Chapter 8, 9, 10 - Electricity

Question 1

Define Current.

Answer 1

Current is defined as the rate of charge per second.
I = ΔQ / Δt
Measured in Amps.

Question 2

Define a coulomb.

Answer 2

One Coulomb (C) is defined as the amount of charge that passes a point per second with a current of 1 amp.

Question 3

What is the elementary charge of an electron?

Answer 3

e = 1.6x10⁻¹⁹

Question 4

Define Potential Difference.

Answer 4

Potential difference is defined as the work done per unit charge moved.
V = W / Q

Question 5

Define a Volt?

Answer 5

1 volt is defined as 1 joule of work moving 1 coulomb of charge through a component.

Question 6

How can the Potential Difference equation relate to Kinetic Energy?

Answer 6

W = VQ
The Work done equals the gain in kinetic energy 
1/2 x m x v² = VQ
Q = e (elementary charge)
1/2 x m x v² = eV

Question 7

What is Mean Drift Velocity?

Answer 7

The Mean Drift Velocity is the average velocity of the Charge Carriers.
It is given by:
I = Anev
Where I: Current (A), A: CS Area Of Wire (m²), n: Number Density of Electrons (m⁻³), e: Elementary Charge (C), v: Mean Drift Velocity (ms⁻¹)

Question 8

What is the number density of electrons?

Answer 8

The number density of electrons the is number of electrons per unit of volume.
Conductor have the highest number density.
Semi-Conductors have a lower number density.
Insulators have a very little if not 0 number density.

Question 9

What are Charge Carriers?

Answer 9

In Solids: The charge carriers are delocalised electrons.
When Molten or Liquid: Charge Carriers can be ions.
Gases: Most gases are insulators, unless a high enough voltage is applied to rip the electrons out the atoms.

Question 10

Define Resistance.

Answer 10

Resistance is the ratio of Voltage and Current.
R = V/I
Its units are Ohms (Ω)

Question 11

Define a Ohm.

Answer 11

1 Ohm is defined as a potential difference of 1V making a current of 1A.

Question 12

Define Resistivity.

Answer 12

Resistivity is defined as resistance multiplied by the CS area of the wire divided by the length of the wire.
Resistivity is the measure how hard it is for current to flow through a circuit.
⍴ = RA/L
Where ⍴: Resistivity (Ωm), R: Resistance (Ω), A: CS Area (㎡), L: Length (m)

Question 13

What is meant by an Ohms Law?

Answer 13

Ohms Law states: Provided the temperature is constant, the current through an ohmic conductor is directly proportional to the potential difference across it.
Resistance is constant in an ohmic conductor.

Question 14

How can you investigate the Resistivity of a metal wire?

Answer 14

• Measure: CS area of metal wire, and the current of through the wire (should be constant throughout), measure the length of the wire and the corresponding resistances.
• Plot a resistance against length graph.
• As R/L = ⍴/A, multiply the gradient by the current through the circuit.
• This will give the resistivity.

Question 15

What is meant by the term IV characteristics?

Answer 15

IV characteristic refer to the graph of Current against Voltage produced by different circuit components.\
You Need to know:
Metallic Conductor
Filament Lamp
Thermistor
LDR 
Diodes

Question 16

How does temperature effect Resistivity?

Answer 16

In metal wire the resistivity increases as temperature increases, this is because the atoms start to vibrate. This means that there are more collisions between the charge carriers and the atoms. Ergo, more resistance.

Question 17

Define Power.

Answer 17

Power is defined as the rate of work done.
P = VI
Measured in Watts (W)

Question 18

Define a Watt.

Answer 18

1 watt is defined as 1 joule of work per second.

Question 19

How can Power be expressed?

Answer 19

P = VI
P = W/t
P = V²/R
P = I²R

Question 20

How can the Work done by a circuit be calculated?

Answer 20

Since Power is the of work done.

W = VIt

Question 21

How do you convert from Joules to Kilowatt-hours?

Answer 21

Work Done = Power x Time
W -> kW is x10³
s -> hrs is x3600
Therefore Joules to KiloWatt-Hours is x3.6x10⁶

Question 22

How do you work out the Cost of electricity?

