Post on 12-Feb-2016
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Topic 4: Oscillations and Waves
TEST
Friday 12th 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
energy
electron
lamp
Electrons
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Hi, I’m Eleanor the electron.
Coulomb of charge (electrons)
Think of it as a “bag of electrons” (containing 6000000000000000000 electrons!)
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Current
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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A
I’m counting how many
coulombs of electrons go
past me every second
1 Amp = 1 coulomb per second
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Let’s build some circuits!
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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I’m checking the difference in energy (per
coulomb) between the 2 red arrows
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
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 circuitThe sum of the p.d.s across the lamps
equals the emf across the cells
9 V
3 V 3 V 3 V
In a parallel circuitIn a simple parallel circuit, p.d. across each lamp equals the e.m.f. across the cells
5 V
5 V
5 V
Stand up!
Resistance
Measures how difficult it is for current to flow. Measured in Ohms (Ω)
VA
Resistance = voltage/current R = V/I
Ohm’s Law
• V = IR
VRI X
Let’s do a practical!
Resistance
• R is proportional to the length of wire – WHY?R α L
• 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 OhmsL = Length of conductor in metresA = 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 10-5mcross 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 voltage-current characteristics of filament lamps) to be handed in Wednesday 14th April.
Resistance of a lamp
AV
Resistance = voltage/current R = V/I
Vary the voltage and current using a variable resistor (rheostat). Plot a graph of resistance against current
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)
VA
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 = VItE = 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
Let’s try some IB questions!