Section 6: Electricity

Question 1

What does Ohm’s Law state?

Answer 1

Provided the physical states such as temperature remain constant, current through an ohmic conductor is directly proportional to the potential difference across it.

Question 2

When the gradient of a IV characteristic graph gets shallower, what happens to the resistance?

Answer 2

The resistance increases as the gradient decreases.

Question 3

What does the IV characteristic graph look like for an ohmic conductor?

Answer 3

Straight line through the origin.

Question 4

What does the IV characteristic graph look like for a filament lamp?

Answer 4

A curve that starts steep but shallower as the voltage rises.

Question 5

What does the IV characteristic graph look like for a diode?

Answer 5

Pretty much level but then a sharp increase.

Question 6

What is resistivity?

Answer 6

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.

Question 7

What is the equation to find the resistivity of a material?

Answer 7

resistivity = (resistance x cross-sectional area) all divided by length.

Question 8

What are semi-conductors?

Answer 8

A group of materials that aren’t as good at conducting electricity as metals because they have far fewer charge carriers (electrons) available.

Question 9

What happens if energy is supplied to a semi-conductor?

Answer 9

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.

Question 10

What are three examples of semi-conductors?

Answer 10

thermistors, diodes and light dependent resistors

Question 11

What is a thermistor?

Answer 11

A component with a resistance that depends on its temperature.

Question 12

How do thermistors work?

Answer 12

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.

Question 13

What experiment can you do to investigate the resistance of a thermistor?

Answer 13

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.

Question 14

How can a material become a superconductor?

Answer 14

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.

Question 15

What are three uses of superconductors?

Answer 15

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.

Question 16

How can you calculate the cross-sectional area of a wire?

Answer 16

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.

Question 17

Describe the method of measuring the resistivity of a wire? (Required Practical 5)

Answer 17

1. Attach the flying lead to the end of the test wire and measure the length of the test wire connected in the circuit.
2. 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.
3. Repeat this process at least one more time and calculate the mean resistance for this length of wire.
4. Reposition the flying lead and repeat steps 2 and 3 to get an average resistance for a range of different lengths of test wire.
5. Plot a graph of average resistance in ohms against length in meters using your results.
6. Use the equation of resistivity to find your resistivity of the wire.

Question 18

What is internal resistance?

Answer 18

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.

Question 19

What is load resistance?

Answer 19

The total resistance of all the components in the external circuit. It is also called external resistance.

Question 20

What is E.M.F and what does it stand for?

Answer 20

E.M.F. –> Electromotive force
It is the amount of electrical energy the battery produces and transfer to each coulomb of charge.

Question 21

What is E.M.F. measured in?

Answer 21

Volts

Question 22

What is lost volts?

Answer 22

The energy wasted per coulomb overcoming the internal resistance.

Question 23

What does the conservation of energy law tell about load resistance and internal resistance?

Answer 23

Energy per coulomb supplied by the source =
energy per coulomb transferred in load resistance +
energy per coulomb wasted in internal resistance.

Question 24

Describe the method of measuring internal resistance and E.M.F.? (Required Practical 6)

Answer 24

1. 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
2. Decrease the resistance of the variable resistor by a small amount and repeat step 1 for this resistance.
3. 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).
4. Plot a V-I graph for your mean data and draw a line of best fit - you get a straight line graph.
5. Make sure all other variables are kept constant when carrying out the experiment, including external factors like temperature (which affects the resistivity of materials).

Question 25

What is Kirchoff’s first law?

Answer 25

The total current entering a junction = the total current leaving it.

Question 26

What is Kirchoff’s second law?

Answer 26

The total EMF around a series current = the sums of the PDs across each component.

Question 27

In a series circuit, how is the total resistance found?

Answer 27

R = R1+R2+R3…

Question 28

In a parallel circuit, how it the total resistance found?

Answer 28

1/R =1/R1 + 1/R2 + 1/R3

Question 29

What is a potential divider?

Answer 29

A circuit with a voltage source and a couple of resistors in series.

Question 30

What do you use potential dividers for?

Answer 30

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.

Question 31

How does a light dependent resistor work?

Answer 31

It has a very high resistance in the dark but a lower resistance in the light.

Question 32

What can be used as one of the resistors in a potential divider circuit?

Answer 32

LDRs or thermistors as they give an output voltage that varies with the light level or temperature.

Question 33

What is a potentiometer and how does it work?

Answer 33

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.