Download File - The Physics Doctor

A –Level Physics:
Electrical Quantities:
Current and Electrical Energy
Objectives:
Additional skills gained:
• Deriving equations
• Accurate definition of quantities
Starter: Experimental Design
Design an experiment that you would
undertake to calculate the Young’s modulus
of a wire.
Include all the measurements etc. that you
would need to take and graphs that you would
draw
You have 15mins ONLY
Extension: Draw a diagram of this experiment and
fully label it
GCSE link: Charge
Copy down the following questions and then answer them
using what you remember from GCSE (20mins)
1) An electrostatic paint sprayer is going to be used to paint a metal
object. When the paint particles leave the sprayer, they are negatively
charged.
Explain the benefits of using this sprayer compared with one that does
not charge the paint
2) Vicky combs her hair with a metal comb. Then she tries to pick up
some small pieces of metal foil with the comb. The metal comb does
not pick up any pieces of metal foil.
Explain why the metal foil is not picked up by the comb.
3) The student rubs a cloth against a balloon and holds the balloon
against a wall. The balloon sticks to the wall when he lets go. Explain
how this happens.
Charge
• The Greek philosopher Thales
(c.600BC) discovered that
when amber was rubbed
with a cloth, it could attract
small pieces of dried leaf.
• The Greek word for amber is
elektron which is the origin of
our words electron and
electricity
• Under normal circumstances
(due to equal number of
protons and electrons), there
is no overall charge on an
object (conservation of
charge) but we can induce
charge via friction
Charge
• Some particles are said to have
an electric charge such as the
electron which has a negative
charge.
The charge on a single electron
is 1.6 x 10-19C
So…how many electrons make up
1C of charge?
6.25 x 1018
• This fundamental charge on a
particle is unchangeable,
remember we can’t destroy nor
create charge (conservation of
charge)
Definition Challenge
There are so many misconceptions with defining electrical
quantities. Copy and fill in the table below. The last
column will be filled in after we have discussed them!
Electrical
Quantity
Symbol
Unit
My original
definition
Official Definition
Current
The rate of flow
of charge
Potential
Difference
The energy
transferred by a
component per
coulomb of charge
Electromotive
Force
The energy
supplied to each
coulomb of charge
(not a force at all!)
Current
We defined current as the rate of flow of charge and this
can be applied to anything that is carrying a charge (charge
carriers) such as conventionally, the electron.
This can be expressed simply mathematically with the
formula:
𝐶𝑢𝑟𝑟𝑒𝑛𝑡=
𝐶ℎ𝑎𝑟𝑔𝑒
𝑇𝑖𝑚𝑒
aka
𝐼=
∆𝑄
∆𝑡
So if 1C worth of charge flows by in 1s, then 1A of current is
flowing! The ampere is the base unit of current but as 1A is
actually a large current, often values are given in mA, so don’t
forget to convert to amperes.
What current is produced when a lightning strike that lasts
for 0.1s transfers 1150 coulombs of charge?
What’s the (potential) difference?
There are four separate quantities that are often
interchanged in GCSE but are subtly different!
Voltage
Electromotive Force
Potential Difference
Electronvolt
Weirdly, less first consider how they are similar and why they
get confused!
Generally speaking, they have almost identical formula to
calculate. It’s just the symbols that alter!
𝑬𝒏𝒆𝒓𝒈𝒚 (𝑱)
𝑸𝒖𝒂𝒏𝒕𝒊𝒕𝒚 =
𝑪𝒉𝒂𝒓𝒈𝒆 (𝑪)
What’s the (potential) difference?
Voltage
Voltage is the generalised term, and is the one you are most
familiar with from previous years.
It is very rarely used at A-level but is simply the measure of
the amount of energy a component uses per unit charge
It’s not specific enough when discussing circuits and current, so
let’s try to avoid it
𝑬𝒏𝒆𝒓𝒈𝒚 𝑻𝒓𝒂𝒏𝒔𝒇𝒆𝒓𝒓𝒆𝒅 (𝑱)
𝑽𝒐𝒍𝒕𝒂𝒈𝒆(𝑽) =
𝑪𝒉𝒂𝒓𝒈𝒆 𝑷𝒂𝒔𝒔𝒊𝒏𝒈(𝑪)
𝑬
𝑽=
𝑸
What’s the (potential) difference?
Potential Difference
Potential difference is the more specific term for voltage, as it
highlights the transformation of energy from electrical into
another type
It is the amount of electrical energy transferred by a
component into a different form per unit charge
In order to transfer the energy, the component is said to have
done work and this is why the symbol has changed.
𝑬𝒏𝒆𝒓𝒈𝒚 𝑻𝒓𝒂𝒏𝒔𝒇𝒆𝒓𝒓𝒆𝒅 (𝑱)
𝑷. 𝒅(𝑽) =
𝑪𝒉𝒂𝒓𝒈𝒆 𝑷𝒂𝒔𝒔𝒊𝒏𝒈(𝑪)
𝑾
𝑽=
𝑸
What’s the (potential) difference?
Electromotive Force
This is basically the supply voltage, i.e. if you were to put a
voltmeter around the power-pack or battery etc, this would be
the reading
It is the energy transferred to each coulomb of charge
from a supply. It is given symbol of epsilon.
𝑬𝒏𝒆𝒓𝒈𝒚 𝑻𝒓𝒂𝒏𝒔𝒇𝒆𝒓𝒓𝒆𝒅 (𝑱)
𝑬𝑴𝑭(𝑽) =
𝑪𝒉𝒂𝒓𝒈𝒆 𝑷𝒂𝒔𝒔𝒊𝒏𝒈(𝑪)
ε=
𝑾
𝑸
What’s the (potential) difference?
Electronvolt
This quantity is likely to be new to you and is only really
considered for sub-atomic particles (i.e. looking at an
individual electron itself)
If an electron is accelerated by a p.d of 1V then it gains
energy equal to its charge.
Voltage(𝑽) =
𝑬𝒏𝒆𝒓𝒈𝒚 𝑻𝒓𝒂𝒏𝒔𝒇𝒆𝒓𝒓𝒆𝒅 (𝑱)
𝑪𝒉𝒂𝒓𝒈𝒆 𝒐𝒏 𝒂𝒏 𝒆𝒍𝒆𝒄𝒕𝒓𝒐𝒏(𝑪)
V=
𝑬
𝑸
Practice Questions
Explain how potential difference is different from
electromotive force
a) A cell provides 76C of charge with 120J of energy, what’s
the emf? b) What is the p.d. across a lamp when the lamp
carries a current of 2.4A for 5.4s transferring 120J of energy?
Convert a) 9.6x10-19 J into elecronvolts and b) 4.8MeV into
joules
Write an evaluation of the strengths and weaknesses of a ski
slope model of an electric circuit including discussion of:
• Representation of the charge carriers
• Analogies of electrical energy
• Ski lift as a cell
• Snowpark obstacles as components
• Differing speeds by different skiers
• How each quantity is represented
Objectives:
Additional skills gained:
• Deriving equations
• Accurate definition of quantities