Circuits are drawn using standard circuit symbols. 2 Electricity Topic overview Circuits Circuits are drawn using standard circuit symbols. A cell is shown as with the long end positive. A battery is a combination of cells, connected positive to negative. Circuits can be connected in series or parallel or a combination of both. © Hodder & Stoughton 2016

Electrical charge and current 2 Electricity Topic overview Electrical charge and current Charge, Q, is measured in Coulombs (C). Potential difference or p.d., V, is the electrical work done per coulomb in a circuit component. Potential difference is measured in Volts (V). Current, I, is the flow of charge, Q, per second in a circuit. Current, I, is measured in Amperes or Amps (A) 𝑄=𝐼×𝑡 or 𝐼= 𝑄 𝑡 where t is the time in seconds. Current is the same at all points in a series circuit. © Hodder & Stoughton 2016

Current and potential difference 2 Electricity Topic overview Current and potential difference Current in a circuit can be changed by changing the p.d. or the resistance. Current is measured by adding an ammeter in series in a circuit. P.d. is measured by adding a voltmeter in parallel with cells or other components. © Hodder & Stoughton 2016

Resistance, R, is measured in Ohms (Ω) 2 Electricity Topic overview Resistance Resistance, R, is measured in Ohms (Ω) Potential difference = current x resistance 𝑉=𝐼×𝑅 Resistance is sometimes measured in kilohms (kΩ) or megohms (MΩ) © Hodder & Stoughton 2016

Resistance 2 Electricity Topic overview In ohmic conductors, the resistance is always the same. For some conductors, e.g. a filament lamp, the resistance increases with temperature. © Hodder & Stoughton 2016

Resistance 2 Electricity Topic overview In a diode, the resistance depends on size and direction of the p.d. For other resistors, e.g. a LDR or most thermistors, the resistance changes with an external property. LDR resistance decreases with increasing light intensity Thermistor resistance decreases with increasing temperature LDRs and thermistors are used in switching circuits, e.g. automatic lights and thermostats. © Hodder & Stoughton 2016

Series Circuits 2 Electricity Topic overview In a series circuit the p.d. is shared between components in the same proportions as their resistances. Total p.d., 𝑉= 𝑉 1 + 𝑉 2 Current is the same at all places, 𝐴 1 = 𝐴 2 Total resistance, 𝑅= 𝑅 1 + 𝑅 2 Current in a series circuit reduces since total resistance is greater © Hodder & Stoughton 2016

Parallel Circuits 2 Electricity Topic overview In a parallel circuit the p.d. across every component in parallel is the same. The total current is equal to the sum of the currents through the separate components. The current is shared between the parallel components in inverse proportion to their resistance. 𝐴 1 = 𝐴 2 + 𝐴 3 The total resistance of resistors in parallel is always lower than that of the lowest resistor. Current in a parallel circuit increases since the total resistance is smaller. © Hodder & Stoughton 2016

Direct and alternating potential difference 2 Electricity Topic overview Direct and alternating potential difference Cells have a steady p.d. and cause direct current (d.c.) in a circuit. Direct current is always in the same direction. An alternating current (a.c.) is caused by an alternating p.d. which changes direction many times per second. In the UK mains electricity has a p.d. of about 230V and changes direction at 50 Hz. © Hodder & Stoughton 2016

Domestic uses and safety Mains cables have three wires 2 Electricity Topic overview Domestic uses and safety Mains cables have three wires Brown – live Blue – neutral Green/Yellow – Earth The live wire changes potential compared to the neutral The neutral is at or near Earth potential The Earth wire is for safety and only carries a current in the case of a fault. This large current will cause a fuse to melt or a circuit-breaker to trip so preventing fire or other danger. © Hodder & Stoughton 2016

Power 2 Electricity Topic overview Electrical power, P, is measured in Watts (W) or kilowatts (kW). Electrical power = p.d. x current 𝑃=𝑉×𝐼 and since 𝑉=𝐼×𝑅 then 𝑃= 𝐼 2 ×𝑅 Energy, E, is transferred in an appliance or component 𝐸=𝑃×𝑡 where t is the time in seconds. and 𝐸=𝑄×𝑉 where Q is the charge that has flowed. Mains appliances give their power in kW A kettle of power 2.2 kW = 2200 W therefore 𝐼= 𝑃 𝑉 = 2200 230 =9.6 A The National grid transfers electrical power from where it is generated to the consumer. Transformers are used to make this transfer of electrical power more efficient by reducing power loss in the transmission lines. © Hodder & Stoughton 2016

Static electricity 2 Electricity Topic overview This is stationary charge on the surface of an object. Often achieved by rubbing an object which adds or removes negative charge (electrons). unlike charges attract like charges repel Charge can jump from one charged object to another oppositely charged object causing a spark, e.g. lightning. A charged object has an electric field around it. A charge experiences a force in an electric field. The direction of the field is the direction of the force on a positive charge. A charged sphere has a radial electric field around it. © Hodder & Stoughton 2016