Electric circuitsElectric circuits

What is the job of an What is the job of an electrical circuit?electrical circuit?

• Electrical energy can be transferred Electrical energy can be transferred through itthrough it

• This electrical energy comes from a This electrical energy comes from a battery or other power supplybattery or other power supply

• The energy can be transferred to The energy can be transferred to components in the circuitcomponents in the circuit

• The unit of energy is the joule (J)The unit of energy is the joule (J)

What is a current?What is a current?

• Current is a Current is a flow flow of electronsof electrons round round the circuitthe circuit

• Current will only Current will only flow through a flow through a component in a component in a circuit if there is a circuit if there is a voltagevoltage across across that componentthat component

What is direct current?What is direct current?

• Direct current is travels in Direct current is travels in one one direction onlydirection only

• CellsCells and and batteriesbatteries supply direct supply direct currentcurrent

• Direct current is also known as d.c. Direct current is also known as d.c. current or d.c. supplycurrent or d.c. supply

Direct currentDirect current

• With d.c. current With d.c. current the voltage is the voltage is constantconstant

• The cathode ray The cathode ray oscilloscope oscilloscope (CRO) trace will (CRO) trace will show d.c. supply show d.c. supply as a horizontal as a horizontal line line

voltage

What is alternating What is alternating current?current?

• Alternating current is also known Alternating current is also known as a.c. current of a.c supplyas a.c. current of a.c supply

• This current is This current is always changing always changing directiondirection, so the voltage goes up , so the voltage goes up and down all the time tooand down all the time too

Alternating currentAlternating current

• a.c. supply will a.c. supply will produce a wave produce a wave on the cathode on the cathode ray oscilloscoperay oscilloscope

• The voltage with The voltage with a.c. supply is a.c. supply is constantly going constantly going up and downup and down

Types of circuits:Types of circuits:

SERIES SERIES • These circuits go in one These circuits go in one

continuous ‘loop’continuous ‘loop’• The current is the same The current is the same

anywhere in the circuitanywhere in the circuit• An ammeter can be An ammeter can be

placed anywhere in the placed anywhere in the circuitcircuit

• If we get one gap in the If we get one gap in the circuit the current stops circuit the current stops flowingflowing

The other type of circuit:The other type of circuit:PARALLELPARALLEL

• These circuits have two or These circuits have two or more routes for the current to more routes for the current to flow roundflow round

• If we get a gap in the circuit If we get a gap in the circuit not all components will go offnot all components will go off

• Parallel circuits are used for Parallel circuits are used for house and car lightshouse and car lights

• The current leaving the power The current leaving the power supply will split up to go supply will split up to go through different parts of the through different parts of the circuitcircuit

• The current returning to the The current returning to the battery is the same as what battery is the same as what had left ithad left it

Circuit symbols:Circuit symbols:

Measuring currentMeasuring current

• An ammeter is used to measure An ammeter is used to measure currentcurrent

• Current is measured in amperes or Current is measured in amperes or amps (A) for shortamps (A) for short

• In physics electrical current is In physics electrical current is shown as (I)shown as (I)

Heating effectHeating effect

• When electrical charge flows When electrical charge flows through a resistor it will generate through a resistor it will generate heatheat

• This heat is useful in the following:This heat is useful in the following:• HairdryersHairdryers

• Immersion heatersImmersion heaters• Light bulbsLight bulbs

VoltageVoltage

• A voltmeter is used to measure A voltmeter is used to measure voltagevoltage

• Voltage is also known as Voltage is also known as potential potential differencedifference

• The unit of voltage is volts (V)The unit of voltage is volts (V)

ResistanceResistance

• Resistance tells us how easy or difficult Resistance tells us how easy or difficult it is for electrical charge to flow through it is for electrical charge to flow through a conductora conductor

• Resistance (R) is measured in ohms (Resistance (R) is measured in ohms ())• We can use the following equation to We can use the following equation to

find out the resistance of a wire or a find out the resistance of a wire or a component:component:

• Voltage = current x resistanceVoltage = current x resistance• V=IRV=IR

What does a variable resistor What does a variable resistor do?do?

• You change You change change the change the resistanceresistance in a circuit by in a circuit by twisting a knob or twisting a knob or moving a slider on an moving a slider on an old-fashioned variable old-fashioned variable resistorresistor

• We can use a variable We can use a variable resistor to change the resistor to change the current flowing through current flowing through a circuita circuit

What is a light dependant What is a light dependant resistor?resistor?

• In In bright lightbright light the the resistance will resistance will drop drop in in an LDRan LDR

• When it is When it is darkdark the the resistance is resistance is highesthighest

• This makes an LDR This makes an LDR useful in automatic useful in automatic circuits such as circuits such as automatic automatic night lightsnight lights and and burglar detectorsburglar detectors

What do thermistors do?What do thermistors do?

• A thermistor is a A thermistor is a temperature dependant temperature dependant resistorresistor

• The resistance The resistance dropsdrops when it is hotwhen it is hot

• When it is When it is coldcold the the resistance resistance goes upgoes up

• Thermistors are useful Thermistors are useful temperature detectors, temperature detectors, e.g. in e.g. in car enginescar engines and and electronic electronic thermostatsthermostats

What else can affect the What else can affect the resistance in a circuit?resistance in a circuit?

• Thick wiresThick wires have a have a low resistancelow resistance as it is as it is easier for current to easier for current to flow throughflow through

• Thin wiresThin wires have a have a high resistancehigh resistance as it as it is harder for current is harder for current to flow throughto flow through

• Long wiresLong wires in a in a circuit have a circuit have a higher resistancehigher resistance than than short wiresshort wires as the current has as the current has further to travel further to travel

What else affects What else affects resistance?resistance?

• Some metals are better conductors Some metals are better conductors than othersthan others

• Therefore Therefore some metals give a some metals give a higher resistance than othershigher resistance than others

• Copper is a Copper is a good conductorgood conductor of of electricityelectricity

• Copper Copper has a has a low resistancelow resistance

Temperature and Temperature and resistanceresistance

• If a If a wire gets warmerwire gets warmer, the , the resistance increasesresistance increases

• The atoms in the wire start to The atoms in the wire start to vibrate more when they have more vibrate more when they have more energy energy

• This makes it harder for the flow of This makes it harder for the flow of electrons to flow through the wire.electrons to flow through the wire.

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