6.3 Electromagnetism Flashcards
What causes magnetic fields? (2)
Moving charges
Permanent magnets
How do magnetic field lines represent magnetic fields? (2)
The arrow points from north to south
The spacing represents the field strength
How is a uniform magnetic field represented?
Equally spaced, parallel lines
Magnetic field patterns - current-carrying wires
Field lines are concentric circles centred on the wire and perpendicular to it
What is the right-hand grip rule? (2)
In a current carrying wire, the thumb points in the direction of the conventional current and fingers point in the direction of the field
In a solenoid, the thumb points in the direction of the field and the fingers point in the direction of the conventional current
What is Fleming’s left hand rule? (3)
Thumb gives direction of force
First finger gives the direction of the external field
Second finger gives direction of conventional current
What does the magnitude of the force experienced by a wire in an external magnetic field depend on? (4)
Angle of wire to field
Current
Length of wire in field
Strength of the magnetic field
When is the force experienced by a wire in an external magnetic field at a maximum?
When the wire is perpendicular to the magnetic field
Define magnetic flux density
The strength of the field
How is magnetic flux density determined experimentally? (5)
Place two magnets on top of each other on a top pan balance with a stiff copper wire between the two poles
Measure the length of the wire in the field with a ruler
Connect the wire in series with an ammeter and variable power supply using crocodile clips
When a current flows through the wire, an upwards force is exerted on it so, according to Newton’s third law, the magnets experience an equal downwards force which can be calculated from the mass reading on the balance
Magnetic flux density can be calculated using B = F/IL
How does a charged particle move in a magnetic field? (4)
As the charged particles enter the field, they experience a force according to Fleming’s left hand rule
They change direction but the force always remains perpendicular to the velocity
The speed of the electrons remains the same as there is no component of force in that direction
This causes the charged particles to move in circular motion until they leave the field and move in a straight line
What is a velocity selector? (2)
A device that uses an electric field and a perpendicular magnetic field to select charged particles of a specific velocity
The forces from the electric and magnetic fields act in opposite directions and only one specific velocity will cause the two forces to cancel out, allowing these particles to travel in a straight line and emerge through the opposite narrow slit
Why does a magnet moving in a coil cause electromagnetic induction? (2)
Some of the work done to move the magnet is transferred into electrical energy
The relative motion of the coil causes the electrons to move as they experience a force from the field, causing a current to be induced
Define magnetic flux
The product of the component of the magnetic flux density perpendicular to the cross-sectional area
Define magnetic flux linkage
The product of the number of turns in the coil and the magnetic flux
When is an EMF induced?
Whenever there is a change in the magnetic flux linking the circuit
State Faraday’s law
The magnitude of the induced EMF is directly proportional to the rate of change of magnetic flux linkage
Explain Lenz’s law (3)
When a magnet and coil are brought close together, the current induced in the end of the coil is such that the polarity at the end of the coil is the same as the polarity of the pole of the magnet
The work done on the magnet = electrical energy produced in the coil
The induced pole cannot be the opposite pole because the principle of conservation of energy would be violated, producing electrical energy out of nothing
State Lenz’s law
The direction of the induced current is always such as to oppose the change producing it
What is a simple AC generator?
A rectangular coil rotating in a uniform magnetic field
What does an flux linkage-time graph show for an AC generator? (2)
The gradient is equal to the induced EMF
If the coil rotates at a steady frequency, the variation is sinusoidal
What does maximum induced EMF depend on? (4)
Magnetic flux density
Cross-sectional area of the coil
Number of turns
Frequency of rotating coil
How does a simple transformer work? (2)
An alternating current in the primary coil produces a varying magnetic flux in the iron core
The varying magnetic flux induces a varying EMF across the secondary coil
What is a step-up transformer?
A transformer with more turns on the secondary coil than the primary coil
What is a step-down transformer?
A transformer with fewer turns on the secondary coil than the primary coil
How are transformers made more efficient? (3)
Low-resistance windings to reduce power loss die to the heating effect of the current
Laminated core minimises eddy currents induced in the core itself
Use soft iron as it is easy to magnetise and demagnetise
Why is electrical power transmitted at high voltage?
To minimise heat losses
