Electricity

Question 1

Electric field

Answer 1

A region in which an electric charge will experience a force.

Question 2

Electric current

Answer 2

The rate of flow of charge.

Question 3

Kirchoff’s 1st Law

Answer 3

The total current into a junction is equal to the total current of the junction.

Question 4

Mean drift velocity

Answer 4

-> Average distance travelled by the electrons along the wire per second.

They move slowly in one direction through the metal lattice, because they are continually colliding with other electrons and the ions in a short distance.

Question 5

Number Density

Answer 5

The number of charge carriers per cubic metre. (m^-3)

Question 6

+How does number density vary for conductors, semi-conductors and insulators?

Answer 6

-> Conductors: Will have a much larger value of n as metals
-> Semi-conductors: Will have a lower value of n compared to metals but higher than insulators
-> Insulators: Will have the smallest value of n compared to semi-conductors and conductors

Question 7

Variation of number density with temperature:

Answer 7

-> A wire can become hot due to the the heating effect of an electric current. For a material such as a conductor the number density will not be changed. But for a semi-conductor (such as a thermistor) if the temperature increases, the agitation of the ions in the material actually releases electrons thus the number density increases. This actually results in the resistance of the thermistor decreasing, for the same applied p.d.

Question 8

Kirchoff’s 2nd Law

Answer 8

Sum of e.m.f.’s is equal to the sum of p.d.’s in a closed loop.
- In Kirchoff’s 2nd Law energy is conserved so this law is called a conservation of energy law.

Question 9

E.m.f.

Answer 9

Work done per unit charge. The electromotive force of a cell is the amount of energy transformed from other forms into electrical per unit charge.

Question 10

P.d.

Answer 10

Work done per unit charge. The potential difference across a component is the amount of energy transformed from electrical into other forms per unit charge.

Question 11

Volt

Answer 11

1 Volt = 1 Joule per Coulomb

Question 12

Resistance

Answer 12

Resistance of a component is the ratio of potential difference across the component through it. Resistance opposes current.

Question 13

Ohm

Answer 13

1 Ohm is 1 Volt per Amp

Question 14

Ohm’s Law

Answer 14

A component obeys Ohm’s law if the current through the component is directly proportional to the potential difference across it provided the temperature of the component remains constant.

Question 15

Threshold Voltage

Answer 15

This is the voltage at which the diode conducts as the current starts to increase.

Question 16

Resistivity

Answer 16

The resistivity of a material is = resistance x cross sectional area of material/ length of material

Question 17

Power

Answer 17

The rate at which energy is transferred.

Question 18

Watt

Answer 18

1 Watt is 1 Joule per second

Question 18

Why does a NTC thermistor’s resistance decrease when temperature increases?

Answer 18

When temperature increases, the amplitude of vibration of ions in the material increases, whuch causes the release of electrons, thus the number density increases. The resistance therefore decreases for the same p.d.

Question 18

Why does an LDR’s resistance decrease as light intensity increases?

Answer 18

The LDR has a resistance affected by light intensity. As the light intensity exposed to the LDR increases, the resistance decreases. This is because the light provides energy to release electrons from the sensitive material thus the number density of the LDR material increases and resistance decreases for the same applied p.d.

Question 19

1kWh

Answer 19

-> 1 kWh is the energy used by a device of power 1kW used for 1 hour
-> 1kWh is equal to 3.6 million Joules

Question 20

-Find energy transferred (kWh)
-Find cost for kWh

Answer 20

-> Energy transferred (kWh) = Power (kW) x Time (hours)

-> Cost = Units used (kWh) x cost per unit

Question 21

How to calculate total resistance in a series circuit?

Answer 21

R = R1 + R2 + R3 +…

Question 22

Internal Resistance

Answer 22

Resistance at the source of emf (e.g. cell) due to its construction.

Question 22

How to calculate total resistance for parallel circuits?

Answer 22

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

Question 23

Terminal P.d.

Answer 23

The potential difference across the terminals of a cell. If there was no internal resistance, the terminal p.d. would be equal to the emf

Question 24

What does 'lost volts' mean?

Answer 24

Work done per unit charge from electrical to thermal energy inside a power source (cell) with internal resistance. Lost volts = E-V. Where E is emf and V is terminal p.d.

Question 25

Under what circumstances is terminal p.d. equal to e.m.f?

Answer 25

When current is zero or when there is negligible internal resistance.

