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Grade Boundaries A* 90% A 80% B 70% C 60% P5 Electric Circuits Model of the Atom • Electrons Negative • Protons Positive • Neurons no charge Static Electricity • Materials can only lose or gain Electrons • Protons don’t move • This will make them positive or negative • Static Electricity only happens between two insulators. • Conductors direct the charges to the Earth. Structure of Metals • Conductors – Electrons free to move • Insulators – Electrons are not free to move Circuit Symbols • • • • • • • • • • • Open Switch Closed Switch Lamp Cell Battery Voltmeter Resistor Ammeter Variable resistor Thermistor Light dependent resistor (LDR) Resistance • Moving electrons can collide with atoms of a conductor. This is called resistance. • Resistance makes it harder for the current to flow. • These collisions also make a conductor hot. EG: this is why a light bulb is hot. Thermistors • Low Temperatures = High Resistance • High Temperatures = Low Resistance Light Dependent Resistors • Darkness = High Resistance • Light = Low Resistance Complete / Incomplete Circuits • Ammeter – Measures Current in Amps. – Must be connected in series. • Voltmeter – Measures Voltage (Potential Difference) in Volts – Must be connect in parallel Series / Parallel Circuits • Series: if lamp breaks, circuit breaks • Parallel: if lamp breaks, the other lamp will still work Series Circuits –Atotal = A1=A2=A3 –Vtotal = V1+V2+V3 Parallel Circuits –Atotal=A1+A2+A3 –Vtotal=V1=V2=V3 Ohm’s Law • The current is directly proportional to the voltage. • IE: if you double the voltage, the current will also double. Resistors in Series and Parallel Electromagnetic Induction • Moving a magnet through a coil of wire produces an electric current. • To increase the voltage you can: 1. Move the magnet faster 2. Use a stronger magnet 3. Increase the number of coils Electric Generator • • • Rotating coil inside a magnet The size of the induced voltage can be increased by: 1. Increasing the speed of rotation of the coil 2. Increasing the strength of the magnetic field 3. Increasing the number of turns on the coil 4. Placing an iron core inside the coil Generators produce Alternating Current AC/DC • Alternating Current: Produced in Generators. Current reverses it’s flow every half turn of the magnet. • Direct Current: Produced by a battery. Current flows in only one direction. Transformers • Change the Voltage of an AC supply • Step-Up Transformer: Increases Voltage • Step-Down Transformer: Decreases Voltage Transformers • Primary coil connected to AC supply • Changing current produces a changing Magnetic Field • This induces an AC in the secondary coil Transformer Equation Secondary Voltage = Number of Coils Secondary Primary Voltage Number of Coils Primary Power Energy Transferred Energy Transferred = Power x Time Energy = kilowatt-hours Power = kilowatts (1000 watts) Efficiency • Energy cannot be created or destroyed. It can only be transferred from one form to another. • Most Electrical Energy is wasted in a lightbulb as Heat Energy. Cost of Electricity Cost = Number of Units x Cost of One Unit Efficiency Efficiency = (useful energy/total energy)x100