Trilogy Physics Chapter 2Electricity
Trilogy Physics Chapter 2Electricity
Warming up1-3 / Getting practice
4-5 / Stretching
6-8
Current - flow of electric charge; the size of the electric current is the rate of flow of electric charge; symbol I, unit amps.
Potential difference - a measure of the energy transferred per unit charge as charges move between two points in a circuit – also called the voltage between two points, symbol V, unit volts.
Resistance – shows how hard the current is being opposed and/or ratio of the potential difference across an electrical component to the current through the component; symbol R, unit ohms, Ω / t = Q / I = 96 / 6 = 16 s / a. series circuit with all components in and drawn correctly.
b.0.4 A – the current through components in seriesis the same.
c.10 Ω + 20 Ω = 30 Ω
d.8 V – the potential difference of the power supply(12 V) is shared between the components (4 Vand 8 V)
e.The current would be half as big because the totalresistance has doubled from 30Ω to 60Ω.
/ The brightness of all of the lamps decreases. The current decreases. / a. parallel circuit with all components in and drawn correctly.
b. 0.6 A – the current from the supply (1.8A) is thesum of the currents through the motor (1.2A) andthe lamp. So the current through the lamp = 1.8 –0.6 = 1.2 A.
c.12 V – the potential difference across eachcomponent in a parallel circuit is the same and isequal to the p.d. of the power supply.
d.It must be smaller than 10 Ω. Connecting resistorsin parallel decreases the total resistance, so
connecting the lamp to the motor in parallel wouldmake the resistance less than the resistance of themotor.
e.Adding the resistor decreases the total resistance,therefore the battery will be providing a biggercurrent.
a V = IR = 10 A × 100 Ω = 1000 V
b V = IR = 5 A × 3000 Ω = 15 000 V / The potential difference across the 3Ω resistor = I× R = 3 V.
The current through the 6 Ω resistor is also 1 A sothe p.d. across the 6 Ω resistor = 1 × 6 = 6 V.
Therefore, the p.d. of the battery = 3V + 6V = 9V. / Adding resistors in series increases the totalresistance of the circuit as this makes it harder for the charge to flow through. Addingresistors in parallel decreases the total resistanceof the circuit as this makes it easier for the chargeto flow through due to the extra path they canfollow.
/ Tungsten obeys Ohm’s law when the temperatureremains constant. In a filament lamp thetemperature increases as more current flowsthrough it (to thousands of degrees), so the lampdoes not obey Ohm’s law. / The current is directly proportional to the potentialdifference.
When the temperature increases, the resistancedecreases. / The resistance increases. / The resistance of a diode is very small when thecurrent goes through it in the allowed direction.This might allow the current to becomedangerously high and damage the diode. An extraresistor prevents the current from becoming too
high.
/ The I-V graph is not a straight line through the origin. / The lamp is not an ohmic conductor. Its resistanceincreases as the current and potential differencebecome higher positive and negative values.
In a direct potential difference, the current isalways pushed in the same direction. In analternating potential difference, the power supplypushes the current so that it keeps on changingdirection. / Frequency = 50 Hz
Potential difference / Voltage = 230 V / If the current becomes too large then thecurrent becomes high enough to melt the wire inthe fuse which breaks the circuit and stops thecurrent flowing.
It would be very dangerous to wire up a plug thewrong way round. Therefore, you need to be ableto identify which wire is which very easily – even ifyou are colour blind.
Brown = live
Blue = neutral
Yellow and green = earth / The charge flows from them to the ground. Thiscreates an electric current. / The earth wire prevents electric shocks if the metal casing becomes live due to a fault. The earth wire provides a path of low resistance so the currentwill be very high. This makes sure that the fuse melts and switches off the circuit.
The National Grid is a collecton of power cables and transformers that connect power stations to factories and homes across GB. / Step-down They are connected between the power cablesand factories or homes.
Step-up So there is only a small amount of energy wastedbetween the power station and thetransformer. / Higher potential difference supply
needs a smaller current (for the same power); acurrent heats the wire and wastes energy; asmaller current means the system is moreefficient. Transformers are needed tochange the pd which makes the system more complicated and prone to faults; a high pd is verydangerous.
aThe electric current turns the motor in the drill.This means that it transfers energy from the mainselectricity store into the kinetic energy stored in the
drill. Energy is also transferred to the thermalenergy stored in the surroundings.
b The power of the drill = 400 W. / E = Pt = 2500 × (45 × 60) = 6 750 000 J (or 6.75MJ) / E = QV = 30 × 230 = 6900 J
P = VI = 230 × 4 = 920 W / P = VI = 230 × 5 = 1150 W / P = I2R = 52 × 1000 = 25 000 W