Electrical Charge
Contents
- 1 Key Stage 3
- 2 Key Stage 4
- 2.1 Meaning
- 2.2 About Charge
- 2.3 Equation
- 2.4 Example Calculations
- 2.4.1 Finding Current from Charge and Time
- 2.4.2 Finding Charge from Current and Time
- 2.4.3 Finding Time from Current and Charge
- 2.4.4 Finding Charge from Potential Difference and Energy Transferred
- 2.4.5 Finding Potential Difference from Charge and Energy Transferred
- 2.4.6 Finding Energy Transferred from Charge and Potential Difference
- 2.5 References
- 3 Key Stage 5
Key Stage 3
Meaning
Charge is a property of matter that can cause an electrostatic force between two objects.
About Charge
- There are two types of charge; positive and negative.
- Like charges repel each other and opposite charges attract each other.
- Charges create an electrostatic field which affects other charged objects.
To see the electrostatic field created by a charge click on the picture below to play a PHET simulation.
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Key Stage 4
Meaning
Charge is a property of matter that can cause an electrostatic force between two objects.
About Charge
- The SI Unit of charge is the Coulomb (C).
- Charge is a scalar quantity as it has magnitude but does not have a direction.
- There are two types of charge; positive and negative.
- Like charges repel each other and opposite charges attract each other.
- Charges create an electrostatic field which affects other charged objects.
- Charge is a conserved quantity which means; "Charge cannot be created or destroyed, it can only be transferred from one place to another."
- A flow of charge is an electrical current.
Equation
Equation linking Charge, Current and Time
NB: You should remember this equation with charge as the subject of the formula.
Charge = (Current) x (time)
\(Q=It\)
Where
\(Q\) = The amount of charge flowing past a point.
\(I\) = The electrical current
\(t\) = The time taken for the charge to flow.
Equation linking Charge, Potential Difference and Energy Transferred
NB: You should remember this equation with energy transferred as the subject of the formula.
Charge = (Energy Transferred)/(Potential Difference)
\(Q=\frac{E}{V}\)
Where
\(Q\) = The amount of charge that moves between two points.
\(E\) = The Energy Transferred by the charge.
\(V\) = The potential difference between two points.
Example Calculations
Finding Current from Charge and Time
| A charge of 15 Coulombs passes through a point in a circuit ever 0.52 seconds. Calculate the current flowing past this point correct to two significant figures. | A capacitor stores a charge of 10C. It discharges in 12ms. Calculate the average current flowing out of the capacitor correct to two significant figures. |
| 1. State the known quantities in correct units.
Q = 15C t = 0.52s |
1. State the known quantities in correct units.
Q = 10C t = 12ms = 12x10-3s |
| 2. Substitute the numbers and evaluate.
\(Q=It\) \(15=I \times 0.52\) |
2. Substitute the numbers and evaluate.
\(Q=It\) \(10=I \times 12 \times 10^{-3}\) |
| 3. Rearrange the equation and solve.
\(I=\frac{15}{0.52}\) \(I=28.846153A\) \(I\approx29A\) |
3. Rearrange the equation and solve.
\(I=\frac{10}{12 \times 10^{-3}}\) \(I=833.3A\) \(I\approx830A\) |
Finding Charge from Current and Time
| A battery supplies 4.7Amps to a bulb over a period of 14 seconds. Calculate the charge leaving the battery in this time correct to two significant figures. | A hairdryer uses a current of 7.2A for 5 minutes to dry a person’s hair. Calculate the charge flowing through the hairdryer in this time correct to two significant figures. |
| 1. State the known quantities in correct units.
I = 4.7A t = 14s |
1. State the known quantities in correct units.
I = 7.2A t = 5min = 300s |
| 2. Substitute the numbers into the equation and solve.
