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Questions and Answers
The high voltages used to transmit ______ power around a country are dangerous.
The high voltages used to transmit ______ power around a country are dangerous.
electrical
When a current flows in a wire or cable, some of the energy it is carrying is lost because of the cable's ______.
When a current flows in a wire or cable, some of the energy it is carrying is lost because of the cable's ______.
resistance
A small current wastes less energy than a ______ current.
A small current wastes less energy than a ______ current.
high
A transformer is a device used to increase or decrease the ______ of an electricity supply.
A transformer is a device used to increase or decrease the ______ of an electricity supply.
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A step-up transformer increases the voltage, so there are more turns on the ______ than on the primary.
A step-up transformer increases the voltage, so there are more turns on the ______ than on the primary.
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Transformers only work with ______ current (a.c.).
Transformers only work with ______ current (a.c.).
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The primary coil has alternating current flowing through it, producing an alternating ______ field.
The primary coil has alternating current flowing through it, producing an alternating ______ field.
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If a transformer is 100% efficient, no power is ______ in its coils or core.
If a transformer is 100% efficient, no power is ______ in its coils or core.
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A device that transforms mechanical energy into electrical energy is called a ______.
A device that transforms mechanical energy into electrical energy is called a ______.
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All generators have three things in common: a magnetic field, a coil of wire, and ______.
All generators have three things in common: a magnetic field, a coil of wire, and ______.
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The process of generating electricity from motion is called ______.
The process of generating electricity from motion is called ______.
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In an a.c. generator, the coil spins around in the magnetic field, inducing an ______.
In an a.c. generator, the coil spins around in the magnetic field, inducing an ______.
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A d.c. motor uses a split ring commutator, whereas an a.c. generator uses ______.
A d.c. motor uses a split ring commutator, whereas an a.c. generator uses ______.
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When one pole of the magnet moves downwards past the wire, a current ______.
When one pole of the magnet moves downwards past the wire, a current ______.
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If the magnet is held stationary next to the wire, no current ______.
If the magnet is held stationary next to the wire, no current ______.
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The principle of electromagnetic induction is crucial for devices like ______.
The principle of electromagnetic induction is crucial for devices like ______.
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Alternating current flows back and forth, unlike __________ current, which always flows in the same direction.
Alternating current flows back and forth, unlike __________ current, which always flows in the same direction.
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The __________ of an a.c. supply is the number of cycles it produces each second.
The __________ of an a.c. supply is the number of cycles it produces each second.
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If the magnet is stationary, there is no cutting of field lines and so no __________ is induced.
If the magnet is stationary, there is no cutting of field lines and so no __________ is induced.
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Using a coil with more __________ can increase the induced e.m.f.
Using a coil with more __________ can increase the induced e.m.f.
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Stronger magnets lead to greater cutting of magnetic field lines, resulting in a larger __________.
Stronger magnets lead to greater cutting of magnetic field lines, resulting in a larger __________.
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Electricity is usually generated at a __________ from where it is used.
Electricity is usually generated at a __________ from where it is used.
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Power lines enter a local distribution centre where the voltage is reduced to a less __________ level.
Power lines enter a local distribution centre where the voltage is reduced to a less __________ level.
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In the substation, transformers reduce the voltage to the local supply voltage, typically __________ V.
In the substation, transformers reduce the voltage to the local supply voltage, typically __________ V.
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Study Notes
Chapter 21: Electromagnetic Induction
- Learning Outcomes: Describe how an electromotive force (e.m.f.) is induced in a circuit, identify factors affecting e.m.f. magnitude and direction, describe the design of an alternating current (a.c.) generator, describe the construction of a transformer, use the transformer equation, explain how transformers work, and use the power equation for a transformer.
21.1 Generating Electricity
- Motor vs. Generator: A motor transforms electrical energy into mechanical energy, while a generator transforms mechanical energy into electrical energy.
- Generator Components: All generators have a magnetic field (from magnets or electromagnets), a coil of wire (fixed or moving), and movement relative to each other.
