Electric circuits

What is the job of an electrical circuit? Electrical energy can be transferred through itElectrical energy can be transferred through it This electrical energy comes from a battery or other power supplyThis electrical energy comes from a battery or other power supply The energy can be transferred to components in the circuitThe energy can be transferred to components in the circuit The unit of energy is the joule (J)The unit of energy is the joule (J)

What is a current? Current is a flow of electrons round the circuitCurrent is a flow of electrons round the circuit Current will only flow through a component in a circuit if there is a voltage across that component

What is direct current? Direct current is travels in one direction onlyDirect current is travels in one direction only Cells and batteries supply direct currentCells and batteries supply direct current Direct current is also known as d.c. current or d.c. supplyDirect current is also known as d.c. current or d.c. supply

Direct current With d.c. current the voltage is constant The cathode ray oscilloscope (CRO) trace will show d.c. supply as a horizontal line voltage

What is alternating current? Alternating current is also known as a.c. current of a.c supplyAlternating current is also known as a.c. current of a.c supply This current is always changing direction, so the voltage goes up and down all the time tooThis current is always changing direction, so the voltage goes up and down all the time too

Alternating current a.c. supply will produce a wave on the cathode ray oscilloscopea.c. supply will produce a wave on the cathode ray oscilloscope The voltage with a.c. supply is constantly going up and downThe voltage with a.c. supply is constantly going up and down

Types of circuits: SERIES These circuits go in one continuous ‘loop’These circuits go in one continuous ‘loop’ The current is the same anywhere in the circuitThe current is the same anywhere in the circuit An ammeter can be placed anywhere in the circuitAn ammeter can be placed anywhere in the circuit If we get one gap in the circuit the current stops flowingIf we get one gap in the circuit the current stops flowing

The other type of circuit: PARALLEL These circuits have two or more routes for the current to flow round If we get a gap in the circuit not all components will go off Parallel circuits are used for house and car lights The current leaving the power supply will split up to go through different parts of the circuit The current returning to the battery is the same as what had left it

Circuit symbols:

Measuring current An ammeter is used to measure currentAn ammeter is used to measure current Current is measured in amperes or amps (A) for shortCurrent is measured in amperes or amps (A) for short In physics electrical current is shown as (I)In physics electrical current is shown as (I)

Heating effect When electrical charge flows through a resistor it will generate heatWhen electrical charge flows through a resistor it will generate heat This heat is useful in the following:This heat is useful in the following: HairdryersHairdryers Immersion heatersImmersion heaters Light bulbsLight bulbs

Voltage A voltmeter is used to measure voltageA voltmeter is used to measure voltage Voltage is also known as potential differenceVoltage is also known as potential difference The unit of voltage is volts (V)The unit of voltage is volts (V)

Resistance Resistance tells us how easy or difficult it is for electrical charge to flow through a conductorResistance tells us how easy or difficult it is for electrical charge to flow through a conductor Resistance (R) is measured in ohms (  )Resistance (R) is measured in ohms (  ) We can use the following equation to find out the resistance of a wire or a component:We can use the following equation to find out the resistance of a wire or a component: Voltage = current x resistanceVoltage = current x resistance V=IRV=IR

What does a variable resistor do? You change change the resistance in a circuit by twisting a knob or moving a slider on an old- fashioned variable resistor We can use a variable resistor to change the current flowing through a circuit

What is a light dependant resistor? In bright light the resistance will drop in an LDR When it is dark the resistance is highest This makes an LDR useful in automatic circuits such as automatic night lights and burglar detectors

What do thermistors do? A thermistor is a temperature dependant resistorA thermistor is a temperature dependant resistor The resistance drops when it is hotThe resistance drops when it is hot When it is cold the resistance goes upWhen it is cold the resistance goes up Thermistors are useful temperature detectors, e.g. in car engines and electronic thermostatsThermistors are useful temperature detectors, e.g. in car engines and electronic thermostats

What else can affect the resistance in a circuit? Thick wires have a low resistance as it is easier for current to flow throughThick wires have a low resistance as it is easier for current to flow through Thin wires have a high resistance as it is harder for current to flow throughThin wires have a high resistance as it is harder for current to flow through Long wires in a circuit have a higher resistance than short wires as the current has further to travel

What else affects resistance? Some metals are better conductors than othersSome metals are better conductors than others Therefore some metals give a higher resistance than othersTherefore some metals give a higher resistance than others Copper is a good conductor of electricityCopper is a good conductor of electricity Copper has a low resistanceCopper has a low resistance

Temperature and resistance If a wire gets warmer, the resistance increasesIf a wire gets warmer, the resistance increases The atoms in the wire start to vibrate more when they have more energyThe atoms in the wire start to vibrate more when they have more energy This makes it harder for the flow of electrons to flow through the wire.This makes it harder for the flow of electrons to flow through the wire.

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