Current
Dec 16, 2015
Current
Electrons Flow- +
Electrons are negatively charged and therefore flow from negative to positive
e-
CurrentCurrent is the FLOW OF ELECTRONS AROUND A CIRCUIT
In metals, current is carried by electrons
We use an ammeter to
measure current
The unit of current is
the ampere (A)
Current in a Series Circuit
2A 2A
2A
The Current in a Series Circuit is the Same at Every
Point
In a series circuit the bulbs are all the same
brightness
If the bulbs are
of the same
rating!
Current Questions
A B C
1)The current reading for bulb B is 2A. What are the readings for bulbs A and C?
2) Bulb suddenly fails. What happens to bulbs A and C?
What Is the Current at This Point?
6A
If a cell is added the current will
increase and the bulb gets brighter
Brightness of Bulbs in a Series Circuit
If a bulb is added the current will
decrease as there is greater resistance in the circuit and the bulbs get
dimmer
Brightness of Bulbs in a Series Circuit
Current in a Parallel CircuitA PARALLEL circuit has more than one path for the current to flow through
Bulb A
Bulb B
Assuming both bulbs are the Same…they will be the same brightness
Current in a Parallel Circuit
Bulb A
Bulb B
The Advantages of a parallel circuit are:
• If one bulb fails ….the other will remain on
• Both bulbs can be switched on separately
Current in a Parallel Circuit
The total flow of current is equal to the total current from all of the branches
2A
2A
2A
6A 6A
What is the ammeter reading?
3A
3A
9A
3A
Voltage (Potential Difference)
Voltage Is the Driving Force That Pushes the Current Around the Circuit
Potential Difference/ Voltage
This is VOLTA. He studied the driving force that pushes current around a circuit. He gave his name to the unit of Potential Difference
The unit of Potential Difference is the VOLT (V)
Voltage in a Series Circuit
12V
12V The voltage in a series circuit is shared across each component. The sum of the voltages across each component equals the source voltage
4V 4V4V
What Is the Voltage?
?V
8V 8V8V
?V
Voltage in a Parallel Circuit
6V
6V
6V
The voltage is the same across all
components in parallel
What Is the Voltage?
9V
? V
? V
Return to menu
Complete the meter readings
V1
V2
12V
6A
A2
A1
V3
A3
Which Is the Series and Parallel Circuit
A * Current is THE SAME at any point
* Voltage SPLITS UP over each component
* Current SPLITS UP down each branch
* Voltage is THE SAME across each branch
B
Resistance
Resistance
It can be calculated using Ohm’s Law:
Resistance = Voltage (V)
(Ohms) Current (A)
V
I x R
Resistance is anything in a circuit that restricts the
flow of current
The unit of Resistance is the
Ohm
Summary Table
Current Is the flow of electrons around a circuit
Voltage Is the driving force that pushes the current
around.
Resistance
Is anything in a circuit which slows the flow
down
Calculate the resistance
12V
3A ?
Electrical quantities in an electric circuit are related to each other by:
OHM’S LAW
OHM’S LAW says: If the source voltage remains constant,increasing the resistance in a circuit will cause a decrease
in current flow in that circuit.In mathematical terms it tells us that current flow is
inversely proportional to resistance.
In equation form it says:Voltage (V) = Current (I) x Resistance (R)
orV(in volts) = I (in amps) x R (in ohms)
PotentialIn volts
(joules / coul)
CurrentIn amperes
(coul / second)
ResistanceIn ohms
(volts / amp)
Drop across a resistance
Current passingthrough the
resistor
VOLTAGE
(volts)
Current (I) in amps
I
v
The slope of theLine gives theResistance (Ω)
Factors Affecting Resistance1. Thickness (diameter) of wire
2. Length of wire
3. Type of material
4. Temperature
Electron flow and Resistance
Electricity in wires is a flow of electrons along the wire. As the electrons move along the wire they collide with the metal atoms. These collision make the atoms vibrate more…which makes the metal hotter.
Resistance is a measure of how much a material tries to stop electricity passing through it.
CROSS SECTIONAL AREA
-Thin wires have more resistance than thick wires
-Halving (1/2) the cross sectional area of a wire doubles its resistance because there is half as much space for the electrons to move
Resistance - Length of Wire
-Doubling the Length of a wire doubles its resistance because the electrons have twice as far to move
Type of MaterialDifferent materials have different resistances.
Example: A nichrome wire has more resistance than a copper wire of the same size (the atoms in nichrome hold the electrons more tightly than copper atoms)
Temperature
As temperature increases, resistance increases.
Voltage –current for a lamp
V/volts I / amps
0 0
0.5 0.04
1.0 0.08
1.5 0.12
2.0 0.15
2.5 0.18
3.03.5
0.190.20
Plot these results
What do they show?
Current-Voltage Graphs Show Resistance
I
V
A Filament
Bulb
As the temperature of the filament increases, the
resistance increases – hence the curve
Current-Voltage Graphs Show Resistance
I
V
A Diode
Current will only flow through in ONE
DIRECTION
(It has very high resistance in the reverse direction)
There is a Balance….
If you increase the Voltage – then more current will flow
If you increases the Resistance – then less
current will flow
The Light Dependent Resistor
1A
Dark
5A
Light
The resistance of the LDR depends on the amount of light falling on it. Its resistance decreases as the amount of light falling on it increases
The Thermistor
1A 5ACOLD WARM
The resistance of a thermistor depends on its temperature.
Its resistance decreases as the temperature of the thermistor increases