Physics module 4.2 (energy, power and resistance)

Question 1

What is the difference between electromotive force and potential difference?

Answer 1

-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

Question 2

Do Revision cards

Question 3

What is Ohms law?

Answer 3

-The current through a conductor is directly proportional to the potential difference, provided physical conditions stay constant.

Question 4

Why is the resistance of wires low? and why does increasing length of wire increase resistance?

Answer 4

-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

Question 5

How does the cross-sectional area of a wire affect resistance?

Answer 5

-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.

Question 6

How does changing the temperature of a wire affect the resistance?

Answer 6

-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.

Question 7

What is resistivity?

Answer 7

-A property that describes the extent at which a material opposes the flow of electric charge
-It is a fixed value for different materials

Question 8

Is resistivity affected by temperature, length or cross-sectional area?

Answer 8

-Resistivity isn’t affected by length or cross-sectional area but it is affected by temperature?

Question 9

Why is electricity carried at high voltage on the national grid?

Answer 9

-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

Question 10

What is a kilowatt-hour?

Answer 10

-A kilowatt-hour is equal to 3,600,000Js
-It is most commonly used when looking at electrical bills