Electricity 1

Question 1

Electric current

Answer 1

The rate of flow of charge, symbol I, measured in amperes, A; normally a flow of electrons in metals or a flow of ions in electrolytes

A large current can be due to a greater number of electrons moving past a given point each second (because of greater cross sectional area, for e.g.) or the same number of electrons moving faster.
–> from I = Anev

Question 2

Coulomb

Answer 2

The derived SI unit of electrical charge, symbol C; 1 coulomb of electric charge passes a point in one second when there is a current of one ampere, 1 C = 1 As

Question 3

Elementary charge

Answer 3

The electric charge equivalent to the charge on a proton, 1.6 x 10^-19 C; symbol e

Question 4

Quantisation of net charge

Answer 4

An electron can’t be broken into smaller pieces, so charge always changes in multiples of the elementary charge. This property of electrons means that charge is quantized and the charge on any object must be an integer multiple of the elementary charge.

Question 5

Conventional current

Answer 5

a model used to describe electric current in a circuit - conventional current travels from positive to negative - it is the direction in which positive charges would travel

Question 6

Kirchhoff’s first law

Answer 6

At any point in an electrical circuit, the sum of currents into that point is equal to the sum of currents out of that point, electrical charge is conserved

Question 7

Mean drift velocity

Answer 7

The average displacement/distance travelled of the electrons along the wire per second. They move slowly in one direction through the metal lattice because they collide with the lattice.
I = Anev

Question 8

Number density

Answer 8

The number of free electrons per cubic metre of a material, symbol n, unit m^-3

Conductors have high number densities, insulators have very low number densities and semiconductors have number densities in between

Question 9

Potential difference (p.d.)

Answer 9

Defined as the energy transferred from electrical energy to other forms (heat, light, etc.) per unit charge

Question 10

Volt

Answer 10

The derived SI unit of potential difference and electromotive force, symbol V; 1V is the p.d. across a component when 1J of energy is transferred per 1C passing through the component; 1V = 1JC^-1

Question 11

Electromotive force (e.m.f.)

Answer 11

Defined as the energy transferred from chemical to electrical energy per unit charge

Question 12

Resitance

Answer 12

A property of a component calculated by dividing the potential difference across it by the current in it, symbol R, unit ohm, Ω

Question 13

Ohm

Answer 13

The derived SI unit of resistance, symbol Ω; a component with a p.d. of 1V per unit ampere; 1Ω = 1VA^-1

Question 14

Ohm’s law

Answer 14

The potential difference across a conductor is directly proportional to the current in the component as long as its temperature remains constant

Question 15

I-V characteristics

Answer 15

Resistor - I ∝ V, shown by the straight line passing through the origin

Filament lamp - as p.d. increases, current increases, so the temperature of the filament in the lamp increases, increasing resistance, causing current to increase at a decreasing rate

Diode - forward bias: zero reading until sharp increase in current when threshold p.d. is reached; reverse bias: zero reading until sharp increase in current when diode breaks down

LED - sharp increase in current is further away from the origin as frequency of light increases

Question 16

Method for IV characteristics

Answer 16

Independent variable = Potential difference, V (change from battery pack)
Dependent variable = Current, I
Control variables:
Use of the same equipment eg. wires, diodes

Question 17

Resistivity

Answer 17

A property of a material, measured in Ωm, defined as the product of the resistance of a component made of the material and its cross sectional area divided by its length

In metals, resistivity increases as temperature increases because the positive ions will vibrate faster, hindering the motion of electrons

In semiconductors, resistivity decreases as temperature increases because the number density increases

Question 18

LDR

Answer 18

As light intensity increases, resistance decreases because the semiconductor it is made of has a low number density in low light intensities, but it increases as light intensity increases

Question 19

Thermistor

Answer 19

As temperature increases, resistance decreases because the semiconductor it is made of has a low number density at low temperatures, but it increases as temperature increases

Question 20

Method for resistivity

Answer 20

Independent variable = length of wire (shifting position of crocodile clips)
Dependent variable = resistance, R (measure p.d. and current)
Control variables = same equipment e.g. wires, ammeter, voltmeter

Question 21

Kilowatt-hour

Answer 21

A derived unit of energy, most often associated with paying for electricity, symbol kWh (1 kWh = 3.6 MJ)

Question 22

Kirchhoff’s second law

Answer 22

In any circuit, the sum of electromotive forces is equal to the sum of p.d.s around a closed loop. Energy is conserved.