3.3 Potential difference and Circuits

• a field is a region where an object experiences a force without being touched.
• An electric field applied to a conductor will cause free electrons to move. This creates an electric current.
• An electrical current is a flow of charges.
• The voltage (or potential difference) of the supply is a measure of the energy given to the charges in a circuit.
• The unit of voltage and potential difference is the Volt (V).
• Photovoltaic (circuit symbol)

  
• Relay (circuit symbol)

  
• LED-Light emitting diode (circuit symbol)

  
• Diode (circuit symbol)

  
• Capacitor (circuit symbol)

  
• Thermistor (circuit symbol)

  
• LDR (Light dependant resistor)

  
• Switch (circuit symbol)

  
• Fuse (circuit symbol)

  
• Variable resistor (circuit symbol)

  
• Resistor (circuit symbol)

  
• Lamp (circuit symbol)

  
• Cell Battery

  
• Voltmeter- connected in parallel with a component

  
• Ammeter- connected ins series with the circuit, measures the current flowing through the circuit

  
• The current is the same at all points in a series circuit
• In a series circuit IA = IB= IC
• The sum of the potential difference across the components in a series circuit is equal to the voltage of the supply.
• Series circuit:VS= V1+ V2
• In a parallel circuit the current has more than one path to follow. This means that

  the supply current splits through each parallel branch.

  IS= IA+ IB
• The voltage in a parallel circuit is the same across all branches.

  VS = V1= V2
• The mains voltage in houses is 230 volts
• Resistance is a measure of the opposition of a circuit component to the current through that component. The greater the resistance of a component, the smaller the current through that component.
• V=IR

  V= voltage (V)

  I= current (A)

  R= resistance (measured in ohms)
• V/I = constant (R)
• In a series circuit

  RT= R1+ R2 + R3
• In a parallel circuit

  1/RT=1/R1+1/R2+1/R3
• A potential divider is a device or a circuit that uses two (or more) resistors or a variable resistor (potentiometer) to provide a fraction of the available voltage (p.d.) from the supply.
• The p.d. from the supply is divided across the resistors in direct proportion to their individual resistances. Take the fixed resistance circuit - this is a series circuit therefore the current is the same at all points.
• A transistor is an electronic component which contains numerous parts a transistor can be used as an electronic switching device.
• A NPN transistor turns on at about 0.7V
• A MOSFET turns on at around 2-2.5V