Transcript Electric potential difference

Topic 4: Oscillations and Waves
TEST
th
12
Friday
March
Topic 5: Electric currents
Can you look
through the contents
and definitions?
Definition TEST
Friday 19th March
Stand
up!
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cell
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energy
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electron
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lamp
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Electrons
Hi, I’m Eleanor the
electron.
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Coulomb of charge (electrons)
Think of it as a “bag of electrons” (containing
6000000000000000000 electrons!)
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Current
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The rate of flow of electric
charge (number of
Coulombs flowing past a
point in the circuit every
second).
I = Q/t
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I’m counting
how many
coulombs of
electrons go
past me every
second
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A
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1 Amp = 1 coulomb per second
Let’s build some circuits!
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In a series circuit
Current is the same at any point in the circuit
2.5 A
2.5 A
2.5 A
2.5 A
In a parallel circuit
The current splits (total current stays the
same)
2.5 A
2.5 A
1.25 A
1.25 A
Voltage(emf)
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V
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I’m checking the
difference in
energy (per
coulomb) between
the 2 red arrows
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1 Volt = 1 Joule per coulomb
Voltage (p.d.)
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I’m checking the
difference in
energy (per
coulomb) before
and after the lamp
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1 Volt = 1 Joule per coulomb
p.d. and e.m.f
Electric potential difference between
two points is the work done per unit
charge to move a small positive charge
between two points.
Electromotive force is the total energy
difference per unit charge around the
circuit (it is the potential difference when
no current flows in a circuit).
Let’s build
some
circuits!
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In a series circuit
The sum of the p.d.s across the lamps
equals the emf across the cells
9V
3V
3V
3V
In a parallel circuit
In a simple parallel circuit, p.d. across
each lamp equals the e.m.f. across the
cells
5V
5V
5V
Stand
up!
Resistance
Measures how difficult it is for current to
flow. Measured in Ohms (Ω)
V
Resistance = voltage/current
A
R = V/I
Ohm’s Law
• V = IR
V
I
X
R
Let’s do a
practical!
Resistance
• R is proportional to the length of wire – WHY?
RαL
• R is inversely proportional to the cross sectional
area of wire – WHY?
R α 1/A
• R depends on the type of material – WHY?
Resistivity
R = ρL
A
where
R = resistance in Ohms
L = Length of conductor in metres
A = cross sectional area of conductor in m2
ρ = resistivity of the material in Ohms.meters
Example
The resistivity of copper is 1.7 x 10-8 Ωm.
What is the resistance of a piece of copper
wire 1 m in length with a diameter of
0.1mm?
Example
The resistivity of copper is 1.7 x 10-8 Ωm. What
is the resistance of a piece of copper wire 1 m in
length with a diameter of 0.1mm?
radius = 0.05mm = 5 x 10-5m
cross sectional area = πr2 = 3.14x(5 x 10-5)2
= 7 x 10-9 m2
R = ρL/A = (1.7 x 10-8 x 1)/ 7 x 10-9 = 2.42 Ω
Let’s do
another
practical!
Homework
• BOTH electricity practicals (resistance of
different thicknesses of wire AND voltagecurrent characteristics of filament lamps)
to be handed in Wednesday 14th April.
Resistance of a lamp
Vary the voltage and current using a variable resistor
(rheostat). Plot a graph of resistance against current
V
Resistance = voltage/current
A
R = V/I
Resistance of a lamp
• As the current in a lamp increases, its
resistance increases. Why?
Ohmic behaviour
• p.d. is proportional to the current
Metal wires
at constant
temperature
Non-Ohmic behaviour
• p.d. is not proportional to the current
Power
The amount of energy used by a device
per second, measured in Watts (Joules
per second)
A
V
Power = voltage x current
P = VI
Power dissipated in a
resistor/lamp
• P = VI
• From Ohm’s law, V = IR
• So P = VI = I2R
• From Ohm’s law also, I = V/R
• So P = VI = V2/R
Total energy
So the total energy transformed by a lamp
is the power (J/s) times the time the lamp
is on for in seconds,
E = VIt
E = energy transformed (J)
V = Voltage (also called p.d.)
I = current (A)
t = time (s)
Electronvolt
• Electronvolt – the energy gained by an
electron when it moves through a potential
difference of one volt.
Questions!
• Page 316 and 317 questions 2, 5, 8, 9, 10,
12, 13, 14, 15, 17, 18.
Internal resistance
• Connecting a voltmeter (VERY high
resistance) across the terminals of a cell
measures the EMF of the cell (no current
flowing)
V
Internal resistance
• We have assumed so far that the power
source has no resistance…….not a good
assumption!
Internal resistance
• In actuality the p.d. across a cell is less
than the EMF due to energy lost in the
INTERNAL RESISTANCE
Internal resistance
• To help us visualize this, a cell is
represented as a “perfect” cell attached in
series to the internal resistance, given the
symbol r.
Internal resistance
• The p.d. across a cell (V) is then equal to
the EMF (ε)minus the voltage lost across
the internal resistance (=Ir)
V = ε - Ir
Example
• A cell of emf 12V and internal resistance
1.5 Ω produces a current of 3A. What is
the p.d. across the cell terminals?
• V = ε - Ir
• V = 12 – 3x1.5
• V = 7.5 V
Another practical!
Adding resistances
In series and parallel
Reading and taking notes
• Pages 320 to 328
• Read and make your OWN NOTES.
• I will collect these in the first lesson back
after the holiday.
Ideal meters
• Voltmeters – infinite resistance!
• Ammeters – Zero resistance!
Potential divider
LDRs, Thermistors in potential
divider circuits
Simple!
Let’s try some IB questions!