4.2 Energy, Power and Resistance Flashcards
What is the name of the component whose circuit symbol is a rectangle with two arrows pointing at it?
Light Dependant Resistor (LDR)
Should voltmeters be connected in series or parallel?
Parallel
What is potential difference? State the units
The work done per unit of charge; the difference in potential between two points in a circuit
Measured in volts
What does “Ę” represent on electricity?
Electromotive Force (e.m.f)
What is the difference between voltage and emf?
Voltage is the energy transferred per coulomb across a load resistance
Emf is the total amount of work done by the battery per coulomb
Give the equation relating energy transferred to voltage
W = VQ
W = work done (energy transferred) V = potential difference Q = charge
True or false: The kinetic energy of an electron is equal to its charge multiplied by the voltage
True
Energy transferred = eV = 1/2 m v^2
What is resistance?
How difficult it is for current to flow through an appliance
The potential difference required for each unit of current that flows through a resistor
R = V/I
What is the definition of 1 Ohm?
When a resistor is subject to a voltage of 1V and allows a current of 1A through, it’s resistance is 1 Ohm
What is meant by an ohmic conductor?
A conductor that obeys ohm’s law
What does Ohm’s law say?
The current through an ohmic conductor is directly proportional to the potential difference across it (ie. resistance doesn’t vary with voltage or current).
(This is only true if temperature is constant)
What does the gradient of a current potential difference graph represent?
1/R
Because the gradient = rise in current / change in potential difference ie. gradient = I/V
As resistance is V/I, gradient is 1/R
Why does the current increasing on a filament lamp cause an increase in resistance?
The flow of current causes collisions between the electrons and the metal lattice
These collisions increase the temperature (vibration) of the lattice
As the temperature increases, more collisions occur, causing the resistance to increase
What is a diode?
A diode is an appliance that only allows current to flow in one direction
What is a Light Dependent Resistor (LDR)?
A semiconductor that is sensitive to light: as the light intensity increases, resistance decreases
How does a thermistor work?
Similar to a LDR, but as the temperature increases, resistance decreases
(Note - thermistors that work in this way are called negative temperature coefficient (NTC) thermistors).
What is resistivity?
The resistivity is a property of a metal which shows how easy it difficult it is for current to flow in the material (at a specific temperature).
p = RA/L
p = resistivity R = resistance A = cross-sectional area L = length
Describe an experiment to determine the resistivity of a metal:
1) Measure the diameter of the wire using a micrometer
2) Use this to calculate the area
3) Set up circuit with ammeter in series, and voltmeter in parallel to the metal wire with a certain length (should be variable)
4) Vary the length and record the voltage and current for each length
5) Use R=V/I to work out the resistance
6) Plot a graph of resistance against the length
7) The gradient = resistivity + are
8) Rearrange to work out resistivity
True or false: The resistivity of both metals and semiconductors increases with temperature
False
The resistivity of a metal increases with temperature because as the metal ions heat up the vibrate more and electrons bang into them and slow down. Some semiconductors get less resistive as temperature increases, because supplying energy actually causes more charge carriers to be released, so current can flow more easily
What is power? state the unit
The rate of energy transfer
Measured in J/s or Watts (W)
Give an equation for power in terms of current and voltage
P=V/I
Is a kilowatt-hour (kWh) a unit of power, potential difference or energy?
Energy because it’s a unit of power multiplied by time
Why do electricity companies use “units” (kWh) rather than joules or watts?
Joules and Watts are both so small that everyone would use tens or hundreds of millions of them each month - it’s impractical to have such large numbers used
How much would this customer pay for their daily electricity if the cost is 15p per unit?
morning reading: 03543kWh
evening reading: 03563kWh
Units used = 3563-3543 = 20
Total cost = units x price = 20 x 15p = 300p / £3
