Physics - Electromagnetic Induction Flashcards
How can you induce a voltage across a conductor?
Relative motion between the conductor and magnetic field or if the field changes
Describe induction*
Occurs due to the ‘cutting’ of field lines or changing flux through a loop of wire
List the factors that affect the size of the induced voltage
Rate of change, strength of field and number of turns on conductor if coiled
Distinguish between the motor effect and electromagnetic induction
The motor effect – force on a current carrying wire in a magnetic field
Electromagnetic induction – potential difference across a wire due to the motion in a magnetic field
How is the induced field affected by the cross-sectional area of the coil and its alignment relative to the field?
Surface area perpendicular to field for maximum potential. Larger the surface the greater the potential
Describe the basic principle of an AC generator
Magnet rotating within a coil or coil rotating within a magnetic field induces an EMF due to the changing magnetic field lines through the coil
Describe the function of the slip-rings and brushes
Slip rings and brushes allows a current to flow through the contacts as the coil rotates
What factors affect the size of the generated voltage?
Rate of rotation, strength of field, number of turns on coil
Where is the AC generator used?
Mains electricity generation in power stations
How are DC generators different to AC generators?
AC generators always have the terminal touching the same slip ring
DC generators use a commutator so the current only flows in one direction.
Draw a diagram of a transformer
(See diagram) Must include: the type of power supply, primary coil, secondary coil, iron core, primary circuit, secondary circuit, load
What are the stages that produce a current in the secondary coil?
- Power supply causes a current to flow in the wire.
- The current causes a magnetic field to be generated in the primary coil
- The iron core directs the magnetic field to the secondary coil
- The current is alternating, so produces an alternating magnetic field in the secondary coil
- The changing magnetic field in the secondary coil induces a current in the secondary coil.
- This current flows round the secondary circuit and powers the device.
Why can we not say the electrical energy from the power supply is transferred directly to the load in the secondary circuit?
There are no electrons passing from the primary to the secondary coil. At no point are they connected
Describe how to change the voltage in the secondary coil relative to the primary coil
Have different numbers of coils in the primary and secondary coils
What is the equation that allows you to calculate how the voltage changes depending on the number of turns in the coils of the primary and secondary coils.
V_p/V_s =N_p/N_s
Primary voltage divided by secondary voltage = number of primary coils divided by number of secondary coils
Describe what a step-up and step-down transformer does
Step-up: Increases the voltage in the secondary relative to the primary
Step-down: decreases the voltage in the secondary relative to the primary
Assuming 100% energy efficiency, write down an equation that links the current and voltage of the primary and secondary coils
P_p=P_s
I_p x V_p=I_s x V_s
Primary power = secondary power
Primary current x primary voltage = Secondary current x secondary voltage
Basically the power is the same
What can cause inefficiency in a transformer and how can these be reduced?
Resistance of the wire in the coil - Better conductors or superconductors
Eddy currents can form in the iron core - This can be reduced by laminating sheets of iron to reduce the current
Leakage of field lines, not all the field lines produced in the primary cut the secondary coil - Reduce any gaps in the coil.
Define flux linkage
It is the amount of magnetic flux going through a coil or loop of wire.
It is therefore the magnetic field strength times the area of the coil/loop
Write down the equation describing Ohm’s Law
V=IR
Write down the equation for power
P = IV = V^2/R = I^2 R
Power = current x voltage = current squared x resistance
Explain how the power lost when passing electricity through a wire depends on the current and the resistance of the wire
The power lost in a wire is the resistance of the wire and this is increased as the current (flow of electrons) increases. The power lost is given by the equation P=I^2 R
Use the equation about power loss to explain whether we want high voltage or low voltage going through power cables
High voltage, as it leads to low current (therefore less resistance and less energy lost)
What type of transformer would we use when taking electrical power form a power station and transferring it through power cables?
Step-up so the voltage is increased and the resistance decreases. This means that when it is travelling through the power lines less energy is lost
Why can we not use the same voltage in the power lines in our homes?
It is too dangerous, because they are more likely to discharge through normally insulating materials (e.g. jump through the air)
What type of transformer would we use when taking electricity from power lines and transferring it to homes?
Step-down so the voltage is reduced to a safe level for the home.
Describe how a DC rectifier works
Uses diodes to only allow current through in one direction
