9 - Energy, power and resistance Flashcards
What are the rules for drawing circuit diagrams?
- Use standard circuit symbols
- No gaps between wires
- Straight lines with pencil and ruler (or careful sketch)
What is polarity?
Whether something is positive or negative.
What does potential difference measure?
The amount of work done per unit charge.
Define 1 volt.
The potential difference when one joule of energy is used to move one coulomb of charge through the component.
What is electromotive force (emf)?
The amount of energy given to each coulomb by the battery.
What is a thermionic emission?
When an object is heated and the electrons gain enough energy to escape from the surface of the metal.
What is a cathode?
A negatively charged electrode.
What is an anode?
A positively charged electrode.
How does an electron gun produce a beam of electrons?
- Metal filament is heated > thermionic emission
- In a vacuum, potential difference is applied to the filament and an anode, causing the electrons to accelerate through a small hole to form a beam.
- Electrons move through a gas which lights up
What does a high resistance mean?
A lot of energy is required to push electrons through a component.
What does Ohm’s law state?
For a metal conductor kept at a constant temperature, the current in the wire is directly proportional to the p.d across the wire.
What happens when the temperature of a metal wire increases and why?
Its resistance increases, which decreases the current because the positive ions gain more energy and vibrate with a greater amplitude which increases the number of collisions between them and the free electrons. This means that more electrons will need to pass through the wire for the same amount to reach the other side, which means that more energy is transferred.
Define the ohm.
The amount of resistance when a potential difference of 1 volt produces a current of 1 amp.
Describe the relationship between voltage and current for an ohmic conductor.
Linear
What is a component that does not follow Ohm’s law called?
A non-ohmic component.
Why are diodes different to other electrical components?
They only allow current to flow in one direction.
Why are LEDs special?
They are made from a material which emits light as it conducts. Electrical energy is transferred directly into light which means that LEDs do not get hot and are extremely efficient.
What are diodes made from?
A semiconductor.
What does a shallow gradient on an IV graph represent?
A high resistance
What does a steep gradient on an IV graph represent?
A low resistance
How do you calculate resistance from the gradient of an IV graph?
Resistance = the inverse of the gradient
Why does a filament lamp increase resistance as more current flows through it?
Because more current means more flow of charge and as more electrons flow through it there are more collisions which causes the ions to vibrate more and gain thermal energy, which increases the number of collisions, etc.
What is resistivity used to describe?
The electrical property of a material.
What is the difference between resistance and resistivity?
Resistance describes a component, such as a resistor whereas resistivity describes the material that a component is made of.
What is the unit of resistivity?
Ohm metre
The resistivity of a material when one cubic metre has a resistance of 1ohm
How can we define resistivity?
By rearranging the equation.
The product of the resistance of a component made from the material and the cross sectional area, divided by the length.
How can we use a resistance, length graph to find the resistivity of a material?
Find the gradient and multiply it by the cross sectional area.
What does a negative temperature coefficient mean?
Resistance decreases as temperature increases
What are the 4 factors that affect resistance?
- Temperature
- Material
- Length of wire
- Cross sectional area of wire
Why does decrease as temperature increases in some semiconductors?
Because as the temperature increases, so does the number density of the material.
What happens to the resistance of a thermistor as the temperature increases / decreases.
Increase temp = decrease resistance
Decrease temp = increase resistance
What is the resistance of a wire directly proportional to?
The length of the wire.
What is the resistance of a wire inversely proportional to?
The cross sectional area if the wire.
What happens to the resistivity of a material as you increase the temperature?
It increases.
Why does an increase in light intensity decrease the resistance of an LDR?
Because the semiconductor they are made from has a high amount number density in bright light and a low number density in darkness.
What is electrical power?
The rate at which energy is transferred.
What are the 4 equations for power?
P = VI P = w/t P = I^2R P = V^2/R
Define the watt.
1 watt = 1 joule per second
Define the kilowatt hour.
The amount of energy transferred by a 1 kw device in one hour.
How many joules is 1 kWh equivalent to?
3.6 mJ
(1kw = 1000J 1h = 3600s
1000 x 3600 = 3.6mJ)
What is the approximate cost of one kilowatt hour?
6-15p
Define the coulomb.
The amount of charge that flows through a point when 1 amp moves past a point in one second.
Define resistance.
The potential difference across a component per unit current through it.
