4.2 ✅ Energy, power and resistance Flashcards
Explain what is Potential Difference
Potential Difference is a measure of the transfer of energy by the charge carriers, or the work done by the charge carriers as they pass through components of a circuit
What causes the potential difference across a filament lamp?
The potential difference across a component like a filament lamp is a result of electrical energy being transferred into heat and light, as charge carriers move through the lamp
Give another name for Potential Difference
Voltage
Explain why high voltages can be very dangerous
High voltages can be very dangerous because charge carriers can transfer enormous amounts of energy through conductors, and if the voltage is high enough, through insulators such as air or people
Define Potential Difference
Potential Difference is defined as the energy transferred from electrical energy to other forms (heat, light, etc.), per unit charge.
Also see flashcard 1
What is the equation for potential difference?
Using the equation, explain what 1 Volt is
One volt is the p.d. across a component when 1J of energy is transferred per unit charge passing through the component.
1V = 1JC⁻¹
What does a p.d. of 1000V mean?
A p.d. of 1000V means that 1000J of energy of energy are transferred per coulomb of charge.
Explain what we use to measure p.d, and how we use it
We use a voltmeter to measure p.d. They are always connected in parallel across a particular component.
Explain what is electromotive force is
Electromotive force is used to describe when work is done on the charge carriers, as they pass through a source such as a cell or a power supply.
The greater the E.M.F….
The greater the e.m.f., the more energy per coulomb has been transferred (often from the form of chemical energy in a cell) into electrical energy. Other sources of e.m.f. include solar cells (from light), dynamos (movement), and thermocouples (heat)
Define electromotive force
Electromotive force is defined as the energy transferred from chemical energy (or another form) to electrical energy per unit charge.
What is the difference between e.m.f and p.d in terms of energy transfer
Potential difference is used to describe when work is done by the charge carriers.
The charges lose energy, as it is transferred, as they move through the component.
Electromotive force is used to describe when work is done on the charge carriers.
The charges gain energy as they pass through a source, such as a cell or power supply
Why do we have potential difference and electromotive force as separate things?
Because at times it is necessary to describe whether the charges in a circuit are losing or gaining energy
Describe what happens when have a higher p.d, in comparison to a higher e.m.f
- The greater the p.d, the more energy charge carriers transfer
- The greater the e.m.f, the more energy the charge carriers gain
- The greater the p.d, the more energy per coulomb is transferred from electrical energy to other forms as the charges move through the component
- The greater the e.m.f, the more energy per coloumb has been transferred into electrical energy
What is the equation for electromotive force?
Linked equations for electromotive force and potential difference
W=VQ or W=εQ
What is an electron gun?
An electron gun is an electrical device that produces a thin beam of electrons, which is accelerated to high speeds
How does an electron gun work?
All electron guns need a source of electrons. In most cases, a small metal filament, which acts as a cathode, is heated by passing an electric current through it
The electrons in this piece of metal wire gain kinetic energy
Some of the electrons gain enough kinetic energy to escape from the surface of the metal
This process is known as thermionic emission - the emission of electrons through the action of heat
If the heated filament is placed in a vacuum, and a high p.d. is applied between the filament and anode, the filament acts as a cathode, and the freed electrons accelerate towards the anode, gaining kinetic energy
If the anode has a small hole in it, then electrons in line with this hole can pass through it, creating a beam of electrons with a specific kinetic energy
A cathode is the negative electrode that gains electrons after attracting positive ions
What is thermionic emission?
Thermionic emission is the emission of electrons through the action of heat
Explain how we derive the equation
Describe what resistance is in your own words
Every electrical component in a circuit “resists” or opposes the flow of current/charge carriers to some degree - including wires, filament lamps and even battery cells. This is known as Resistance.
Define the resistance of a component
The resistance of a component is defined as:
- The ratio between the potential difference across the component and the current in the component
OR ALTERNATIVELY
- The potential difference across the component, divided by the current in the component
What is the formula for Resistance?
Describe what 1 Ohm is
One ohm can be described as the resistance of a component when a p.d. of 1V is produced per ampere of current
1Ω = 1V A⁻¹
Describe what a component with a resistance of 500Ω would have
A component with a resistance of 500Ω would have a p.d. of 500V when there is a current of 1A in it
What is 1Ω equal to?
