Physics module 4.2 (energy, power and resistance) Flashcards
What is the difference between electromotive force and potential difference?
-The electromotive force is the energy transferred to each unit of charge
-The potential difference is the work done by each unit of charge
-Both have a unit of one joule per coulomb or volts
What is Ohms law?
-The current through a conductor is directly proportional to the potential difference, provided physical conditions stay constant.
Describe an experiment showing Ohms law?
1.Set up apparatus as shown
2. Keep current at low value so heating effect is negligible
3. Add more cells to the battery and record current and voltage
4.Plot a graph with I against V and they should be directly proportional (Ohms law)
Describe the I-V characteristic graph of a resistor? and a filament bulb?
-Resistor: behaves in accordance to Ohms law current is directly proportional to voltage in all directions
-Filament lamp: in accordance to Ohms law at low currents, however as the current increases more electrons collide with metal ions which increases resistance and the graph flattens
What is the I-V characteristic graph of a diode and graphs for a LDR and a thermistor?
-A diode is said to have infinite resistance in the opposite direction so the diode will only allow current to flow in the forward bias direction
Why is the resistance of wires low? and why does increasing length of wire increase resistance?
-They contain free electrons which move throughout the structure when under the influence of P.D so have low resistance
-As the length of the wire increases however the charge carriers experience a smaller potential gradient and therefore have a lower drift velocity, smaller current and higher resistance
How does the cross-sectional area of a wire affect resistance?
-Increasing the cross-sectional area increases the number of electrons available to flow at the same drift velocity so resistance is inversely proportional to cross-sectional area of the wire.
How does changing the temperature of a wire affect the resistance?
-Increasing the temperature will lead to more collisions between the vibrating metal ions and the conducting electrons which will lead to an increase in electrical resistance
-However some materials have structures(semiconductors like thermistors) which lead to a decrease in electrical resistance.
What is resistivity?
-A property that describes the extent at which a material opposes the flow of electric charge
-It is a fixed value for different materials
Describe an experiment to determine the resistivity of a wire
1.Set up apparatus as shown and measure values for current and voltage at different lengths
2.Plot a graph of resistance against length of wire
3.Find the cross sectional area using a micrometer(make sure to take measurements at different parts of the wire and work out a mean)
4. Find the gradient of the graph and multiply by cross sectional area to get resistivity (derived from p=RA/l)
What are the two equations for resistivity of a wire?
Is resistivity affected by temperature, length or cross-sectional area?
-Resistivity isn’t affected by length or cross-sectional area but it is affected by temperature
What are the equations for electrical power?
Why is electricity carried at high voltage on the national grid?
-High power is needed and as power=current*voltage a high voltage is used, this is because a high current causes lots of energy to be lost via heat because more coulombs of charge will be flowing through the wire
What is a kilowatt-hour?
-A kilowatt-hour is equal to 3,600,000J
-It is most commonly used when looking at electrical bills
What is an electron volt and the equation for kinetic energy gained of an electron when accelerated through a P.D.?
-ElectronVolt: the energy transferred to one electron when accelerated through a P.D. of one volt. 1eV=1.6*10^-19
