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?
LDR light dependent resistor
Should voltmeters be connected in series or parallel?
Parallel
What is potential difference? State the units
The work done per unit charge; the difference in potential between two points in a circuit
Measured in volts
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 multiplied by its charge multiplied by the voltage
True
Energy transferred=eV=1/2mv^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 currents of 1A through, it’s resistance is 1 Ohm
What is meant by an object conductor?
A conductor that obeys Ohm’s law
What does Ohm’s law state in words?
The current through an ohmic conductors is directly proportional to the potential difference across it. (Ie. Resistance doesn’t vary with voltage or current)
(Only true if temperature is constant)
What does the gradient of a current potential difference graph represent?
1/R
The gradient=rise in current/change in potential difference
The gradient= I/V
So resistance is 1/R
Why does the current increasing on a filament lamp cause an increase in the 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?
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, the resistance decreases
How does a thermistor work?
Similar to an LDR, but as the temperature increases, the resistance decreases (NTC thermistors)
What is resistivity?
A property of a metal which shows how easy or difficult it is for current to flow in the material (at a specific temperature)
p=RA/L
Describe an experiment to determine resistivity of a metal
-measure diameter of wire using micrometer
-halve and calculate area
-description of circuit set up
- ray length record voltage and current for each length
-R=V/I to work out resistance
-gradient= resistivity/area
Rearrange for 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, they vibrate more and the electrons bang into the, and slow them down
Some semiconductors get less resistive as temperature increases, because supplying energy causes more charge carriers to be released, so current can flow more easily
What is power?
The rate of energy transfer
J/s or W
Give an equation for power in terms of current and voltage
P=IV
What is a kilowatt-hour(kWh) a measure for?
Energy
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 monthly- it’s impractical to have such large numbers used
