Summary AQA GCSE triple higher physics study notes key topic 2

Physics – Key Topic 2
4.2 Electricity
4.2.1 Current, potential difference and resistance
4.2.1.1 Standard circuit diagram symbols




 A switch enables the current in a circuit to be switched on or off

 A heater is designed to transfer the energy from an electric current to heat the
surroundings

 A cell is necessary to push electrons around a complete circuit. A battery
consists of two or more cells

 A variable resistor allows the current to be varied

 An ammeter is used to measure current

 A light emitting diode (LED) emits light when a current passes through

 An indicator such as a bulb is designed to emit light as a signal when a current
passes through it

 A fixed resistor limits the current in the circuit

 A voltmeter measures the potential difference

 A diode allows current through in one direction

 A fuse is designed to melt and “break” the circuit is a current through it is
greater than a certain amount

,  An LDR has low resistance in bright light

 A thermistor has low resistance when warm

4.2.1.2 Electrical charge and current
 For electrical charge to flow through a closed circuit, the circuit must include a source
of potential difference
- Potential difference is the driving force that pushes charge around
- Resistance in the circuit is anything that slows the flow down
 Electric current is a flow of electrical charge. The size of the electric current is the rate
of flow of electrical charge.
- charge flow = current x time (Q=IT) (C = A x s)
 A current has the same value at any point in a single closed loop

4.2.1.3 Current, resistance and potential difference
 The current through a component depends on both the resistance of the component
and the potential difference across the component. The greater the resistance of the
component, the smaller the current for a given potential difference across the
component
 Potential difference = current x resistance (V=IR) (V = I x )

4.2.1.4 Resistors
 Resistance is caused by electron colliding with the positive ions in the lattice
structure. It causes them to slow down and so reduces the current
 The current through an ohmic conductor (at a constant temperature) is directly
proportional to the potential difference across the resistor. This means that the
resistance remains constant as the current changes




 The resistance of components such as lamps, diodes, thermistors and LDRs is not
constant; it changes with the current through the component