Current Electricity

Question 1

What is Electric Current

Answer 1

The flow of charge per unit time

Question 2

Electric Current Equation

Answer 2

I = Q /t

Current (A) = Charge (C) / Time (s)

Question 3

What directions do electrons move in a wire

Answer 3

Electrons normally drift in random directions but there is no net movement of charge

Question 4

What is Potential Difference

Answer 4

The energy transferred per unit charge

Question 5

Potential Difference Equation

Answer 5

V = W /Q

Volatge or Potential Difference (V) = EnergyTransferred (J) / Charge (C)

Question 6

Why do you want your voltmeter to have infinite resistance and your ammeter to have zero resistance

Answer 6

• Voltmeter - current takes the path of the least resistance so if the voltmeter has infinite resistance no current will flow through it, therefore all the current will flow through the appliance
• Ammeter - if there is zero resistance that means there would be zero voltage across the ammeter so no energy would be lost across it

Question 7

What is a Conventional Current

Answer 7

• The idea that potential difference always goes from positive to negative
• Even though in reality the negative electrons are attracted towards the positive

Question 8

What is Resistance

Answer 8

The ratio of voltage to current

Question 9

What is the Resistance Equation

Answer 9

R = V / I

Resistance = Voltage (V) / Current (A)

Question 10

What factors effect the resistance of a material

Answer 10

• Cross Sectional Area
• Length of a Wire
• Temperature
• Material of Wire

Question 11

What does Ohm’s Law state

Answer 11

For an ohmic conductor current is directly proportional to the voltage, provided that the physical conditions such as temperature are kept constant

Question 12

What is a Non-Ohmic Component

Answer 12

A component that does not obey Ohm’s Law meaning the current and voltage are not directly proportional

Question 13

Describe the Ohmic Conductor Graphs

Answer 13

Question 14

Describe the Semiconductor Diode Graph and Explanation

Answer 14

The diode will allow current to flow in the forward bias past the thresold voltage (0.7v). In the direction of the reverse bias, the resistance of the diode is extremely high meaning that only a very small current can flow

Question 15

Filament Lamp Graph and Explination

Answer 15

• At low current Ohm’s Law is obyed
• As the current increases, the temperature increases, increasing the frequency of electron collisons, this causes the electrons to slow down
• Therefore current decreases and so resistance increases

Question 16

What is the Resistivty Equation

Answer 16

p = RA / L

Resistivty (Ωm)= Resistance (Ω) x Cross Sectional Area (m^2) / Length (m)

Question 17

Why do we use Resistivty

Answer 17

It is useful to compare different materials because resistivty gives the value of resistance through a material independent of other factors (apart from temeprature)

Question 18

Describe how you would find the resistivity of a wire and describe the labelled circuit diagram

Answer 18

• Measure the diameter the wire at various points, using the micrometer and find and record the mean diameter
• Set up the apparatus as shown in the diagram.
• Adjust the length to 0.1m using the crocodile clips, measured using the metre ruler.
• Read and record the current on the ammeter and the voltage on the voltmeter
• Calculate the resistance using R=V/I
• Repeat this at 0.1m intervals from 0.1 - 0.8m
• Calculate the cross-sectional area of the wire using A = πr^2
• Plot a graph a graph of Resistance against Length and draw a line of best fit, the resistivity will be the gradient multiplied by the cross-sectional area of the wire

Question 19

Explain what happens to resistance when the temperature of a metal conductor increases (Explain in terms of energy of metal ions)

Answer 19

• When the temperature increases the positive metal ions gain more kinetic energy
• This means the positive ion cores vibrate more and more
• Increasing the frequency of electron collisons
• This causes the electrons to slow down
• Therefore current decreases and so resistance increases

Question 20

What is a Thermistor

Answer 20

A component those resistance varies with temperature (when temperature increases, their resistance decreases)

Question 21

Thermistor Graph and Explanation

Answer 21

• As the temperature of a thermistor increases, its resistance decreases.
• This is because increasing the temperature of a thermistor causes electrons to be emitted from atoms, therefore the number of electrons increases and so current increases causing resistance to decrease

Question 22

What is an Application of a Thermistor

Answer 22

A temperature sensor, for example, it could be used to turn on the heating once room temperature drops below a specific value

Question 23

What is a Superconductor

Answer 23

A material which below the critical temperature, has zero resistiance (the critical temperature depends on the material)

Question 24

What are the Application of Superconductors

Answer 24

• Power cables - superconductors would mean energy loss would be zero beacuse the cables would not heat up at all
• Strong magnetic fields - superconductors would not require a constant power source. In magnetic trains there would be no friction between the train and rail
• Less fuel is needed when using superconductors which reduced greenhouse gases produced

Question 25

Superconductor Graph and Explination

Answer 25

Below the critical temperature, the material has zero resistivity