Contents
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
- 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.
\(E\) = 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 15kC 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 = 15kC = 15x103 |
| 2. Substitute the numbers and evaluate.
\(Q=It\) \(245 = 150 \times 10^{-3} \times t\) |
2. Substitute the numbers and evaluate.
\(Q=It\) \(15 \times 10^3 = 3.1 \times 10^7 \times t\) |
| 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.
\(t=\frac{15 \times 10^3}{3.1 \times 10^7}\) \(t = 4.8387 \times 10^{-4}s\) \(t\approx4.8 \times 10^{-4}s\) |
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.
\(Q=\frac{E}{V}\) |
2. Substitute the numbers into the equation and solve.
\(Q=\frac{E}{V}\) |