curent and potential difference

• Electric current is defined as the rate of flow of charge
• The size of an electric current is the amount of charge passing through a component per second
• Metal wires

  Good conductor of electric current
• In metal wires, the current is a flow of negatively charged electrons
• The wires in an electric circuit are made of metal because metal is a good conductor of electric current
• The current in metal wires is a flow of negatively charged electrons
• Calculating Electric Charge: Q = It (Q = charge measured in Coulombs (C), I = current measured in amps (A), t = time measured in seconds (s))
• Worked example: When will 8 mA of current pass through an electrical circuit? Answer: B
• Electric currents in everyday circuits tend to be quite small, so it's really common for examiners to throw in a unit prefix like 'm' next to quantities of current, e.g. 10 mA (10 milliamperes). Make sure that you are on the lookout for these prefixes and that you can convert them into standard units, so 10 mA = 10 x 10^-3 A
• Current is measured using an ammeter
• Ammeters should always be connected in series with the part of the circuit being measured
• Connecting ammeters

  Ammeters should always be connected in series with the part of the circuit you wish to measure the current through
• Ammeters


  • Measure the amount of charge passing through them per unit time
  • Need to be in series so that all the charge flows through them
• An ammeter can be used to measure the current around a circuit
• Voltage is the potential difference across a circuit
• Potential difference

  The amount of energy transferred per unit of charge passing through the terminals
• One volt is equivalent to one joule per coulomb
• Equation linking energy transferred, voltage, and charge: E = V × Q
• Worked example: The normal operating voltage for a lamp is 6 V. Calculate how much energy is transferred in the lamp when 4200 C of charge flows through it. Energy transferred = 25,200 J
• Exam Tip: Potential difference symbol (V) is the same as its unit (volt, V). One volt is equivalent to 'a joule per coulomb'
• Potential difference is measured using a voltmeter
• Voltmeters


  • Can be digital or analogue
  • Connected in parallel with the component being tested
  • Connected to two points in the circuit to measure the difference in electrical potential between them
• Voltmeters are connected in parallel with the component being tested
• Potential difference

  The difference in electrical potential between two points
• Connecting a voltmeter


  The voltmeter has to be connected to two points in the circuit
• When building a circuit, save the voltmeter until last. First, make the whole circuit and check it works. Then connect the voltmeter to the component being measured
• Resistance is defined as the opposition to current. The higher the resistance of a circuit, the lower the current
• Resistance

  Symbol: R, Measured in Ohms (Ω), Defined as one volt per ampere (1 V / A)
• The resistance of a circuit can be increased by adding resistors (or variable resistors) to it. Every electrical component has a resistance, even wires
• In exam questions, the resistance of the wires and batteries are assumed to be negligible
• The greater the resistance R of the component, the lower the current I for a given potential difference V across the component
• The lower the resistance R of the component, the greater the current I for a given potential difference V across the component
• The current, resistance, and potential difference of a component in a circuit are calculated using the equation V = IR
• Calculate the voltage across a resistor of resistance 10 Ω if there is a current of 0.3 A through it: V = 0.3 × 10 = 3 V