Physics circuits

• Common circuit components


  • Cell/battery
  • Switch
  • Fixed resistor
  • Variable resistor
  • Thermistor
  • Light-dependent resistor (LDR)
  • Diode
  • Light-emitting diode (LED)
  • Ammeter
  • Voltmeter
• Cell/battery


  Provides the circuit with a source of potential difference. A battery is two or more cells
• Switch


  Turns the circuit on (closed), or off (open)
• Fixed resistor


  Limits the flow of current. A fixed resistor has a resistance it cannot change
• Variable resistor


  A resistor with a slider that can be used to change its resistance. These are often used in dimmer switches and volume controls
• Thermistor


  The resistance of a thermistor depends on its temperature. As its temperature increases, its resistance decreases and vice versa
• Light-dependent resistor (LDR)


  The resistance of an LDR depends on the light intensity. As the light intensity increases, its resistance decreases and vice versa
• Diode


  Allows current to flow in one direction only. Used to convert AC to DC current
• Light-emitting diode (LED)


  Equivalent to a diode and emits light when a current passes through it. Used for aviation lighting and displays (TVs, road signs)
• Ammeter


  Used to measure the current in a circuit. Connected in series with other components
• Voltmeter


  Used to measure the potential difference of an electrical component. Connected in parallel with the relevant component
• Being able to draw and interpret circuit diagrams using circuit symbols is an essential skill in the electricity topic
• Electric circuit diagrams


  • Require an energy source (cell, battery, or power supply)
  • Require a closed path or complete circuit
  • Require electrical components (sensors, measuring devices, energy transfer devices)
• An ammeter is always connected in series
• A voltmeter is always connected in parallel to the component the voltage is being measured
• The direction of current flow is always from the positive to the negative terminal of the power supply
• Resistance


  The opposition to current
• The higher the resistance of a circuit, the lower the current</b>
• Good conductors have a low resistance and insulators have a high resistance
• Resistance unit


  Ohms (Ω)
• Resistance of a circuit can be increased by adding resistors (or variable resistors)
• Every electrical component has a resistance, even wires
• In exam questions, the resistance of the wires and batteries are assumed to be negligible
• Current through a component

  Depends on both the resistance R of the component and the potential difference V across the component
• Resistance R of a component

  The greater the resistance, the lower the current for a given potential difference V
• Potential difference (V)


  The energy transferred per unit charge flowing from one point to another
• Potential difference is measured in volts (V), which is the same as a Joule per coulomb (J/C)
• Potential difference (or voltage) is measured using a voltmeter
• A voltmeter is always set up in parallel to the component you are measuring the potential difference across
• Electric current


  The flow of electrical charge, measured in amperes (A)
• In metals, the electrical charge that flows is electrons, so the current in a circuit is a flow of electrons
• Charge unit


  Coulomb (C)
• Conventional current is defined as the flow of positive charge from the positive terminal of a cell to the negative terminal, which is opposite to the direction of electron flow
• Current is measured using an ammeter, which should always be connected in series with the part of the circuit you wish to measure the current through
• In a closed-loop circuit, the current is the same value at any point
• Types of resistors


  • Fixed resistors
  • Variable resistors
• Fixed resistors have a resistance that remains constant
• Variable resistors can change the resistance through the circuit, varying the amount of current
• For fixed and variable resistors, once the resistance is set, it will stay at this value no matter how the current changes
• The resistance of components such as lamps, diodes, thermistors and LDRs changes with the current through the component