\(Q=It\) \(Q=4.7 \times 14\) \(Q = 65.8C\) \(Q \approx 66C\) |
2. Substitute the numbers into the equation and solve.
\(Q=It\) \(Q=7.2 \times 300\) \(Q = 2160C\) \(Q \approx 2200C\) |
Finding Time from Current and Charge
| A battery charger uses a current of 150mA to deliver a charge of 245 Coloumbs to a battery. Calculate the time taken to charge this battery correct to two significant figures. | A cloud in a thunderstorm loses 15C in one lightening strike. At a current of 31,000kA. Calculate how long this lightning strike lasts correct to two significant figures. |
| 1. State the known quantities in correct units.
I = 150mA = 150x10-3A Q = 245C |
1. State the known quantities in correct units.
I = 31,000kA = 3.1x107A Q = 15C |
| 2. Substitute the numbers and evaluate.
\(Q=It\) \(245 = 150 \times 10^{-3} \times t\) |
2. Substitute the numbers and evaluate.
\(Q=It\) \(15= 3.1 \times 10^7 \times t\) |
| 3. Rearrange the equation and solve.
\(t=\frac{245}{150 \times 10^{-3}}\) \(t=1633.3s\) \(t\approx1633.3s\) |
3. Rearrange the equation and solve.
\(t=\frac{15}{3.1 \times 10^7}\) \(t = 4.8387 \times 10^{-7}s\) \(t\approx4.8 \times 10^{-7}s\) |
Finding Charge from Potential Difference and Energy Transferred
| The potential difference of 12V is placed across a resistor increasing its thermal energy store by 3.7J as a result. Calculate the charge that has flowed through the resistor in this time correct to two significant figures. | A circuit transfers 2.8kJ of energy electrically to a motor. The potential difference across the motor is 1.5V. Calculate thecharge that has flowed through the motor in this time correct to two significant figures. |
| 1. State the known quantities in correct units.
V = 12V E = 3.7J |
1. State the known quantities in correct units.
V = 1.5V E = 2.8kJ = 2.8x103J |
| 2. Substitute the numbers into the equation and solve.
\(Q=\frac{E}{V}\) \(Q=\frac{3.7}{12}\) \(Q=0.3083C\) \(Q\approx0.31C\) |
2. Substitute the numbers into the equation and solve.
\(Q=\frac{E}{V}\) \(Q=\frac{2.8 \times 10^3}{1.5}\) \(Q=1866.7C\) \(Q\approx1900C\) |
Finding Potential Difference from Charge and Energy Transferred
| A charge of 84C transfers an energy of 20kJ. Calculate the potential difference correct to two significant figures. | 170J of energy is transferred by a charge of 92mC. Calculate the potential difference correct to two significant figures. |
| 1. State the known quantities in correct units.
Q = 84C E = 20kJ = 20x103J |
1. State the known quantities in correct units.
Q = 92mC = 92x10-3C E = 170J |
| 2. Substitute the numbers and evaluate.
\(Q=\frac{E}{V}\) \(84=\frac{20 \times 10^3}{V}\) |
2. Substitute the numbers and evaluate.
\(Q=\frac{E}{V}\) \(92 \times 10^{-3}=\frac{170}{V}\) |
| 3. Rearrange the equation and solve.
\(V=\frac{20 \times 10^3}{84}\) \(V=238.0952V\) \(V\approx 240V\) |
3. Rearrange the equation and solve.
\(V=\frac{170}{92 \times 10^{-3}}\) \(V=1847.826V\) \(V\approx 1800V\) |
Finding Energy Transferred from Charge and Potential Difference
| A bolt of lightning with a potential difference 31,000kV transfers a charge of 15C. Calculate the energy transferred by this bolt of lightning correct to two significant figures. | A 9V battery is able to mobilise a charge of 4.3kC during its operation. Calculate the total amount of energy stored in this battery correct to two significant figures. |
| 1. State the known quantities in correct units.
V = 31,000kV = 3.1x107V Q = 15C |
1. State the known quantities in correct units.
V = 9V Q = 4.3kC = 4.3x103 |
| 2. Substitute the numbers and evaluate.
