil the wire with more turns.
What do the slip rings and brushes do in an AC generator (Physics Only)
The slip rings allow the frame(coils of wire) to turn with out permanent wire connections getting tangled
How alternators work (alternating current) (Physics Only)
The magnet rotates, meaning the direction of the magnetic field is changing, thus inducing an alternating current
DC Generator graph (Physics Only)
The current is always positive (always going in the same direction) but increases and decreases in value as the wire is upright to the magnetic field before rotating back into being parallel with the current still going in the same direction, due to the commutator
AC Generator graph (Physics Only)
The current alternates between positive and negative (switches directions) as the magnet continuously turns, thereby changing the direction of the magnetic field, thus changing the direction of the current
Transformers
Transformers are used to increase the current from a power input to an output
How transformers work
A transformer consists of an iron core with a primary coil and a secondary coil
Why do transformers only work with an alternating current?
The alternating current, creates a magnetic field which is continuously changing direction, which provides the movement required to induce the voltage in the secondary coil
Step up transformers have more/fewer turns on the secondary
more
Step down transfers have more/fewer turns on the secondary
fewer
How to work out the change in voltage
[No. of turns (primary coil)] ÷ [No. of turns (secondary coil)] = [Voltage (primary coil)] ÷ [Voltage (secondary coil)]
