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QUIZ
1. Define electric force
The amount of attraction or repulsion between two
charged objects
2. What is charge measured in?
C
3. If two charges are opposite, what happens to those charges?
They will attract each other
4. The force between two charges can be calculated using the
following equation
F = k Q1Q2
r2
What two things affect the amount of Force between 2 charges?
The size of the charge and
the distance between them
5. Draw the electric fields of two negative charges and a positive
and neutral charge
When a battery is connected to a conductor;
• One end of the conductor becomes positive, the other
negative
• Excess positive charge at one end, excess negative charge at
other end
• This causes an electric field to form, directed away from
positive towards negative
• Charges in the wire will experience a force due to this field
• Free electrons in the wire will tend to move (in direction
opposite to the field)
• This movement of charge is called ELECTRIC CURRENT
ELECTRIC CURRENT
Electric Current is the flow of electric charge
Magnitude of current is defined as the rate of transfer of charge
Amount of charge that flows past any point in a conductor in one
second
A current of 1 A flows when 1 C flows past a point in 1 second
1 A = 1 C s-1
ELECTRIC CURRENT
Electric Current is the rate of transfer of charge
I=q
t
I = Current in Amps
q = Charge in C
t = Time in seconds
1 Ampere (A) = 1 C s-1
1C=1As
Determine the charge that has flowed through a torch battery
producing a current of 300mA if it has been left on for 20
minutes.
I=q
t
q=Ixt
300 x 10-3 x 1200s = 360 C
CURRENT FLOW
• Electric charge can be positive or negative
• The direction of an electric current is the direction of the flow of POSITIVE
CHARGE
• Positive charge can be transferred either by positive charges moving in the
direction of the current or by negative charges moving in the opposite
direction
• In a metal wire, the current is carried by electrons moving in the direction
opposite to that of the current
• Two types of current, electron current and conventional current
• CONVENTIONAL current is the one we use because it is the transfer of
positive charge
I
t=0
I
t = later
---------------
-q
++++
++++
++++
+q
EMF & ELECTRIC POTENTIAL
In order to drive a current the charges must be given energy
EMF stands for Electromotive Force, it is not a force but it is the
energy given to charges
EMF – concentration of charge
Concentrated charges have potential energy, when the charges
are allowed to move the electrical energy can be recovered
EMF is electric potential given to charges by a device
EMF & ELECTRIC POTENTIAL
A battery uses chemical energy to give charges on its terminals
potential energy
A generator uses the kinetic energy of rotation to give charges
potential energy
A solar cell uses sunlight to give potential energy to charges
ELECTRIC POTENTIAL
SI unit for Potential is Joules per Coulomb
1J
-1
C
= 1 Volt
The alternator of a car being driven at night with the headlights on is
producing a 50 A current at an EMF of 12V
How many coulombs of charge flow from the alternator each second?
1 A = 1 J s-1
50 A = 50 C per second
How many joules of energy does each coulomb of charge obtain?
1 V = 1 J C-1
12V = 1C of charge is given 12J of energy
How many joules of energy does the alternator produce each second?
1 C is given 12J of energy therefor
50 C x 12 J = 600J
Where does this energy go?
The energy goes to the headlights, the ignition and other electrical devices. Some
may also go and recharge the battery
ELECTRIC CIRCUITS AND POTENTIAL DIFFERENCE
A electric circuit consists of at least one source of EMF,
conductors and circuit elements (bulbs, switches)
In these circuit elements is where electric potential energy is
converted into heat, light, motion
When a battery is connected to a circuit it creates a potential
difference
When switch is off all connected to positive terminal will have
positive charge concentration, all those connected to negative
side will have negative charge concentration
When switch turned on, charges move through circuit using up
their potential energy as they move through circuit elements.
When charge moves through a circuit element it loses potential energy
This is known as
Potential Difference
A charge q moving through a potential difference ∆V will lose energy
E = qV
E = energy in J
q = charge in C
V = voltage in V
The potential difference across a torch bulb is found to be 2.7V.
The current flowing through it is 0.2A
How much charge flows through the torch in 1 minute?
q = It
q = 0.2A x 60 s
q = 12C
How much energy is lost by this charge?
E = qV
E = 12C x 2.7V
E = 32.4 J
Complete Q 7.3, page 289 in
Heinemann Physics
1.What direction does current flow in a circuit?
Current flows in the direction of the transfer of
positive charge
2. A source of EMF does what to a charge?
Gives electric potential energy to a charge
3. 1 V = ?
1 V = 1 J C-1