Section 6: Electricity Flashcards
What does Ohm’s Law state?
Provided the physical states such as temperature remain constant, current through an ohmic conductor is directly proportional to the potential difference across it.
When the gradient of a IV characteristic graph gets shallower, what happens to the resistance?
The resistance increases as the gradient decreases.
What does the IV characteristic graph look like for an ohmic conductor?
Straight line through the origin.
What does the IV characteristic graph look like for a filament lamp?
A curve that starts steep but shallower as the voltage rises.
What does the IV characteristic graph look like for a diode?
Pretty much level but then a sharp increase.
What is resistivity?
This is a measure of how much a particular material resists current flow. It depends on the structure of the material as well as the environmental factors such as temperature and light intensity. It is a property of a material.
What is the equation to find the resistivity of a material?
resistivity = (resistance x cross-sectional area) all divided by length.
What are semi-conductors?
A group of materials that aren’t as good at conducting electricity as metals because they have far fewer charge carriers (electrons) available.
What happens if energy is supplied to a semi-conductor?
More charge carriers can be released and the resistivity of the material decreases. This means that they can make excellent sensors for detecting changes in their environment.
What are three examples of semi-conductors?
thermistors, diodes and light dependent resistors
What is a thermistor?
A component with a resistance that depends on its temperature.
How do thermistors work?
The resistance decreases as the temperature goes up. Warming the thermistor gives more electrons enough energy to escape from their atoms. This means that there are more charge carriers available, so the resistance is lower.
What experiment can you do to investigate the resistance of a thermistor?
Set up a circuit with the battery, thermistor and ammeter all in series. Place the thermistor in a beaker and pour enough boiling water into the beaker to cover the thermistor. Measure and record the temperature of the water using a digital thermometer and current through the circuit - the potential difference across the thermistor needs to be kept constant throughout the experiment. Continue to record the current and temperature for every 5 degrees Celsius drop in temperature. You should find that as the temperature decreases, the resistance increases.
How can a material become a superconductor?
You can lower the resistivity of many materials like metals by colling them down. If you cool some materials down to below a critical temperature called the ‘transitional temperature’, their resistivity disappears entirely and they become a superconductor. Without any resistance, none of the electrical energy is turned into heat, so none of it is wasted.
What are three uses of superconductors?
Power cables that transmit electricity without any loss of power, really strong electromagnets that have lots of applications such as medicine, electronic circuits that work really fast with minimal energy loss because there’s no resistance to slow the current down.
How can you calculate the cross-sectional area of a wire?
Use a micrometer to measure the diameter of the wire along at least three places on the wire to then find an average. Half the diameter to find the radius and then use the equation of the area of a circle to find the cross-sectional area of the wire.
Describe the method of measuring the resistivity of a wire? (Required Practical 5)
- Attach the flying lead to the end of the test wire and measure the length of the test wire connected in the circuit.
- Close the switch and measure the current through the circuit and the potential difference across the test wire. Open the switch again once you’ve taken your measurements and use these values to calculate the resistance of the wire.
- Repeat this process at least one more time and calculate the mean resistance for this length of wire.
- Reposition the flying lead and repeat steps 2 and 3 to get an average resistance for a range of different lengths of test wire.
- Plot a graph of average resistance in ohms against length in meters using your results.
- Use the equation of resistivity to find your resistivity of the wire.
What is internal resistance?
Resistance comes from electrons colliding with atoms and losing energy. In a battery, chemical energy is used to make electrons move. As they move, they collide with atoms inside the battery, so batteries must have resistance.
What is load resistance?
The total resistance of all the components in the external circuit. It is also called external resistance.
What is E.M.F and what does it stand for?
E.M.F. –> Electromotive force
It is the amount of electrical energy the battery produces and transfer to each coulomb of charge.
What is E.M.F. measured in?
Volts
What is lost volts?
The energy wasted per coulomb overcoming the internal resistance.
What does the conservation of energy law tell about load resistance and internal resistance?
Energy per coulomb supplied by the source =
energy per coulomb transferred in load resistance +
energy per coulomb wasted in internal resistance.
Describe the method of measuring internal resistance and E.M.F.? (Required Practical 6)
- Set the variable resistor (the load resistance) to its highest resistance. Close the switch and record the current through and the potential difference across the circuit. Open the switch and close it again to get two more sets of current and potential difference readings for this load resistance. Calculate the mean current and potential difference for this resistance from your results
- Decrease the resistance of the variable resistor by a small amount and repeat step 1 for this resistance.
- Repeat steps 1 and 2 until you have a set of mean currents and potential differences for 10 different load resistances (over the widest possible range).
- Plot a V-I graph for your mean data and draw a line of best fit - you get a straight line graph.
- Make sure all other variables are kept constant when carrying out the experiment, including external factors like temperature (which affects the resistivity of materials).
What is Kirchoff’s first law?
The total current entering a junction = the total current leaving it.
What is Kirchoff’s second law?
The total EMF around a series current = the sums of the PDs across each component.
In a series circuit, how is the total resistance found?
R = R1+R2+R3…
In a parallel circuit, how it the total resistance found?
1/R =1/R1 + 1/R2 + 1/R3
What is a potential divider?
A circuit with a voltage source and a couple of resistors in series.
What do you use potential dividers for?
To supply a potential difference between zero and the potential difference across the power supply. This can be helpful when you need varying potential difference or one that is at a lower potential difference than the power supply.
How does a light dependent resistor work?
It has a very high resistance in the dark but a lower resistance in the light.
What can be used as one of the resistors in a potential divider circuit?
LDRs or thermistors as they give an output voltage that varies with the light level or temperature.
What is a potentiometer and how does it work?
A potentiometer has a variable resistor replacing R1 and R2 of the potential divider, but it uses the same idea. You move a slider or turn a knob to adjust the relative sizes of R1 and R2, which is useful when you want to change a voltage continuously, like in the volume control of a stereo.
