Year 9

Physics GCSERevision Guide

Term 1

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Contents

1. Circuit Components (Page 3)2. Series and Parallel Circuits (Page 4)3. Current Voltage (Page 5 and 6)4. Cells and Circuits (Page 7)5. Resistance and Ohm’s Law (Page 8, 9 and 10)6. LED’s, Thermistors and LDR’s (Page 11)7. Measuring Resistance (Page 12 and 13)8. Power, Current and Voltage (Page 14)9. Electrical Energy Calculations (Page 15)10.Calculating the cost of electricity (Page 16)

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Circuit Components

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Series and parallel circuits

You should know the difference between series and parallel connections in circuits

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Series connectionsComponents that are connected one after another on the same loop of the circuit are connected in series. The current that flows across each component connected in series is the same

Parallel connections

Components that are connected on separate loops are connected in parallel. The current is shared between each component connected in parallel. The total amount of current flowing into the junction, or split, is equal to the total current flowing out. The current is described as being conserved.

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Current and voltage

You need to know how to measure the current that flows through a component in a circuit. You also need to know how to measure the potential difference, also called voltage, across a component in a circuit

CurrentA current flows when an electric charge moves around a circuit. No current can flow if the circuit is broken, for example, when a switch is open.

Measuring current:

Current is measured in amperes Amperes is often abbreviated to amps or A The current flowing through a component in a circuit is measured using an ammeter The ammeter must be connected in series with the component

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Potential difference - voltageA potential difference, also called voltage, across an electrical component is needed to make a current flow through it. Cells or batteries often provide the potential difference needed.

Measuring potential difference:

Potential difference is measured in volts, V Potential difference across a component in a circuit is measured using a voltmeter The voltmeter must be connected in parallel with the component

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Cells and circuits

You should know what happens to the potential difference and current when the number of cells in a circuit is changed.

Potential differenceA typical cell produces a potential difference of 1.5 V. When two or more cells are connected in series in a circuit, the total potential difference is the sum of their potential differences. For example, if two 1.5 V cells are connected in series in the same direction, the total potential difference is 3.0 V.

CurrentWhen more cells are connected in series in a circuit, they produce a bigger potential difference across its components. More current flows through the components as a result.

The diagrams below shows what happened to the reading on the ammeter when more cells are connected in a series circuit.

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Resistance and Ohm's Law

Why do we get resistance?

An electric current flows when charged particles called electrons move through a conductor. The moving electrons can collide with the atoms of the conductor. This makes it more difficult for the current to flow, and causes resistance. Electrons collide with atoms more often in a long wire than they do in a short wire. A thin wire has fewer electrons to carry the current than a thick wire.

This means that the resistance in a wire increases as:

the length of the wire increases the thickness of the wire decreases

Ohm's LawResistance is measured in ohms. The symbol for an ohm looks like this:  Ω 

The greater the number of ohms, the greater the resistance.

The equation below shows the relationship between voltage, current and resistance:

potential difference (volt, V) = current (ampere, A) × resistance (ohm, Ω )

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Ohm’s Law: Resistor

The current flowing through a resistor at a constant temperature is directly proportional to the voltage across the resistor. So, if you double the voltage, the current also doubles. This is called Ohm's Law. The graph shows what happens to the current and voltage when a resistor follows Ohm's Law.

The filament lamp

The filament lamp is a common type of light bulb. It contains a thin coil of wire called the filament. This heats up when an electric current passes through it, and produces light as a result.

The resistance of a lamp increases as the temperature of its filament increases. The current flowing through a filament lamp is not directly proportional to the voltage across it. This is the graph of current against potential difference for a filament lamp.

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The diode

Diodes are electronic components which can be used to regulate the potential difference in circuits. Diodes only allow current to flow in one direction.

The diode has a very high resistance in one direction. This means that current can only flow in the other direction. This is the graph of current against potential difference for a diode:

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LED’s, Thermistors and LDR’s

LEDs

A light-emitting diode, LED, produces light when a current flows through it in the forward direction. LEDs are often used for indicator lights in electrical equipment such as computers and television sets. As LEDs use a much smaller current than other types of lighting, their use is increasing.

Thermistor

Thermistors are used as temperature sensors - for example, in fire alarms. Their resistance decreases as the temperature increases:

At low temperatures, the resistance of a thermistor is high and little current can flow through them

At high temperatures, the resistance of a thermistor is low and more current can flow through them

LDR

Light dependent resistor (LDR)

LDRs (light-dependent resistors) are used to detect light levels, for example, in automatic security lights. Their resistance decreases as the light intensity increases:

In the dark and at low light levels, the resistance of an LDR is high and little current can flow through it

In bright light, the resistance of an LDR is low and more current can flow through it

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Measuring Resistance

You should know how to work out the resistance in a series circuit, and also how to change the resistance in a circuit.

Resistor in series

When resistors are connected in series, the current through each resistor is the same. In other words, the current is the same at all points in a series circuit.

When resistors are connected in series, the total potential difference across all the resistors is equal to the sum of the potential differences across each resistor.

In other words, the potential differences around the circuit add up to the potential difference of the supply.

The total resistance of a number of resistors in series is equal to the sum of all the individual resistances.

For example, if a 2Ω resistor, a 1Ω resistor and a 3Ω resistor are connected side by side, their total resistance is 2 + 1 + 3 = 6Ω.

Sum of resistance is 6 ohms

If you increase the number of lamps in a series circuit, the total resistance will increase and less current will flow.

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Resistors in parallel

When resistors are connected in parallel, the supply current is equal to the sum of the currents through each resistor. In other words the currents in the branches of a parallel circuit add up to the supply current.

When resistors are connected in parallel, they have the same potential difference across them. In other words, any components in parallel have the same potential difference across them.

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Power, current and voltage

The power of an electrical appliance can be calculated from the current that flows through it and the potential difference across it.

Calculating powerYou can work out power using this equation:

Power = voltage × current

Power is measured in watts, W Voltage (potential difference) is measured in volts, V Current is measured in amperes (amps), A

Example Calculation:

What is the power of a 5 A 1.5 V lamp?

Power = 5 × 1.5 = 7.5 W

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Electrical energy calculations

The amount of electrical energy transferred to an appliance depends on its power and the length of time it is switched on. The amount of mains electrical energy transferred is measured in kilowatt-hours, kWh. One unit is 1 kWh.

E = P × t

E is the energy transferred in kilowatt-hours, kWh P is the power in kilowatts, kW T is the time in hours, h.

Note that power is measured in kilowatts instead of watts. To convert from W to kW you must divide by 1,000. For example, 2,000 W = 2,000 ÷ 1,000 = 2 kW.

Also note that time is measured in hours instead of seconds. To convert from seconds to hours you must divide by 3,600. For example, 7,200 s = 7,200 ÷ 3,600 = 2 h.

Example calculation:

A 2 kW electrical fire is switched on for 3 hours. It uses 2 × 3 = 6 kWh of electrical energy.

You also use the equation;

E = P × t when:

E is the energy transferred in joules, J P is the power in watts, W T is the time in seconds, s.

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Cost of electricity

Electricity meters measure the number of units of electricity used in a home or other building. The more units used, the greater the cost. The cost of the electricity used is calculated using this equation:

total cost = number of units × cost per unit

For example, if 5 units of electricity are used at a cost of 8p per unit, the total cost will be 5 × 8 = 40p.

Remember that the number of units used can be calculated using this equation:

units (kWh) = power (kW) × time (h) … so …

total cost = power (kW) × time (h) × cost per unit

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