- Electromagnetic Induction: The process of generating electricity from motion.
- Moving Magnet/Stationary Wire: Moving a magnet near a wire induces a current in the wire, changing direction when the magnet is moving in the opposite direction.
- Stationary Magnet/Moving Wire: Moving a wire near a magnet produces a current, and the direction of the current changes depending on the direction of the wire movement.
- A.C. Generator Principle: A simple a.c. generator is similar to a d.c. motor operating in reverse. A spinning coil within a magnetic field generates an alternating current.
- Slip Rings: A.C. generators use slip rings to connect the coil to the external circuit, unlike the split ring commutator used in d.c. motors.
- Alternating Current (A.C.) Characteristics: A.C. current changes direction periodically and is represented by a sinusoidal wave. The frequency is the number of cycles per second.
21.1 Induction and Field Lines
- Field Lines and E.M.F: Field lines help understand factors affecting the magnitude and direction of the induced electromotive force (e.m.f.).
- Stationary Magnet: No cutting of field lines, no induced e.m.f.
- Moving Magnet: Faster movement = greater e.m.f.
- Distance and Field Lines: Further distance between magnet and wire = fewer field lines cut, smaller e.m.f.
- Coil vs. Single Wire: A coil generates a larger e.m.f. because each turn cuts field lines.
21.1 Increasing Generator Voltage
- Methods to Increase Voltage: Increasing the rotation speed of the coil, increasing the number of turns in the coil, using a coil with a larger area, or using stronger magnets, all increase the rate of cutting magnetic field lines, resulting in a larger induced e.m.f.
- Direction of Induced E.M.F: An induced current always flows in a direction such that its magnetic field opposes the change that causes it (Lenz's Law).
21.2 Power Lines and Transformers
- High Voltage Transmission: Electricity is generated remotely and transmitted via high-voltage lines to reduce energy loss during transmission.
- Local Distribution: The high voltage is reduced to a safer level for local use.
- Substations and Transformers: Transformers at substations reduce the voltage to a level suitable for homes and businesses (typically 230 V).
21.2 High Voltage Use
- Power Loss Relationship: Power loss in cables is proportional to the square of the current flowing in the cables. Reducing current for the same power, by increasing voltage, reduces the loss.
- Why High Voltages? High voltages allow for lower transmission currents without sacrificing power. They minimize energy loss.
21.2 Transformers
- Transformer Function: A device used to increase or decrease voltage in an AC power supply.
- Transformer Parts: Primary coil, secondary coil, and an iron core.
- Primary vs. Secondary Coils: The incoming voltage is connected to the primary coil; the secondary coil provides the output voltage.
- Iron Core: The iron core links the two coils, providing a path for the magnetic field.
21.2 Step-Up/Step-Down Transformers
- Step-Up Transformer: Increases voltage (more turns on the secondary coil).
- Step-Down Transformer: Decreases voltage (fewer turns on the secondary coil).
- Voltage Ratio: The ratio of secondary voltage to primary voltage equals the ratio of secondary turns to primary turns.
21.3 How Transformers Work
- Alternating Current (AC) Necessity: Transformers only operate with AC because they rely on a changing magnetic field for induction in the secondary coil.
- Magnetic Field Generation: The alternating current in the primary coil produces a changing magnetic field in the core.
- Induction in the Secondary: The changing magnetic field induces a current in the secondary coil.
- Power Transfer: Power going into the primary coil equals the power coming out of the secondary coil in ideal conditions.
21.3 Transformer Power Efficiency
- Efficiency: Well-designed transformers are very efficient, losing only about 0.1% of the transmitted power.
- Power Equation: Power transferred to primary coil equals power transferred from secondary coil. (P=IV).
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Description
Explore the principles of electromagnetic induction in this quiz based on Chapter 21. Learn about the induction of electromotive force (e.m.f.), the differences between motors and generators, and the workings of transformers. Test your understanding of how electricity is generated from motion and the key components involved.
