Electromagnetism Flashcards
What 2 things happens when an electric current flows through a wire?
Heats up
Creates a magnetic field
Give 3 rules of electromagnetic fields caused by wires?
As current increases, magnetic field gets stronger
Magnetic field gets weaker further away from wire
Direction of lines can be found using compass
Give 2 rules about magnetic fields created by coils?
In a coil, the magnetic field can be made stronger by increasing the current
The magnetic field can be made stronger by increasing the number of turns in a coil
What is a solenoid?
A wire that is wrapped in to a coil, and has current passed through it creating a magnetic field
Give 3 ways the strength of a magnetic field created by a solenoid can be increased?
Using larger current
Using more turns in wire
Putting soft iron rod in to middle of solenoid
What does putting a soft iron rod in to the middle of a solenoid turn the coil in to?
A simple electromagnet
Give advantages of electromagnets over magnets?
Strength of an electromagnet can be varied
The electromagnet can be switched off
Why is steel not a suitable core for an electromagnet? And why is a soft iron rod better?
Steel would be permanently magnetised after current passed through the coil. Soft iron is easily magnetised and demagnetised
Give 2 uses of electromagnets?
Separate metals from non metallic materials
To lift magnetic materials in scrapyards
What is the motor effect when a magnetic force acts on a current carrying wire?
A copper wire placed between 2 magnets of opposite poles has no force acting upon it as it is not magnetic. But when current passes through, a force is acted upon the wire
Explain the motor effect?
When current passes through the non magnetic wire, it creates its own magnetic field, which distorts the magnetic field of the 2 magnets. The distorted magnetic field usually then straightens, pushing against the magnetic field around the wire, and causing the wire to move
How can the direction of the wire in the motor effect be predicted?
Flemings left hand rule
In Flemings left hand rule, what does the;
Thumb represent
Index finger represent
Middle finger represent?
Thumb is motion of wire
Index is magnetic field of magnets
Middle is current in wire
Give 3 ways force of motor effect can be increased?
Current in wire increased
Stronger magnets
Length of wire exposed to magnetic field increased
What direction to magnetic field lines go?
From north to south
What direction does current flow in a circuit?
From the positive to the negative
What is the turning effect?
Basically the motor effect, but with a loop of wire between the 2 magnets. Opposite directions of current on either side of the loop cause one side of the loop to go up, and the other to go down, causing the loop to turn
Give 3 ways the turning effect can be increased?
Increasing the number of turns in the loop, to form a coil
Increasing current in loop
Increasing area of loop
Give 1 application of the turning effect?
The d.c. Electric motor
What is d.c.?
Direct current
What is electromagnetic induction?
When a conductor moves through a magnetic field, producing current
Give 4 factors that affect the current produced through electromagnetic induction?
Direction of movement
Speed of movement
Magnet strength
Creating loops of a conductor to make a coil
Explain how direction of movement can affect electromagnetic induction?
The conductor must cross the magnetic field lines, and so moving it vertically produces current, but laterally will not
Apart from moving a conductor through a magnetic field, how else can electromagnetic induction be caused?
By keeping the conductor stationary and moving the magnetic field
Why is current produced through electromagnetic induction?
The work done/ energy used when moving the conductor or the magnetic field, creates current.
Give an application of electromagnetic induction?
The a.c. generator, AKA the alternator
What are magnets?
These are materials which attract magnetic substances, such as iron, which is ferromagnetic
How is electricity generally generated and transmitted?
It’s generated at power stations, which use generators. This electricity is then fed to a huge step - up transformer which transfers power to overhead cables at a greatly increased voltage. These cables feed power to the grid, which is then distributed to substations. These contain step - down transformers which reduce the voltage. This power is then fed in to the homes
Why are step up transformers used to increase the voltage of the electricity generated in power stations?
They are used to reduce the current flowing in the cables, so as to waste less energy. This is because when current flows through a conductor ( the cables ) some energy is lost due to resistance
Describe the number of primary and secondary coils in a step up transformer? Why is this?
Primary coil has very few turns, but secondary coil has many turns. This is because of the transformer equation. So the more turns on the secondary coil than on the primary coil, the higher the voltage will have to be
Describe the number of turns in the primary and secondary coils in a step down transformer?
Primary has many turns, secondary has few turns
What core is often used in transformers? How does it work?
Laminated iron core, connects the coils magnetically
Why does current get lower as the voltage increases?
This is due to the law of conservation of energy, so the power entering the transformer, must equal the power leaving it
Give the transformer equation?
V(Primary) V ( Secondary )
————————- = ————————
No. Turns in coil (Primary) No. Turns in coil ( secondary )
How do transformers work?
Alternating current in primary coil converts coil in to electromagnet, which produces an alternating magnetic field. Core transports alternating field to secondary coil. Secondary coil is a conductor in changing magnetic field, so electricity is produced through electromagnetic induction.
What is the electromagnetic induction in transformers also known as?
Mutual induction
What type of current is the only current transformers work with?
a.c. Alternating current
Why do transformers not work with direct current?
No alternating current in primary coil, means no alternating magnetic field. This means no electromagnetic induction, and therefore no output voltage in secondary coil
Give 2 equations to calculate how much electrical power is lost due to the heating effect in resistors?
Power = Current2 X Resistance P = I2 X R
Power = Voltage2
——–
Resistance
P = V2
—
R