1Ω = 1V A⁻¹
1Ω = 1 Volt per Ampere of current
What does Ohm’s Law state?
Ohm’s Law states that:
For a metallic conductor kept at a constant temperature, the current through it is directly proportional to the potential difference across it
What does Ohm’s Law suggest when the p.d. across a wire is doubled?
Ohm’s law suggests that when the p.d. across a wire (that is kept at a constant temperature) is doubled, the current in the wire also doubles.
Describe the formula that can be used to represent Ohm’s Law?
V = IR
Explain how increasing the temperature affects resistance?
- When the temperature of a wire increases, the positive ions inside the wire have more internal energy and vibrate with greater amplitude about their mean position
- This causes the frequency of collisions between the charge carriers (which are the free electrons in the metal) and the positive metal ions to increase
- So the charge carriers do more work, and transfer more energy as they travel through the wire - which in effect increases the resistance
As the temperature of a wire increases, so does the….
As the temperature of a wire increases, so does the resistance
Describe an experiment proving how the resistance increases as the temperature increases
What do you call a component that follows Ohm’s Law?
An ohmic conductor
What do I-V Characteristics or or I-V Graphs for any electrical component show?
The I-V Characteristics or I-V Graphs for any electrical component shows the relationship between the electric current I in a component and the potential difference V across it
Give 2 examples of an ohmic conductor
A wire and a fixed resistor
What is the IV Graph for a fixed resistor or metallic wire?
A straight line
Give 3 conclusions we can give about a straight line IV Graph?
Such as a fixed resistor or wire
- The potential difference across the component is directly proportional to the current in it
- The component obeys Ohms law and is an ohmic conductor
- The resistance across the component is constant and fixed, as V=IR
Which one has a lower resistance and which one has a higher resistance?
Most wires and other metallic conductors can be though of as resistors with….
Most wires and other metallic conductors can be though of as resistors with a very low resistance
What is the IV Graph of a Filament Lamp?
What are 3 conclusions we can draw from the IV graph of a filament lamp?
- The potential difference across a filament lamp is not directly proportional to the current through it
- Therefore it is a non-ohmic component
- The resistance of a filament lamp is not constant
As the current increases in the filament lamp, what happens to the resistance?
Why?
As the current inside a filament lamp increases, so does the resistance (this can also be determined by different by calculating V/I at different points on the graph )
Thiis increase of resistance is caused by the wire getting so hot that it glows. As the current increases, so does the rate of flow of charge through the filament - more electrons per second pass through it, so more collisions occur between the electrons and the positive metal ions per second. When the electrons collide with the ions, they transfer the energy to the ions, causing the ions to vibrate more and increase temperature, and to collide with still more electrons.
What is a diode?
A diode is a component that only allows current in one particular direction
What is the IV graph for a diode?
What are diodes made of that allow current only flow through one direction?
Diodes are made from semiconducting material
See flashcards on semiconductors, chapter 4.1
What are 5 conclusions you can draw from the IV graph of a diode?
- The potential difference across a diode is not directly proportional to the current through it
- Therefore a diode does not obey Ohm’s Law and can be described as a non-Ohmic component
- The resistance of the diode is not constant
- The diode’s behaviour depends on its polarity. If the diode is reverse biased (i.e. the diode is swapped or the polarity of the battery or power supply is swapped), then the diode does not conduct at all and the resistance across it is infinite.
- At point A, the resistance of the diode is very high - infinite for practical purposes. At point B, as the p.d. increases, the resistance gradually begins to decrease. This point is where the diode finally begins to conduct and is known as the threshold p.d. At point C, the diode has very little resistance.
Light-Emitting Diodes
LEDs
Same I-V characteristics as a diode
What are LDRs
LDRs are small electric components which change their resistance depending on the light intensity
Explain how a Light Dependent Resistor (LDR) works
A typical LDR is made from a semiconductor in which the number density of the charge carriers changes depending on the intensity of the incident light
- In dark conditions, the LDR has a very high resistance. This is because the number density of the free electrons inside the semiconductor is very low, so the resistance is very high.
- When light shines onto an LDR, the number density of the charge carriers increases dramamtically, leading to a rapid decrease in the resistance of the component.
What is the Resistance - Light Intensity graph for a LDR (Light Dependent Resistor)?
