AQA Physics Unit 2

Circuits

Current:

• Current is the number of coulombs per second (amps)
• The strength of an electric current is measured in amps
• Amps are a measure of how much electric charge is flowing round the circuit 
• Current flowing in a circuit can be measured using an ammeter
• Ammeters must be connected in series with the components 

Current (A) = Charge (C) / Time (s)

Resistance:

• Electric current flows when electrons flow and collide with atoms in a conductor
• These collisions make the conductor hot
• This is called resistance and it makes it harder for current to flow
• Resistance is measured in ohms

Resistance (ohms) = Voltage (volts) / Current (amps)

Potential Difference:

• p.d is measured in volts 
• When two or more cells are connected in series, the total p.d is the sum of both cells
• p.d across a component in a circuit is measured using a voltmeter 
• The voltmeter must be connected in parallel with the components
• The p.d across a device equals the amount of energy changed (in joules) per coulomb of charge passing through it 

e.g. the voltage across a lamp is 3V. What does this mean? 

• That 3J per coulomb are passing through the lamp and are converted into electrical energy.

Potential Difference (V) = energy transferred (J) / charge passes (C)

Current : Voltage graphs:

Resistor

The current flowing through a resistor at a constant temperature is directly proportional to the voltage across the resistor.

So, if you double the voltage, the current doubles too.

Filament…