Answer 22

Cost = Price per Unit x Number of Units

Question 23

Why do we need to Cut down on energy usage?

Answer 23

• Fossil Fuels release carbon dioxide and other greenhouse gases, and is finite.
• Nuclear Power produces radioactive waste.
• Renewable energy requires lots of space (land for site or land to grow crops used in process).

Question 24

How can we reduce the amount of energy we use?

Answer 24

• Use LEDs or energy saving light bulbs as they use less energy and last much longer.
• When buying domestic devices look at the efficiency rating, and buy the appliance with the best efficiency.

Question 25

What is Internal Resistance?

Answer 25

Internal resistance is the inside a battery or power unit. It exists due the collisions between the charge carriers and the atoms making up the power unit.

Question 26

What is E.M.F?

Answer 26

E.M.F stands fro electromotive force. It is the total work done on each coulomb of charge by the battery.
Measure in volts.
𝜀 =W/Q

Question 27

What is Terminal P.D?

Answer 27

Terminal P.D is the potential difference actually going through the circuit.

Question 28

What are Lost Volts?

Answer 28

Lost volts are is the energy lost per coulomb to internal resistance.

Question 29

What are the equation for E.M.F?

Answer 29

𝜀 = V + v
𝜀 = I(R + r)
𝜀 = V + Ir
where: 𝜀: EMF (V), V: Terminal P.D (V), v: Lost Volts (v), I: Current (A), R: Load Resistance (Ω), r: Internal Resistance (Ω).

Question 30

What is Load Resistance?

Answer 30

Load resistance is the total resistance of all the components in the external circuit.

Question 31

How do you add EMF in Series and Parallel?

Answer 31

For both series and parallel circuit the total EMF is the sum of all the individual EMFs.
𝜀Total = 𝜀1 + 𝜀2 + 𝜀3 + …

Question 32

How can you investigate the EMF in a circuit?

Answer 32

• Set up a circuit with ammeter, variable resistor, and voltmeter in parallel across the resistor.
• Vary the current by changing the load resistance, measure the p.d and the current at defined intervals of current.
• Plot a Voltage against current graph.
• 𝜀 = V + Ir rearranges to V = -rI + 𝜀
• Therefore the gradient is negative value of internal resistance, and the y-intercept is the EMF.

Question 33

What is Kirchhoff’s First Law?

Answer 33

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

Question 34

What is Kirchhoff’s Second Law?

Answer 34

The total e.m.f going around a series circuit is equal to the sum of the potential differences across each component.
In parallel circuits this means that the total EMF is equal is to sum of the potential difference across each component in a loop.

Question 35

How do Kirchhoff’s Laws apply to a Series Circuit?

Answer 35

• Current is constant throughout the circuit.
• EMF equal the sum of potential differences.
• Total resistance equals the sum of all resistance in the circuit.

Question 36

How do Kirchhoff’s Law apply to a Parallel Circuit?

Answer 36

• Current is split at each junction, in-proportion of the resistance of each pathway.
• Total Pd in each loop is equal to the EMF.
• Rtotal⁻¹ = R₁⁻¹ + R₂⁻¹ + …

Question 37

What is a potential divider?

Answer 37

A Potential Divider is a circuit with a high voltage and a couple of resistors in series. One of which leading to an external circuit. That circuit is provided with a fraction of the voltage.

Question 38

How is the Voltage in a Potential Divider split between the resistors?

Answer 38

In series current is constant.
I₁ = I₂
V₁/R₁ = V₂/R₂
V₁R₂ = V₂R₁
V₁/V₂ = R₁/R₂

Question 39

What is the equation for Voltage Out in a Potential Divider?

Answer 39

Vout = R₂/(R₁+R₂) x Vin

Where R₂ is the resistor which the potential divider is over.

Question 40

How can Sensory Resistors be used in Potential Dividers?

Answer 40

If you replace R₂ with Thermistor or a LDR you can make a sensory Circuit. That will heat up or brighten up. As the Voltage going to circuit will change as the conditions change.