Question 26

Derive the equation for total resistance for resistors in parallel:

Answer 26

I = I1 + I2 + I3 +... (due to Kirchoff 1st Law) V/R = V/R1 + V/R2 + V/R3 +... (p.d. is the same due to Kirchoff's 2nd Law)

Question 27

How is internal resistance represented in a circuit diagram?

Answer 27

By a resistor in series with the power supply both enclosed with a dashed box.

Question 28

Derive the equation E=I(r+r)

Answer 28

E = V + lost volts and V = IR, E = V + Ir , E = IR + Ir. Since I is the same due to Kirchoff's 1st Law then E = I(R+r)

Question 29

What is the advantage of a potentiometer over a potential divider?

Answer 29

Output V has a wider range (can be varied from 0V to the maximum source p.d.)

Question 30

Derive the equation for total resistance for resistors in series:

Answer 30

V = V1 + V2 + V3 + ... (due to Kirchoff's 2nd Law) IV = IR1 + IR2 +IR3 + ... (current is the same at all points due to Kirchoff's 1st Law) so R = R1 + R2 + R3 +...

Question 31

How should a voltmeter be connected in a circuit?

Answer 31

In parallel with the component you're measuring the p.d. through

Question 32

Which conservation law leads to Kirchoff's first law?

Answer 32

Conservation of charge

Question 33

Define the coulomb

Answer 33

The charge that passes in 1 second when the current is 1 ampere (1C=1As)

Question 34

Descrive the I-V graph for a diode or LED

Answer 34

Zero I for any V at all negative V and small positive V, gradient increases rapidly as V increases in positive direction.

Question 35

A metal has a positive temperature coefficient for resistance, what does this mean?

Answer 35

Its resistance increases with temperature

Question 36

Explain what is meant by charge being quantised:

Answer 36

The charge on an object is always a multiple of elementary charge, e

Question 37

What are the similarities between emf and pd?

Answer 37

-> They are both work done per unit charge ->They are both measured in volts

Question 38

What is the name for a conductor that obeys Ohm's Law?

Answer 38

Ohmic conductor

Question 39

Compare the direction of conventional current and electron flow in a circuit:

Answer 39

-> Conventional : Positive to negative -> Electron flow: Negative to positive

Question 40

What are charge carriers in a) metals b) electrolytes?

Answer 40

-> Metals: Delocalised electrons -> Electrolytes: Ions

Question 41

Describe the motion of electrons in a wire with a potential difference across it:

Answer 41

Random and high speed colliding with positive ions, with overall low average speed in one direction (mean drift velocity)

Question 42

What's the difference between emf and pd?

Answer 42

-Emf is work done per unit charge on the charge carriers -Pd is work done per unit charge by the charge carriers Emf is the energy transferred from other forms to electrical, whereas pd is energy transferred from electrical to other forms.

Question 43

Derive I = Anev for a conductor length L and cross-sectional area A:

Answer 43

-> Number of charge carriers = ALn -> Total charge = Q = ALne -> v=s/t=L/t so t=L/v -> I =▲Q/▲t -> I = ALne/(L/v) -> so I = Anev

Question 44

For electrons accelerated through a potential difference give an expression for kinetic energy:

Answer 44

eV = 1/2 mv^2

Question 45

What is the resistance of ideal ammeters and voltmeters?

Answer 45

-> Ideal ammeters have zero resistance -> Ideal voltmeters have infinite resistance

Question 46

Explain how resistance changes for a filament lamp as V increases:

Answer 46

Resistance increases because V increases, I increases and therefore temperature increases (heating effect of a current) so R increases

Question 47

Explain how resistance changes for a diode or LED:

Answer 47

For negative V, R is infinite(or very high), for positive V, R is initially very high and then decreases rapidly because the number density of free electrons increases.

Question 48

How does the average speed of a free electron compare to mean drift velocity?

Answer 48

Electron speed >> Mean drift velocity

Question 49

Why is the average speed of an electron higher than the mean drift velocity?

Answer 49

- Average speed : This refers to the overall speed of electrons as they move randomly within a conductor. They move fast due to thermal energy. -Mean drift velocity: This is the average velocity of the electrons in a specified direction when an electric field is applied. While the electrons do have a high average speed, they collide with atoms and other obstacles which slows their net movement down.

Question 50

State the charge on a proton and an electron in terms of elementary charge:

Answer 50

Electron : -e (-1.6x10^-19) Proton : +e (+1.6x10^-19)