We are learning; Revisit yr 7 work on series and parallel circuits Look at examples of Ohms law

Revisit yr 7 work on series and parallel circuits

Look at examples of Ohms law

Current

Electrons Flow in the Opposite way to the Conventional Current We normally say that the current flows from positive to negative - + Electrons are negatively charged and therefore flow from negative to positive e-

Current Current 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)

Ampere This is the scientist AMPERE. He studied current in circuits and gave his name to the unit of current

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 The current reading for bulb B is 2A. What are the readings for bulbs A and C? Bulb suddenly fails. What happens to bulbs A and C?

The current reading for bulb B is 2A. What are the readings for bulbs A and C?

Bulb suddenly fails. What happens to bulbs A and C?

What Is the Current at This Point? 6A

Brightness of Bulbs in a Series Circuit 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

Current in a Parallel Circuit A 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

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

Voltage (Potential Difference)

What is the ammeter reading? 3A 3A 9A 3A

Voltage Is the Driving Force That Pushes the Current Around the Circuit We need Energy to push the electrons around the circuit. In this case the energy comes from the lemon (in the form of chemical energy)

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 4V 4V

What Is the Voltage? ? V 8V 8V 8V ? 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

Resistance

Resistance It can be calculated using Ohm’s Law: Resistance is anything in a circuit that restricts the flow of current The unit of Resistance is the Ohm Resistance = Voltage (V) (Ohms) Current (A) V I x R

Complete the meter readings V 1 V 2 12V 6A A 2 A 1 V 3 A 3

Calculate the resistance 12V 3A ?

Current-Voltage Graphs Show Resistance A Resistor at constant temperature The current through a resistor at constant temperature is proportional to the voltage Different wires have different resistances I V

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.

Resistance - Length of Wire Doubling the Length of a wire Doubles its resistance because the electrons have twice as far to move

Resistance in Wires – Cross Sectional Area Thin wires have more resistance than thick wires Halving the cross sectional area of a wire doubles its resistance because there is half as much space for the electrons to move

Resistance - Material Different materials have different resistances. For 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)

Voltage –current for a lamp Plot these results What do they show? 0.19 0.20 3.0 3.5 0.18 2.5 0.15 2.0 0.12 1.5 0.08 1.0 0.04 0.5 0 0 I / amps V/volts

Current-Voltage Graphs Show Resistance A Filament Bulb As the temperature of the filament increases, the resistance increases – hence the curve I V

Current-Voltage Graphs Show Resistance A Diode Current will only flow through in ONE DIRECTION (It has very high resistance in the reverse direction) I V

Summary Table Is anything in a circuit which slows the flow down Resistance Is the driving force that pushes the current around. Voltage Is the flow of electrons around a circuit Current

There is a Balance…. If you increase the Voltage – then more current will flow If you increases the Resistance – then less current will flow

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

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 5A COLD WARM The resistance of a thermistor depends on its temperature. Its resistance decreases as the temperature of the thermistor increases