9 Energy, Power and Resistance Flashcards
9.1 What is a cell?
Push for electrons to flow
What is a light detecting resistor
The resistance changes depending on the light exposure
What is a fuse?
Breaks/melts when current is too high
What is a resistor?
Limits electric current in a circuit
What is an LED?
Emits light when it passes through it
What is an open switch?
Open switch means that current can’t flow
What is a thermistor
Resistor depends on temperature
What is a variable resistor?
Changes the current that passes through per second
What is the voltage or potential difference?
It is a measure of the electrical force transferred per unit charge. In other words the energy provided to the charge carriers. It can be defined as the amount of energy transferred per unit charge and is calculated using V=W/Q
Why must a voltmeter be connected by a parallel circuit?
Voltage is measured as a difference in energy between two points. Thus a voltmeter must be connected in parallel and is used to measure the difference in potential across a device.
What is one volt defined as?
It is defined as the energy transferred per coulomb of charge as the charges move between two points in a circuit.
What is the electromotive force?
EMF is the energy transferred to the charge carriers by the cell/battery.
Note: This is NOT a force
It is always used across a cell/battery/power supply
What is the potential difference?
The potential difference is the energy transferred by the charge carriers in the circuit.
It is only used in components in the circuit such as a bulb.
What is the formula for potential difference which includes work done and charge
P.d = work done/charge
E = /\W / /\Q
9.4 Define resistance
The component resists the flow of charge carriers through it
Define the resistance of a component?
The resistance of a component is defined as the ratio between the potential difference across a component and the current flowing through it.
Resistance formula
R = Voltage/current V/I
Ohm’s law
For a metallic conductor at constant temperature the potential difference across the conductor is directly proportional to the current flowing through the conductor.
9.2 What is voltage?
It is a measure of the electrical force transferred per unit charge. In other words, the energy provided to the charge carriers.
Proper definition: The amount of energy transferred per unit charge.
How can you calculate voltage?
With energy transferred and unit charge
V=W/Q Voltage = Energy transferred/ unit charge
How do you use a voltmeter
As voltage is measured as a difference in energy between two points. A voltmeter must be connected in parallel and is used to measure the difference in potential across a device.
What should the resistance of a voltmeter should be?
The resistance should be infinite so current can pass through.
What is one volt defined as?
One volt is defined as the energy transferred per coulomb of charge as the charges move between two points in the circuit.
1V = 1JC^-1
What is the electromotive force?
- It is NOT a force
- EMF is the energy transferred TO the charge carriers by (the cell/battery). It happens across a cell/battery/power supply.
What is a potential difference
Potential difference is the energy transferred BY the charge carriers. It is only used for components in the circuit.
What is the electromotive force?
The term used to describe when work is done on the charge carriers.
Formula for electromotive force?
EMF = Work done/charge
9.7 Resistance or resistivity?
R is proportional to length l
Resistance is proportional to length l.
How is resistance affected by cross sectional area? include formula as well
Resistance is inversely proportional to cross sectional area A.
Resistance formula
With resistivity, length and area
pl/A
P = RESISTIVITY
L = LENGTH
A = area
What is resistivity?
The measure of how much a particular material opposes electron flow is called the resistivity of the material.
Resistivity formula
p=RA/L RESISTANCE X CROSS-SECTIONAL AREA/LENGTH
Relationship of the resistivity of metals and temperature
The resistivity of many metals is directly proportional to temperature.
Although the cross-sectional area and length i______ with temperature, this is negligible when compared to the increase in r_____ due to the increased kinetic energy of the atoms.
Increase and resistance
Relationship between of a metal and temperature
Therefore the resistivity of a metal increases in proportion to temperature in kelvin.
What is kelvin?
Kelvin is the absolute unit of temperature where 0k = -273.15 degrees celsius
I______ in semiconductors such as silicon significantly increase conduction.
Impurities
9.5 How to set up a filament lamp experiment to measure the voltage and current? (practical)
Procedure:
1. Set up the circuit
2. Take a minimum of readings for the p.d. (V) across and current (I) through the lamp over the range 0 to 6.0V.
3. Reverse the connections to the lamp and repeat step 2.
Analysis:
1. Plot a graph of current (y-axis) against voltage (x-axis). Include both positive and negative values of voltage.
2. Predict the shape of a resistance (y-axis) against voltage (x-axis) graph for a filament lamp.
How to set up a resistor experiment to measure the voltage and current?
