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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V

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

V

RI 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 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 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

Simple!

Let’s try some IB questions!