Electricity

Electricity

Conventional current

Flows from the positive terminal (anode) to the negative terminal (cathode)

Electron flow

Flows from negative terminal (cathode) to the positive terminal (anode)

How to calculate the charge passing a point in a certain time

charge = current x time

The definition of the coulomb

The charge that passes a point when a current of 1A flows for 1 second

The definition of 8 coulombs

The charge that passes a point when a current of 8A flows for 1 second

Why must an ammeter be placed in series (an ideal ammeter)

Its resistance is negligible

How do you calculate the number of electrons passing a point in a certain time

total charge passing a point in a time / charge of an electron

Q/ e or I t / e

How do you calculate the number of ions passing a point in a certain time

Q/ charge of ion

or I t / charge of ion

How do you calculate the number of alpha particles passing a point in a certain time

Q/ charge of alpha

or I t / 2e

What is conserved in Kirchhoff's first law

Conservation of charge

State Kirchhoff's first law

The sum of currents entering a point is equal to the sum of currents leaving the point

The definition of potential difference

The work done per unit charge transferred from electrical energy to other forms - when moving a charge between two points (you may need more detail with regards to the points but this will depend on the question)

Define the volt

The p.d when 1 Joule of energy is transferred per unit coulomb

Define eg 8V

The p.d when 8 Joules of energy is transferred per unit coulomb

Define a p.d of Z

The p.d when Z Joules of energy is transferred per unit coulomb from electrical to other forms

Define E.M.F

The energy given from the source per unit charge from chemical to electrical.

The energy per unit charge in moving the unit charge across the entire circuit (including the internal resistance)

Define an EMF of 12V

When 12 Joules of energy is transferred per unit coulomb from the source. This is from other forms to electrical energy.

When 8 J of energy is required to move a unit charge across the entire circuit (including the internal resistance)

Why is the p.d across the external resistance not the same as the E.M.F across the battery

The p.d is shared between the external resistor and the internal resistance. The energy given from the E.M.F is shared between the energy dissipated across the external circuit and the heat lost in the internal resistance

Define resistance

The potential difference per unit current (R = V/I)

Define the ohm

The resistance associated with the ratio of one volt per amp

State Ohms law

Current is proportional to the applied p.d if the temperature is constant

State what component is Ohmic

A fixed resistor / metallic wire ay constant temperature

State two components which are non Ohmic

A lamp or semi conductor such as a diode or thermistor

State three benefits of using an L.E.D over a conventional bulb

The L.E.D is more efficient (less energy dissipated as heat) The L.E.D emits more intense light The L.E.D is smaller and more robust

Define resistivity

(Resistance x Cross sectional area) / length

How does the resistivity of a metal wire change as temperature increases due to current

The resistance and hence resistivity increases with temperature due to the increase in vibrations of the ions in the lattice

How does the resistivity of a semiconductor (diode / thermistor) change as temperature increases

The resistance and hence resistivity decreases as the temperature increases

Define power

The energy transfer per unit time

How does a fuse work

A surge of current (i.e. when a device is first switched on) melts the wire in the fuse stopping the current and protecting the component

How would you determine the correct fuse for the job

Determine the current (eg use P = IV) and use the fuse which is a little larger than this

Define Kirchhoff's 2nd law

The sum of E.M.F's is equal to the sum of P.D's around a closed loop

What conservation law is Kirchhoff's 2nd law based upon

Conservation of energy

Cells in series

The E.M.F's of cells in series add if connected the same way BUT subtract if connected the opposite way. Internal resistances ADD regardless of whether the cells orientation.

Cells in parallel

The E.M.F's do not add up. The total internal resistance will decrease using (1/Rt = 1/R1 + 1/R2).

State the formulas used to determine energy transferred via an electrical component

W = QV (CHARGE X P.D)

W = ItV (CURRENT X TIME X P.D)

W = I^2 R t (CURRENT SQUARED X RESISTANCE X TIME)

W = V^2 t / R (P.D SQUARED X TIME / RESISTANCE) W = P t (POWER X TIME)

State the power formulas

P = W.D X time

P= IV

P=I² R

P = V² / R

What happens to the overall resistance when you add more resistors in series

The total resistance increases

What happens to the overall resistance when you add more resistors in parallel

The total resistance decreases

How do you work out the total resistance of a parallel combination

1/Rt = 1 / R1 + 1/R2

What is meant by terminal P.D

The p.d across the external resistance. Work done per unit charge in moving a charge across the terminals of a battery.

From electrical to other forms across the external resistance

State an advantage of a low internal resistance

Less energy loss per second within the cell. A lower resistance would increase the current (rechargeable cells can therefore charge faster)

State the formula involving e.m.f, terminal p.d, V, and internal resistance, r

EMF = internal p.d + terminal p.d

E.M.F = Ir + V

What graph is required to determine emf and internal resistance

Graph terminal p.d (y - axis) Vs current (x - axis)

Gradient = - r (internal resistance)

Y - intercept = EMF

What happens to the resistance of an L.D.R as light intensity increases

The resistance decreases

State three advantages of using data loggers in potential divider circuits

They can log data over very small time intervals and very long time intervals. The observer does not have to be present. They can be used to plot graphs and determine gradients / areas. They can use remote sensing

Describe the properties of a superconductor

As temperature decreases so does the resistance until it reaches the critical temperature at which the resistance decreases to zero. A superconductor has the property of producing magnetic fields.

State three practical applications of superconductors

The Maglev train uses magnetic fields set up via the superconductors. MRI machines use the intense magnetic fields set up via superconductors. Particle accelerators use the magnetic fields set up via the superconductors.