2 - ELECTRICITY 

Circuits

1 - Current

• the rate of flow of charge round the circuit 
• will only flow through a component if there is a voltage across that component 
• unit = amp, A

2 - Voltage

• driving force that pushes the current round 
• 'electrical pressure' 
• unit = volt, V 

3 - Resistance

• anything in the circuit that slows down the flow
• if you add more components to the circuit, there will be a higher overall resistance 
• units = ohm, Ω 

4 - Balance 

• voltage is trying to push the current around the circuit 
• resistance is opposing voltage 
• relative sizes of voltage and resistance decide how big the current will be 

INCREASE VOLTAGE = MORE CURRENT FLOWS 
INCREASE RESISTANCE =  LESS CURRENT FLOWS 

Standard test circuit - 

• Basic circuit used for testing components and getting V-I graphs 
• Component, ammeter and variable resistor all in series (can be put in any order in main circuit) 
• Voltmeter only placed in parallel around component being tested 
• Varying variable resistor alters current flowing - can take several pairs of readings from ammeter and voltmeter 
• Plot current and voltage on V-I graph

1 - Ammeter 

• Measures current (amps) flowing through the component 
• Must be placed in series 
• Anywhere in main circuit, never in parallel 

2 - Voltmeter 

•  Measures voltage (volts) across component 
• Must be placed in parallel 
• Around the component 

Mains supply/battery supply - 

• a.c. (alternating current - constantly changing direction) = mains 
• d.c. (direct current - current keeps flowing in same direction) = battery 
• UK - mains electricity = 230 volts 

Series circuits -

• Different components connected in a continuous line 
• Can't control which components current flows through - flows through all or none 
• If one component is removed/broken, the circuit is broken 
• eg: fairy lights 
• Same current flows through all parts of the circuit 
• A1 = A2 
• Size of current determined by total voltage of the cells and total resistance of circuit (I = V/R)
• Total resistance depends on the number of components and the type of components 

Parallel circuits -

• Each component is separately connected 
• If one component is removed/disconnected, hardly affects others 
• Diagram - each component in its own loop 
• How most things are connected (eg: household electrics - lights) 

Exceptions to series and parallel -

• Ammeters always connected in series 
• Voltmeters always connected in parallel with a component 

LEDs (light emitting diodes) -

• Emit light when a current flows through them in a forward direction 
• Numbers on digital clocks, traffic lights, remote controls 
• Don't have a filament that can burn out 
• Indicate the presence of current in a circuit - often used in appliances to show when they are switched on 

LDRs (light dependant resistors) -

• Type of resistor that changes its resistance based on how much light falls on it 
• Bright light - resistance falls 
• Darkness - resistance is highest 
• Electronic circuits (eg: burglar detectors) 

Thermistors -

• Temperature-dependant resistor 
• Hot - resistance drops
• Cool - resistance increases
• Temperature detectors (eg: car engine temperature sensors) 

Resistance 

VOLTAGE = CURRENT X RESISTANCE (V = I X R) 

Voltage/current graphs -

1 - Metal filament lamp

• As the temperature of the metal filament increases, the resistance increases 
• (x axis = V, y axis = I) 

2 - Wire

• Current through a wire (at a constant temp.) is proportional to voltage

3 - Diode

• Current will only flow through a diode in one direction 

4 - Different resistors

• The current through a resistor (at a constant temp.) is proportional to voltage 
• Different resistors have different resistances - different gradients

Charge, voltage, energy change 

• Current = rate of flow of electrical charge (A) 
• In solid metal conductors (eg: copper wire) charge is carried by negatively charged electrons 
• More charge passes around a circuit when  bigger current flows 
• (time must be in seconds - charge is measured in coulombs, C)

CHARGE = CURRENT X TIME