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Coulomb of charge (electrons)
Think of it as a “bag of electrons” (containing 6000000000000000000 electrons!)
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Current
The 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
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
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
In a series circuitThe sum of the voltages across the lamps
equals the voltage across the cells
9 V
3 V 3 V 3 V
In a parallel circuitIn a simple parallel circuit, voltage across each lamp equals the voltage across the cells
5 V
5 V
5 V
Resistance
Measures how difficult it is for current to flow. Measured in Ohms (Ω)
VA
Resistance = voltage/current R = V/I
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
V(V) I(A) R(Ω)
I(A)
R(Ω)
Resistance of a lamp
• As the current in a lamp increases, it gets hotter, and its resistance increases. Why?
Ohm’s Law
• V = IR
V
RI X
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
The amount of energy used by a device per second, measured in Watts (Joules per second)
VA
Power = voltage x current P = VI
☺Can you copy this please?
Example
• A 200 W television is plugged into the 110V mains. What is the current in the television?
PIV X
Example
• A 200 W television is plugged into the 110V mains. What is the current in the television?
• I = P/V = 200/110 = 1.8A
PIV X
Example
• A kettle uses 240V and 8A. What is its power?
PIV X
Example
• A kettle uses 240V and 8A. What is its power?
• P = VI = 240x8 = 1920W (=1.9kW)
PIV X
Electric symbols
Cell and battery of cells
Lamp
Switch
Voltmeter
Resistor
Variable resistor (rheostat)
Thermistor
• Resistance decreases with increasing temperature
Temp
Resistance
Light-dependent resistor (LDR)
• High resistance in the dark but a low resistance in light
Diode
• Allows current to flow in one direction only (from + to – in the direction of the “arrow)
Captain Jack Sparrow smoking a cigar
Two cowboys on a tandem
Two fried eggs in a frying pan
3 wizards looking down a well
Let’s try some MORE questions!