What are the factors that affect the resistance across a wire?
- Temperature
- The material of the wire
- The length of the wire L
- The cross-sectional area of the wire A
Describe what happens to the potential difference for any given current when the length of the wire doubles
For any given current, doubling the length of the wire will double the p.d. across it as the length of the wire is directly proportional to the p.d. across it
Explain how increasing the length of the wire will affect the resistance
For any given current, increasing the length of the wire will increase the p.d. across it, as the length of the wire is directly proportional to the p.d. across it.
So doubling the length of the wire doubles the p.d., so consequently the resistance doubles as V = I R.
Describe the relationship between the resistance of a wire and the length of a wire
Explain what happens to the resistance when the cross sectional area of a wire increases
When the cross-sectional area of a wire increases, the resistance drops
Describe the relationship between the Resistance and Cross-Sectional Area
Using the relationship between the Cross-Sectional Area of a wire, and the resistance, explain what happens when the cross sectional Area increases
For any given p.d, doubling the cross-sectional area of a wire will double the current in the wire, so the resistance must be halved
As V = I R
Describe and define what resistivity is, using one example.
Resistivity is used to describe the electrical property of a material. It tells us the extent to which a material opposes the flow of electric current through it
Every material has their own unique resistivity - which remains the same regardless of the length or shape of that material.
For example, different components made from copper may have different resistances, as their lengths and cross sectional areas may differ, but copper has a unique resistivity.
The resistivity of a material at a given temperature is defined as the product of the resistance of a component made of the material and its cross-sectional area, divided by its length:
Explain how we derive the following formula:
What is the unit of measurement for resistivity?
The unit of measurement for resistivity is Ohm metre (Ω m)
When is the condition in order to be able to use the formula for resistivity:
The equation may only be used for a known constant temperature
As the resistivity of a material is affected by its temperature too.
Explain the relationship between the resistivity of a material and the temperature
The resistivity of a material increases with temperature the same way the resistance of most components varies with temperature
Compare the resistivity of conductors, insulators and semiconductors
- Good conductors like metals have a low resistivity of 10 ⁻⁸Ω m
- Insulators have a high resistivity of 10 ¹⁶Ω m
- Semiconductors have a resistivity in between
Explain what it means if a semiconductor has a negative temperature coefficient?
If a semiconductor has a negative temperature coefficient, then it means that as the temperature of the semiconductor increases, the resistance decreases
Which materials have a negative temperatue coefficient?
Some semiconductors have a negative temperature coefficient
Explain how negative temperature coefficient works in semiconductors, in terms of number densities
In some semiconductors, as the temperature increases, the number density of the charge carriers also increases.
Explain what is a thermistor and its function
- A thermistor is an electrical component made from a semiconductor with a negative temperature coefficient.
- As the temperature of a thermistor increases, the resistance drops.
In which applications would thermistors be useful in?
In temperature sensing circuits, such as thermometers, or monitoring the temperature inside electrical devices so they can power them down before overheating damages them
Describe the Resistance-Temperature graph of a thermistor
Describe an experiment to find out how the resistance of a thermistor changes as the temperature changes.
A simple experiment to see how the the resistance of a thermistor changes with temperature change can be carried out using an ohmmeter and a water bath. (See pic below)
What is the I-V graph of a thermistor
What can we tell from the I-V graph of a thermistor?
- Like most semiconducting components, thermistors are non-ohmic. - The I-V characteristics of a thermistor is similar to those of a filament lamp, except for the fact that for filament lamps, as when the current increases, electrons transfer energy to the positive ions, which raises the temperature. This causes an increase in resistance. However for thermistors, when the increase in current increases the temperature, it leads to a drop in resistance because as the temperature increases for semiconductors, the number density of charge carriers increases.
What are the I-V Characteristics of a thermistor most similar to?
The characteristics of a thermistor are most similar to that of a filament lamp, with one crucial difference, which is that for filament lamps, as when the current increases, electrons transfer energy to the positive ions, which raises the temperature. This causes an increase in resistance. However for thermistors, when the increase in current increases the temperature, it leads to a drop in resistance because as the temperature increases for semiconductors, the number density of charge carriers increases.
What is power?
Power is the rate of energy transfer
What are the 3 power equations + the equation linking V, W, I and t?