\(Q=\frac{E}{V}\) \(15=\frac{E}{3.1 \times 10^7}\) |
2. Substitute the numbers and evaluate.
\(Q=\frac{E}{V}\) \(4.3 \times 10^3 =\frac{E}{9}\) |
| 3. Rearrange the equation and solve.
\(E = 15 \times 3.1 \times 10^7\) \(E = 4.65\times10^8 J\) \(E\approx4.7\times10^8 J\) |
3. Rearrange the equation and solve.
\(E = 4.3 \times 10^3 \times 9\) \(E = 38700J\) \(E \approx 39000 \times 10^4J\) |
References
AQA
- Charge (electrical), pages 46-52, 71, 74-5, GCSE Physics; Student Book, Collins, AQA
- Charge; electric, page 24, GCSE Physics; The Revision Guide, CGP, AQA
- Charge; electric, pages 180, 189, 190, GCSE Combined Science; The Revision Guide, CGP, AQA
- Charge; electric, pages 61, 88-90, GCSE Combined Science Trilogy; Physics, CGP, AQA
- Charge; electric, pages 63, 91-93, GCSE Physics; The Complete 9-1 Course for AQA, CGP, AQA
- Electric charge, page 24, GCSE Physics; The Revision Guide, CGP, AQA
- Electric charge, pages 180, 189, 190, GCSE Combined Science; The Revision Guide, CGP, AQA
- Electric charge, pages 61, 88-90, GCSE Combined Science Trilogy; Physics, CGP, AQA
- Electric charge, pages 63, 91-93, GCSE Physics; The Complete 9-1 Course for AQA, CGP, AQA
- Electric charge; ions, page 112, GCSE Combined Science Trilogy; Physics, CGP, AQA
- Electric charge; ions, page 124, GCSE Physics; The Complete 9-1 Course for AQA, CGP, AQA
- Electric charge; of an atom, page 111, GCSE Combined Science Trilogy; Physics, CGP, AQA
- Electric charge; of an atom, page 123, GCSE Physics; The Complete 9-1 Course for AQA, CGP, AQA
- Electric charge; relative charge, page 111, GCSE Combined Science Trilogy; Physics, CGP, AQA
- Electric charge; relative charge, page 123, GCSE Physics; The Complete 9-1 Course for AQA, CGP, AQA
- Electric charge; static, pages 99, 103, GCSE Physics; The Complete 9-1 Course for AQA, CGP, AQA
Edexcel
- Charge (electric), page 221, GCSE Physics, CGP, Edexcel
- Charge, electrical, pages 144, 160-161, GCSE Physics, Pearson Edexcel
- Charge, electrical; energy, page 145, GCSE Physics, Pearson Edexcel
- Charge, electrical; point charges, page 164, GCSE Physics, Pearson Edexcel
OCR
Key Stage 5
Meaning
Electrical charge is a conserved quantity and a property of particles which allow them to interact via the electromagnetic interaction.
About Electrical Charge
- The SI Unit of charge is the Coulomb (C).
- Charge is a scalar quantity as it has magnitude but does not have a direction.
- Charges create an electrostatic field which affects other charged objects.
- Charge is a conserved quantity which means; "Charge cannot be created or destroyed, it can only be transferred from one place to another."
- A flow of charge is an electrical current.
About Electrical Charge at the Particle Level
- Charged particles may exchange virtual photons causing a change in their momentum. This results in the force of attraction or repulsion between charged particles.
- As a conserved quantity charge cannot be created or destroyed, but it can be transferred from one particle to another. This occurs in the weak interaction via the W-boson.
- The electrical charge passed between subatomic particles during the weak interaction has a magnitude of 1.60x10-19 Coulombs (the elementary charge).
- The electrical charge of the proton is +1.60x10-19 Coulombs and the electrical charge of the electron is -1.60x10-19 Coulombs.