Procedure:
1. Set up the circuit shown below for different resistors.
2. Take a minimum of readings for the p.d. (V) across and current (I) through the lamp over the range 0 to 6.0V.
3. Reverse the connections to the lamp and repeat step 2.
Analysis:
1. Plot a graph of current (y-axis) against voltage (x-axis). Include both positive and negative values of voltage.
2. Predict the shape of a resistance (y-axis) against voltage (x-axis) graph for a resistor.
Describe the graph for a resistor
The potential difference across the resistor is directly proportional to the current in the resistor.
As a result:
- A resistor obeys Ohm’s law and so can be described as an ohmic conductor.
- The resistance of the resistor is constant.
The resistor behaves in the same way regardless of the polarity.
Describe the graph for a filament lamp
The potential difference across a filament lamp is not directly proportional to the current through the resistor. In other words:
- A filament lamp does not obey Ohm’s law and so it can be described as a non-ohmic component.
- The resistance of the filament lamp is not constant.
The filament lamp behaves in the same way regardless of the polarity.
The resistance of the filament lamp increases as the p.d. across it increases. You can confirm this by determining V/I at different points on the graph.
9.6 What direction does a diode go in?
Diode only allows a current in one particular direction.
Describe the graph of diode?
- If the p.d. is in the reverse the direction. The diode does not conduct as the resistance of the diode is very high.
- At the threshold p.d. (when the voltage starts to increase) the resistance gradually starts to drop.
- Above this value, the resistance drops sharply for a very small increase in p.d. Above this point the diode has very little resistance.
What is a thermistor?
A thermistor is an electrical component made from a semiconductor with a negative temperature coefficient (NTC). As the temperature of the thermistor increases, its resistance drops.
When are thermistors used?
- Simple thermometers
- In thermostats to control heating and air-conditioning units
- To monitor the temperature of components inside electrical devices like computers and smartphones so that they can power down before overheating damages them.
- To measure temperature in a wide variety of electrical devices like toasters, kettles, fridges, freezers and hair dryers.
- To monitor engine temperatures to ensure the engine does not overheat.
What is an LDR?
LDR is made from a semiconductor in which the number density of charge carriers changes depending on the intensity of the incident light.
When using a thermistor what happens to the resistance when the temperature increases?
When the temperature increases the resistance increases.
When using an LDR? what happens to the resistance to the light intensity?
As the light intensity increases, the resistance of the thermistor increases. However, if the light intensity decreases, the resistance of the thermistor increases.
9.10 What is electrical power?
The rate of energy transfer by each electrical component is called electrical power.
This depends on the current I in the component, and potential difference V across it.
Equation of electrical power
Electrical power = p.d. x current
P=VI
What are the two other equations of power
One has V and R
Other has I and R
P=V^2/R
P=I^2R
Definition of power
The rate of energy transfer (watch out for this in questions)
Energy transferred in a given time can be determined by the general equation for power and electrical power. Show this
P = W/t
P=VI
substituting P = VI gives
W=VIt
9.11 Explain why a powerful microwave oven may be cheaper to run than a less powerful one, despite transferring more energy per second.
- The power of the device
- How long the device is used for
How to find the energy transferred by an electrical device can be calculated as
W = Pt
Energy transferred = Power x time
What is a kWh
It is a kilowatt-hour (kWh) and it is defined as the energy transferred by a device with a power of 1kW operating for a time of 1 hour.
How to find the energy transferred in a kWh?
Power of device (kW) x time for which the device is used (h)
9.3 What is thermionic emission?
The filament is hot therefore it gives out electrons.
Process of the electron gun
- A small metal filament is heated, known as a cathode.
- The electrons in the wire gain kinetic energy
- Some gain enough KE to escape the surface of the metal
4, This process is called Thermionic emission - A high PD is applied between the filament and the anode inside a Vacuum
- The free electrons accelerate towards the anode gaining KE.
- A small hole in the anode allows a line of electrons.
Equation which links kinetic energy and electrons
eV=0.5mv^2
