#### Key Terms for this topic (Tier 3 vocabulary)

Diode – thermistor – resistor – charge – ohmic conductor – potential difference – series circuits – parallel circuits – alternating current – transformer.

Electricity

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#### What everyone needs to know:

You need to learn the circuit diagram components, don’t forget the more difficult ones like: LDR and thermistors. Circuits must be closed for current to flow, this current i a flow of charge and it can be calculated using:

Charge flow = current x time or Q= I t

Charge is in Coulombs (C), current in Amperes (A) and time in seconds (s).

Current is impeded by the resistance of each component, this can be calculated using V=I R, The potential difference in volts is equal to the current flowing through multiplied by the resistance in ohms (Ω).

Required practical 15 – Using circuit diagrams, construct circuits to investigate the factors that affect resistance in a circuit.

Resistors have a constant value in ohms (Ω) so as the PD increases, so does the current, however, in real life devices such as bulbs warm up so the resistance increases to the graphs are no longer straight lines. Current only flows in one direction through a diode. R for a thermistor decreases as temperature increases and they can be used as thermostats. The R in a light dependent resistor (LDR) decreases and light levels increase, these can be used to turn lights on when it gets dark. You need to be able to design circuits, explain what happens in these conditions and calculate V, I, R and Q.

Required practical 16- Construct circuits to investigate the I-V characteristics of components like filament lamps, diodes and resistor.

Series circuits have all of the components in a line with no branching wires. The current is the same through every component, however, the PD drops at each component and the total is equal to that of the power supply. Resistance in a series circuit can be measured by:

Rtotal = R1 + R2 + R3

In a parallel circuit, the potential difference across each component is the same as the power supply, however, the current varies. The total current through each branch is equal to the current through the power supply.

You need to be able to draw circuits and check other drawn circuits, add components in the correct places (ammeter in series, voltmeter in parallel) and describe the differences between series and parallel. Explain how resistance varies in series and parallel and finally, complete all of the calculations. You will only be expected to calculate the resistance in a series circuit, not a parallel one.

Domestic electricity in the UK is 230V alternating current (ac) which has a frequency of 50 Hz. Ensure that you can describe the differences between dc and ac current. Mains electricity at home has colour coded wires to help to increase safety.

• Live = brown
• Neutral = blue
• Earth = yellow and green stripes.

You should be aware of the dangers of electricity and the safety features that are available – fuse, RCCBs etc.

Electrical power is the the rate of transfer of energy, the power of an appiance can be measured using these equations:

Power (W) = potential difference (V) x Current (A) or P=VI

Power (W) = Current2 (A) x Resistance (Ω) or P= I2R

Work is done when charge flows, it then transfers energy to heat or kinetic energy in the appliance. The energy increases if the appliance is left on longer or has a higher power output. You need to be able to calculate the total energy transferred:

Energy (J) = Power (W) x time (s) or E=Pt

Energy (J) = Charge flow (Q) x Potential Difference (V) or E=QV

Our National Grid is the means of transporting electrical power from a power station to our homes. It begins with step-up transformers which increase the voltage and decrease the current to reduce energy loss as heat in the wires. Step down transformers return it to the safe levels that we have at home.

#### Extra topics needed for the Higher papers:

When describing the transformers used in the national grid, you need to be able to calculate an unknow from this equation which you are given:

PDprimary coil x Iprimary coil = PDSecondary coil x ISecondary coil

This shows that the power going into a coil is equal to the power coming out. If you rearrange this, you will be able to make the unknown quantity the subject of the equation and calculate its answer.