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ELECTRIC CURRENT 2018 19

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ELECTRIC CURRENT
• A flow of charge is called an electric current.
• The higher the current, the greater the flow of charge
• Water molecules tend to flow from areas of
high gravitational potential energy to low
gravitational potential energy.
• Electric currents flow from high
electric potential to low electric potential.
• and the greater the difference between them,
the more current flows!
 In a majority of electric currents, the moving charges are electrons.
 Electrons flow from negative terminal to positive terminal of the battery.
 However, due to historical reasons dating back to Ben Franklin, we say that conventional current flows in
the direction positive charges would move. It was defined before the electron was discovered and
scientist realised that the positive charge did not flow through wires.

Ammeters are tools used to measure the current in a circuit.

The ammeter is connected in series with the circuit,
so that the current to be measured flows directly through the ammeter
• Ammeter have very low resistance (we accept it as zero in calculations) to minimize the potential drop
through the ammeter and the ammeter's impact on the circuit, so inserting an ammeter into a circuit in
parallel can result in extremely high currents and may destroy the ammeter.
All the ammeter show the same value in the circuit !!!
1
3
2
 If the electric potential difference between two points is 1 volt, then 1 coulomb of charge will
gain 1 joule of potential energy when moved between those two points.
 If the electric potential difference between two points is 3 volts, then one coulomb of charge
will gain 3 joules of potential energy when moved between those two points.
• The figure below shows a mechanical equivalent of the flashlight circuit.
• The person lifting the water corresponds to the …………………..…….,
the paddle wheel corresponds to the ………………………………, and the
water is like the electric c………………...
© 2014 Pearson Education, Inc.
• The figure below shows a mechanical equivalent of the flashlight
circuit. The person lifting the water corresponds to the battery, the
paddle wheel corresponds to the lightbulb, and the water is like the
electric charge.
© 2014 Pearson Education, Inc.
Parallel Circuits and Overloading
• Electricity enters home by two wires called lines, which then
branch into parallel circuits connecting wall outlets, ceiling lights
– so these have same voltage put across them (220-V)
• As more devices are plugged in, overall resistance of circuit in
home decreases, and draws more current. (c.f picture)
• Sum of the currents in the branches = line current, so this can
get too large to be safe, if too many devices plugged in.
Circuit is overloaded, overheated, may start fire.
Resistors A and B are in
1. series
2. parallel
3. neither
Resistors A and B are in
1. series
2. parallel
3. neither
Resistors A and B are in
1. series
2. parallel
3. neither
Resistors A and B are in
1. series
2. parallel
3. neither
Resistors A and B are in
1. series
2. parallel
3. neither
Resistors A and B are in
1. series
2. parallel
3. neither
• Rheostat
• Rheostat is a variable resistor, which is used to control the flow of
electric current by manually increasing or decreasing the resistance.
• The electric current flowing through an electrical circuit is
determined by two factors: the amount of voltage applied and the
total resistance of the electrical circuit.
• By placing the rheostat in the electrical circuit, we can control
(increase or decrease) the flow of electric current in the circuit.
 Linear rheostats
 If we use the terminals A and B in the rheostat, the minimum resistance is achieved
when we move the slider or wiper close to the terminal A, because the length of the
resistive path decreases.
 In the similar way, the maximum resistance is achieved when we move the slider close to
terminal C, because the length of the resistive path increases.
• Rotary rheostats
• The rheostat consists of three terminals: terminal A, terminal B
and terminal C.
• However, we use only two terminals: either A and B or B and C.
• Terminal A and terminal C are the two fixed terminals connected
to both ends of the resistive element called track and terminal B
is the variable terminal connected to the sliding wiper or slider.
Joule’s Law
Work Done by Voltage Source- Energy dissipated (consumed)
• The voltage source supplies a current into an electrical circuit.
• A power supply should do work, (W), to cause a charge q to flow.
• The total work done per unit charge by the power supply is called “ its voltage“.
𝑾= 𝒒𝑽
𝑽=
𝑾𝒕𝒐𝒕𝒂𝒍
𝒒𝒕𝒐𝒕𝒂𝒍
• W total : total work done by power supply (J: Joule)
• q total : total charge given into the circuit by voltage supply (C :Coulomb)
• V: voltage of power supply (V: volt)
• The total power (P) of a power supply is the total work done (Wtotal) per unit time (t).
• On the circuit the battery gives electrons
potential energy.
• In the bulb, this changed into thermal energy
(heat) and then radiated.
• The battery is supplying 5 joules of energy
every second, so its power is 5 watts.
• The bulb is taking energy at the same rate ,so
its power is also 5 watts.
• Appliances such as toasters, irons and TV’s
have a power rating marked on them.
• The power of a resistor is the amount of energy dissipated (consumed) per unit time,
𝑾𝒐𝒓𝒌 𝒅𝒐𝒏𝒆(𝑬𝒏𝒆𝒓𝒈𝒚 𝒅𝒊𝒔𝒔𝒊𝒑𝒂𝒕𝒆𝒅)
𝑬
𝑷𝒐𝒘𝒆𝒓 =
= 𝑷= ,
𝒕𝒊𝒎𝒆
𝒕
• The units of power;
1 kW = 1000 W
• The units of energy; 1 kW-h = 1000 W-h
• What do we pay for on the bills?
• We pay for the energy we use
• 1 kW-h = 1000 W x 1h = 1000 W x 3600 s = 3.6 x 106 J
𝟐
𝑽
𝑬 = 𝑾 = 𝑷𝒕 = 𝑽𝑰𝒕 = 𝑰𝟐 𝑹𝒕 =
𝒕
𝑹
Ex 26: The power of a hair dryer is 2500 watts.
a) How much electrical energy will be consumed when it operates for 6 min?
b) How much does it cost for the energy consumption of the hair dryer,
if 1kW-h of electrical energy is 0.50 TL?
Ex 27: The power of a vacuum cleaner is 1500 watts.
a) How much electrical energy will be consumed when it operates for 30 min
every week?
b) How much does it cost monthly, if 1kW-h of electrical energy is 0.50 TL?