Download UNIT: 12 ELECTRICAL ENERGY

UNIT: 12 ELECTRICAL
ENERGY
12.1 BATTERIES
Batteries in Action
We use devices that contain batteries all the time.
¨  Name 3 devices that you have used TODAY that run
on batteries.
¨ 
Batteries in Action
A battery consists of electric cells connected
together. A cell (or a battery) is a source of
electrical energy. Each cell transforms non-electrical
energy into electrical energy.
¨  One terminal of a cell is positively charged and the
other is negatively charged.
¨ 
Batteries in Action
A chemical cell transforms chemical energy into
electrical energy. This happens because substances
inside the cell react with each other.
¨  This diagram shows a simple non-rechargeable cell
containing a zinc rod and a copper rod in dilute
sulfuric acid.
¨ 
Batteries in Action
¨ 
The zinc rod becomes negatively charged and the
copper becomes positively charged as a result of
chemical reactions in the cell.
Batteries in Action
A solar cell transforms light energy into electrical
energy. This happens because atoms in the solar cell
release electrons as a result of absorbing light.
¨  The movement of electrons causes one terminal of
the cell to become positively charged and the other
negatively charged.
¨ 
Electromotive Force
¨ 
¨ 
When a cell is in a circuit, charge is forced to flow
round the circuit by the cell. The flow of charge
transfers energy from the cell to the circuit
components.
The electromotive force (or ‘emf’) of a cell or a
battery is a measure of how much ‘push’ the cell or
battery can provide to force charge around the
circuit. It is measured in volts.
Electromotive Force
The emf of the cell is sometimes referred to as its
‘voltage’.
¨  For any given battery-operated device, the battery
in it must be the correct voltage.
¨  If the battery emf is too low, the device is unlikely to
work and if it is too high, the device is likely to be
damaged.
¨ 
EMF
All batteries have an emf.
¨  Emfs convert chemical, mechanical, and other forms
of energy into electrical energy.
¨  EMF is how much energy can be converted into
electrical energy by each coulomb of charge
(6.24x1018 electrons)
¨ 
Electromotive Force
¨ 
The electromotive force of a source of electrical energy is
the electrical energy it produces per coulomb of charge.
emf of a battery in volts = electrical energy produced in joules
charge in coulombs
¨  EMF = E / Q
¨  For every coulomb of charge that passes through a battery,
a cell of 12V produces 12J of electrical energy. Therefore,
a 12V battery can deliver twice as much energy per
coulomb of charge as a 6V battery
¨  1V = 1J/1C
Voltmeter
¨ 
A voltmeter is used to measure the emf of a
battery.
Practice
¨ 
¨ 
¨ 
If a battery produces 36J of electrical energy from
12C of charge, what is the voltage of the battery?
How much energy can a 9V battery produce from
15C of charge?
What is the EMF of 6 1.5V batteries?
12.2 POTENTIAL
DIFFERENCE
Energy Transfer in a Circuit
In a circuit containing a light bulb, variable resistor,
and a battery; charge from the battery has the
potential to deliver energy to the circuit
components.
¨  When it flows around the circuit, it transfers energy
from the battery to the light bulb and the variable
resistor.
¨ 
Energy Transfer in a Circuit
A voltmeter is connected across the light bulb. We
say that it is in parallel with the bulb.
¨  A voltmeter can be connected to any two points in a
circuit.
¨ 
Energy Transfer in a Circuit
Its reading gives the potential difference
(abbreviated to ‘pd’ and sometimes referred to as
‘voltage’) between those two points.
¨  This is a measure of
the energy
transferred by each
electron as it passes
between those two
points.
¨ 
Energy Transfer in a Circuit
If a voltmeter is connected across the terminals of
the battery, it measures the emf of the battery in
volts, provided no energy is ‘wasted’ as heat.
¨  If a voltmeter is connected across the bulb or the
variable resistor, its reading in volts is a measure of
the electrical energy supplied to the lamp or
resistor by each electron.
¨ 
Potential Difference Equation
¨ 
The potential Difference across an electrical
component is:
potential difference (V) = electrical energy supplied (J)
Charge (C)
¨ 
If you have a circuit with 2 bulbs in series:
¤  EMF
of the battery = the pd of the first bulb + the pd
of the second bulb