P5 – Electric Circuits

Static Electricity• When two objects are rubbed together and become charged,

electrons are transferred from one object to the other

• Repulsive forces between similar charges• Attractive forces between opposite charges• Explain simple electrostatic effects in terms of attraction

and repulsion between charges

Electric Current• Current is a flow of charge• Measured in amperes (Amps)• Wires, bulbs, etc are full of charges that are free to move • The battery causes these free charges to move• These charges are not used up but flow in a continuous

loop

Conductors and Insulators• Conductors– Electrons free to

move – EG: Metals

• Insulators– Electrons are not

free to move– EG: Plastic

Resistance• Components (for example, resistors, lamps, motors) resist the

flow of charge through them• The larger the resistance in a given circuit, the smaller the current

will be• The resistance of connecting wires is so small that it can usually

be ignored

Resistors Get Hot• Resistors get hotter when electric current passes

through them• This heating effect is caused by collisions between the

moving charges and stationary atoms in the wire• This heating effect makes a lamp filament hot enough to

glow

Thermistors• Low Temperatures = High Resistance• High Temperatures = Low Resistance

Light Dependent Resistors

• Low Light = High Resistance• Bright Light = Low Resistance

Circuit Symbols1. Open Switch2. Closed Switch3. Lamp4. Cell5. Battery6. Voltmeter7. Resistor8. Ammeter9. Variable resistor10. Thermistor11.Light dependent resistor (LDR)

Series and Parallel• Two (or more) resistors in series have more resistance

than one on its own, because the battery has to push charges through both of them

• explain that two (or more) resistors in parallel provide more paths for charges to flow along than one resistor on its own, so the total resistance is less and the current is bigger

Ohm’s Law

When the Resistance is Constant

• The current is directly proportional to the voltage.

• IE: if you double the voltage, the current will also double.

Voltage / Potential Difference• The larger the voltage of the battery the bigger the current

• The voltage of a battery (measured in V) provides a measure of the ‘push’ of the battery on the charges in the circuit

• Voltmeters are connected either side of the component you are measuring.

Voltage / Potential Difference• The energy given or taken away from the

charge as it moves between two points

Adding Batteries

• When you add batteries in Parallel, the Voltage and the Current stay the same

• When you add batteries in Series the Voltage and Current increase

Series Circuits1. Current through each component is the same2. The voltage across the components add to the

voltage across the battery (The total energy transferred to each unit of charge by the battery must equal the amount transferred from it to other components)

3. The voltage is largest across the component with the greatest resistance (more energy is transferred by the charge passing through a large resistance)

Parallel Circuits1. Voltage across each component is equal to the

voltage of the battery2. Current through each component is the same as

if it were the only component present3. Total current from (and back to) the battery is

the sum of the currents through each of the parallel components

4. Current is largest through the component with the smallest resistance, because the same battery voltage causes more current to flow through a smaller resistance than a bigger one.

Transformers• Changing Current in a coil = Changing Magnetic Field

• This changing magnetic field can induce a voltage in a neighbouring coil

Transformers• Transformer = two coils of wire wound on an iron core

• A transformer can change the size of an Alternating Voltage

Transformer Equation

Secondary Voltage = Number of Coils SecondaryPrimary Voltage Number of Coils Primary

Generator• A magnet or electromagnet is rotated within a coil of wire (or

Rotating coil inside a magnet) to induce a voltage across the ends of the coil

• The size of the induced voltage can be increased by:1. Increasing the speed of rotation of the magnet (or coil)2. Increasing the strength of the magnetic field3. Increasing the number of turns on the coil4. Placing an iron core inside the coil

Alternating Current• The induced voltage across the coil of a

generator changes during each revolution of the magnet or electromagnet and explain that the current produced in an external circuit is an Alternating Current (a.c.)

Alternating v Direct Current• When the current is always in the same

direction, it is a direct current (d.c.), e.g. the current from a battery

• Mains electricity is an a.c. Supply• a.c. is used because it is easier to generate than

d.c., and can be distributed more efficiently

Mains Electricity• Mains electricity is produced by generators• Mains supply voltage to our homes is 230 volts.

Energy Transfer• When electric charge flows through a

component (or device), energy is transferred to the component

• Energy (J, kWh)• Power (W, kW)• Time (s, hr)

• A joule is a very small amount of energy, so a domestic electricity meter measures the energy transfer in kilowatt hours

Power• Power is a measure of the rate at which an

appliance or device transfers energy

Cost of Electricity• Calculate the cost of electrical energy given the

power, the time and the cost per kilowatt hour• Multiply the Energy by the by the cost per kWh.

Efficiency of Electrical Appliances• Efficiency (%) = (Useful Energy / Total Energy) x 100